JOURNAL OF MORPHOLOGY 00:1–20 (2014)

Anatomy and Early Development of the Pectoral Girdle, Fin, and Fin Spine of Sturgeons (Actinopterygii: Acipenseridae) Casey B. Dillman* and Eric J. Hilton Virginia Institute of Marine Science, College of William & Mary, Gloucester Point, Virginia ABSTRACT Acipenseriformes hold an important place in the evolutionary history of bony fishes. Given their phylogenetic position as extant basal Actinopterygii, it is generally held that a thorough understanding of their morphology will greatly contribute to the knowledge of the evolutionary history and the origin of diversity for the major osteichthyan clades. To this end, we examined comparative developmental series from the pectoral girdle in Acipenser fulvescens, A. medirostris, A. transmontanus, and Scaphirhynchus albus to document, describe, and compare ontogenetic and allometric differences in the pectoral girdle. We find, not surprisingly, broad congruence between taxa in the basic pattern of development of the dermal and chondral elements of the pectoral girdle. However, we also find clear differences in the details of structure and development among the species examined in the dermal elements, including the clavicle, cleithrum, supracleithrum, posttemporal, and pectoral-fin spine. We also find differences in the internal fin elements such as the distal radials as well as in the number of fin rays and their association with the propterygium. Further, there are clear ontogenetic differences during development of the dermal and chondral elements in these species and allometric variation in the pectoral-fin spine. The characters highlighted provide a suite of elements for further examination in studies of the phylogeny of sturgeons. Determining the distribution of these characters in other sturgeons may aid in further resolution of phylogenetic relationships, and these data highlight the role that ontogenetic and comparative developmental studies provide in systematics. J. Morphol. 000:000–000, 2014. VC 2014 Wiley Periodicals, Inc.

KEY WORDS: Acipenseriformes; appendicular skeleton; ontogeny; character; phylogeny; Acipenser; Scaphirhynchus

INTRODUCTION Acipenseriformes (sturgeons, paddlefishes, and their fossil relatives) hold an important position in the evolutionary history of bony fishes. As nonteleostean actinopterygians (ray-finned fishes), acipenseriforms are among the earliest extant lineages to have branched from the lines leading to the neopterygians (gars, bowfins, and teleosts, the group comprising over half of all living vertebrates) as well as near the split between actinopterygian fishes and sarcopterygians (lobe-finned “fishes” and tetrapods). It is generally held, therefore, that detailed understanding of their morpholC 2014 WILEY PERIODICALS, INC. V

ogy will inform our understanding of the evolutionary history and the origin of diversity for these major osteichthyan clades, as well as contribute necessary character information for comparative analyses and phylogenetic reconstruction. Extant Acipenseriformes are represented by two families: Polyodontidae and Acipenseridae (Bemis et al., 1997). Polyodontids are composed of two extant species, each in a separate genus, one of which, Psephurus gladius, is likely extinct (Fan et al., 2006). Acipenserids are more diverse than polyodontids and are represented by approximately 25 extant species, which are broadly recognized as being classified in four genera. Diversity of the Acipenseriformes from the fossil record is also quite extensive (Grande and Bemis, 1996; Jin, 1999; Grande et al., 2002; Grande and Hilton, 2006; Hilton and Forey, 2009). The family Acipenseridae traditionally has been divided into two subfamilies: Acipenserinae (including Acipenser and Huso) and Scaphirhynchinae (including Pseudoscaphirhynchus and Scaphirhynchus, Mayden and Kuhajda, 1996). Alternatively, Findeis (1997) defined the Husinae (including only Huso) and Acipenserinae (including Acipenser, Pseudoscaphirhynchus and Scaphirhynchus, the latter two genera forming the tribe Scaphirhynchini). Recently, the genus Pseudoscaphirhynchus was shown to be a possible paraphyletic grade in a monophyletic assemblage that

Grant number: U.S. National Science Foundation DEB-0841691. *Correspondence to: Casey B. Dillman; Virginia Institute of Marine Science, College of William & Mary, Gloucester Point, VA. E-mail: [email protected] *Present address: Casey B. Dillman is currently at Department of Vertebrate Zoology, National Museum of Natural History, PO Box 37012, Smithsonian Institution, Washington, D.C. 20013 Received 20 March 2014; Revised 12 August 2014; Accepted 14 September 2014. Published online 00 Month 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jmor.20328

2

C. B. DILLMAN AND E. J. HILTON

included the genus Scaphirhynchus (Mayden and Kuhajda, 1996) or sister to Acipenser stellatus (Birstein et al., 2002; Hilton, 2005; Hilton et al., 2011). Although there remains a poor understanding of relationships within the family, its monophyly has never been questioned. Mayden and Kuhajda (1996), Findeis (1993, 1997), and Hilton et al. (2011) investigated the systematic relationships of the family Acipenseridae based on morphological character data, and determined that it is united by 10 or 11 synapomorphies (Findeis, 1997, Hilton et al., 2011), one of which is the presence of a pectoral-fin spine (Mayden and Kuhajda’s (1996) study was focused on the relationships of Scaphirhynchus and Pseudoscaphirhynchus, and therefore, did not sample characters densely at a more general level). The pectoral-fin spine is formed by a layer of dermal bone that covers the fin rays associated with the propterygium (Findeis, 1993). Other Acipenseriformes have a pectoral-fin spine, for example, †Chondrosteus (Hilton and Forey, 2009), †Protopsephurus (Grande et al., 2002), and †Yanosteus (Jin, 1999), but those of Acipenseridae appear to be unique among Acipenseriformes in their form. Acipenserid pectoral-fin spines are often recovered in the fossil record and have been the basis for several species names, although these have recently been regarded as nomina dubia based in part on lack of known differentially diagnostic characters of the pectoral-fin spines (Hilton and Grande, 2006). However, there is substantive variation in their overall form within the family (Findeis, 1997; this study). Findeis (1993) investigated the structure of the pectoral-fin spine as part of an in-depth investigation into the ontogeny and anatomy of the genus Scaphirhynchus, which served as the basis for his study of the phylogeny of Acipenseridae. In that study, Findeis (1997) noted that spine “robustness,” that is the thickness of the pectoral spine, was likely the result of the total number of fin rays eventually incorporated under the layer of dermal bone encompassing the fin rays. Hilton et al. (2011) noted that the thickness of the fin rays incorporated also likely contributes to fin spine robustness, and that future work on the pectoral-fin spine should involve both ontogenetic and species diversity components for comparative study to determine the actual number of fin rays incorporated into the developing spine. The primary goal of this article is to document the development of the pectoral girdle and the pectoral-fin spine for four North American species of sturgeon: A. fulvescens, A. medirostris, A. transmontanus, and Scaphirhynchus albus. Based on these new morphological data, we revisit the question of, “What is a sturgeon pectoral-fin spine?” and propose characteristics from the dermal and chondral elements of the pectoral fin and girdle Journal of Morphology

that differ among these species to be included in future investigations of morphological characters across the family Acipenseridae. MATERIALS AND METHODS Developmental series of four species of Acipenseridae (A. fulvescens, A. medirostris, A. transmontanus, and S. albus) were collected. Specimens were preserved by hatchery personnel by direct immersion in 4% neutrally buffered paraformaldehyde at regular intervals after hatching (e.g., once per day for the first 14 days post hatch [DPH], once every other day from 16 to 36 DPH, and weekly from 36 DPH through 68 DPH). Once received from the hatchery, specimens were transferred to 70% ethanol for storage. Observations from these developmental series were supplemented by larger juvenile specimens available and collected from other hatcheries and from museum collections (UAIC, University of Alabama Ichthyology Collection, Tuscaloosa, AL; VIMS, Virginia Institute of Marine Science, Gloucester Point, VA). Individuals of each species were selected from each stage and were cleared and double-stained for bone and cartilage following a modified protocol based on Dingerkus and Uhler (1977). Individuals from these cleared and stained series were selected to document changes and relative timing of development during ontogeny of the dermal elements of the pectoral girdle, pectoral fin, the propterygium, metapterygium, the proximal and distal radials, pectoral-fin rays, and the pectoral-fin spine. Observations were made on the morphology of the scapulocoracoid as well, although this element was not emphasized in our descriptions. All measurements reported in this article are total lengths (TL) unless otherwise indicated. Measurements of the left pectoral-fin spine were taken across a broader growth trajectory of the series of cleared and double-stained specimens to determine the fin spine length relative to TL. A total of 16 A. fulvescens (30– 133 mm TL), 20 A. medirostris (17.9–75 mm TL), 56 A. transmontanus (19.1–121 mm TL), and 31 S. albus (17.9–125 mm TL) were measured for pectoral-fin spine length. Additionally, earliest appearance of each element was determined for each taxon to compare patterns of early ontogeny of the pectoral skeleton. Specimens were examined using a Zeiss D20 microscope with an attached high-resolution Axiocam camera, and were imaged in dorsal, lateral, and ventral orientation with anterior facing left. Final images were saved as .tif files, backgrounds were cleaned, and color and contrast were adjusted in Adobe Photoshop CS2.

Material Examined A. fulvescens Rafinesque, 1817. VIMS 13577 (developmental series of 303 specimens, representing 1-68 DPH; 87 c&s); VIMS 17717 (1 c&s). A. medirostris Ayres, 1854. VIMS 17715 (developmental series of 273 specimens, representing 1-68 DPH and with 84 c&s). A. transmontanus Richardson, 1837. VIMS 17716 (developmental series of 748 specimens, representing 1-62 DPH and with 82 c&s). S. albus (Forbes and Richardson, 1905). UAIC 12581.01 (2 of 4, c&s); UAIC 12583.01 (2 of 4, c&s); UAIC 12582.01 (2 of 4, c&s); VIMS 12193 (113 specimens, including 21 c&s); VIMS 17714 (developmental series of 122 specimens, representing 1-68 DPH and with 39 c&s). Other Acipenseridae. Materials listed in Hilton et al. (2011), including early life history stages of A. brevirostrum, were used for comparative purposes.

RESULTS General Form of the Pectoral Fin and Girdle The pectoral fin and girdle of Acipenseridae is composed of both ossified and cartilaginous

ANATOMY AND EARLY DEVELOPMENT

3

Fig. 1. Generalized pectoral girdle of a sturgeon based on Acipenser brevirostrum. Bone is light gray and cartilage is dark gray. A. Lateral view of the pectoral girdle and fin elements. B. Medial view of the pectoral girdle and fin elements. C. Dorsal view of the pectoral girdle and fin elements. D. Ventral view of the pectoral girdle. E. Lateral view of the pectoral girdle and dermal skull bones. After Hilton et al. (2011: Figs. 88, 90, 92, 38, and 26, respectively). br, branchiostegal; cl, cleithrum; clv, clavicle; cor, coracoid; dr, distal radials; ds1, first dorsal scute; dpt, dermopterotic; excm, median extrascapular; iclv, interclavicle; mcor, mesocoracoid; mtg, metapterygium; pa, parietal; pcl, postcleithrum; pfs, pectoral-fin spine; pt, posttemporal; ptg, propterygium; ra, radials; sc, scapula; scc, scapulocoracoid cartilage; scl, supracleithrum; sop, subopercle; sscc, suprascapular cartilage.

elements. A recent study of A. brevirostrum by Hilton et al. (2011) provides the basis for the outline of our comparative ontogenetic data (Fig. 1). Within our descriptions, first we treat the dermal skeleton in dorsal to ventral sequence: the paired posttemporal, supracleithrum, cleithrum, postcleithrum, clavicle, and the median interclavicle. Next, we describe the chondral portion of the pectoral girdle, including the suprascapular cartilage and the scapulocoracoid cartilage and its ossifications (scapula, coracoid, mesocoracoid). These ossi-

fications are typically only found in larger specimens, as are ossifications of the radials, propterygium and metapterygium; none were observed in the early ontogenetic stages described herein. Finally, we describe the development of the pterygial (propterygium and metapterygium) and radial elements (proximal and distal radials), the fin rays, and the fin spine. Our descriptions of each element, and our figures, which include lateral and ventral developmental sequences (Figs. 2–9), are arranged taxonomically with the three species Journal of Morphology

4

C. B. DILLMAN AND E. J. HILTON

Fig. 2. Pectoral girdle and fin spine development of Acipenser fulvescens. All images in lateral view with anterior facing left. A. from VIMS 13577 (vial Af10–15); 22.0 mm; 16 DPH. B. from VIMS 13577 (vial Af10_jar3); 34.8 mm; 68 DPH. C. from VIMS 13577 (vial Af10_jar3); 41.1 mm; 68 DPH. D. from VIMS 13577 (vial Af10_jar2); 45.2 mm; 56 DPH. E. from VIMS 13577 (Af10_jar3); 47.0 mm; 68 DPH. F. from VIMS 17717; 170.0 mm; unknown DPH. br, branchiostegal; cl, cleithrum; clv, clavicle; dr, distal radials; ds1, first dorsal scute; iclv, interclavicle; pcl, postcleithrum; pfs, pectoral-fin spine; prp, propterygial restraining process; pt, posttemporal; ra, radials; scc, scapulocoracoid cartilage; scl, supracleithrum; sop, subopercle.

of Acipenser treated first and in alphabetical order by species name followed by the genus Scaphirhynchus. The fin itself is variable in shape, as is the length and thickness of the spine. The fin extends Journal of Morphology

laterally from the girdle, and is supported by the propterygium anteriorly and the metapterygium posteriorly; these both articulate directly with the scapulocoracoid cartilage along a distinct ridge. The propterygium and metapterygium flank the

ANATOMY AND EARLY DEVELOPMENT

5

Fig. 3. Pectoral girdle and fin spine development of Acipenser fulvescens. All images in ventral view with anterior facing left. A. from VIMS 13577 (vial Af10–15); 22.0 mm; 16 DPH. B. from VIMS 13577 (vial Af10_jar3); 34.8 mm; 68 DPH. C. from VIMS 13577 (vial Af10_jar3); 41.1 mm; 68 DPH. D. from VIMS 13577 (vial Af10_jar2); 45.2 mm; 56 DPH. E. from VIMS 13577 (Af10_jar3); 47.0 mm; 68 DPH. F. from VIMS 17717; 170.0 mm; unknown DPH. br, branchiostegal; cl, cleithrum; clv, clavicle; dr, distal radials; iclv, interclavicle; mtg, metapterygium; pfs, pectoral-fin spine; prp, propterygial restraining process; ptg, propterygium; ra, radials; sc, scapula; scc, scapulocoracoid cartilage; scl, supracleithrum; sop, subopercle.

proximal and distal radials and all elements support the fin rays. These elements ossify perichondrally in large individuals of acipenserids

(Findeis, 1993, 1997, Hilton et al., 2011). A robust fin spine, formed by the fusion of fin rays and by dermal bone surrounding the fin-ray base, makes Journal of Morphology

6

C. B. DILLMAN AND E. J. HILTON

Fig. 4. Pectoral girdle and fin spine development of Acipenser medirostris. All images in lateral view with anterior facing left. A. from VIMS 17715 (vial Am11-10); 19.0mm; 10 DPH. B. from VIMS 17715 (vial Am11–13); 23.5 mm; 13 DPH. C. from VIMS 17715 (vial Am11–20); 27.0 mm; 26 DPH. D. from VIMS 17715 (vial Am11–21); 35.0 mm; 28 DPH. E. from VIMS 17715 (vial Am11–23); 41.5 mm; 36 DPH. F. from VIMS 17715 (vial Am11-Jar3); 69.0 mm; 68 DPH. br, branchiostegal; cl, cleithrum; clv, clavicle; dr, distal radials; ds1, first dorsal scute; iclv, interclavicle; pcl, postcleithrum; pfs, pectoral-fin spine; prp, propterygial restraining process; pt, posttemporal; ra, radials; scc, scapulocoracoid cartilage; scl, supracleithrum; sop, subopercle.

up the anterior edge of the pectoral fin; and the number of fin rays incorporated into the spine varies among taxa (Findeis, 1997, Hilton et al., 2011). Journal of Morphology

Ossifications of the Pectoral Girdle Posttemporal bone. The posttemporal bone of Acipenseridae is incorporated into the skull roof and receives the sensory canal from the

ANATOMY AND EARLY DEVELOPMENT

7

Fig. 5. Pectoral girdle and fin spine development of Acipenser medirostris. All images in ventral view with anterior facing left. A. from VIMS 17715 (vial Am11-10); 19.0 mm; 10 DPH. B. from VIMS 17715 (vial Am11–13); 23.5 mm; 13 DPH. C. from VIMS 17715 (vial Am11-20); 27.0 mm; 26 DPH. D. from VIMS 17715 (vial Am11-21); 35.0 mm; 28 DPH. E. from VIMS 17715 (vial Am11-23); 41.5 mm; 36 DPH. F. from VIMS 17715 (vial Am11-Jar3); 69.0 mm; 68 DPH. br, branchiostegal; cl, cleithrum; clv, clavicle; dr, distal radials; iclv, interclavicle; mtg, metapterygium; pfs, pectoral-fin spine; prp, propterygial restraining process; ptg, propterygium; ra, radials; sc, scapula; scc, scapulocoracoid cartilage; scl, supracleithrum; sop, subopercle.

supracleithrum; the canal continues anteriorly into the dermopterotic. The posttemporal (Fig. 2B) first appears in A. fulvescens at 24.4 mm TL and by 30 mm TL, it is a large bone, roughly in the

shape of a flattened J, with the posterior portion directed dorsomedially from the point it contacts the supracleithrum. The posterior and lateral edges of the posttemporal meet posteriorly at a Journal of Morphology

8

C. B. DILLMAN AND E. J. HILTON

Fig. 6. Pectoral girdle and fin spine development of Acipenser transmontanus. All images in lateral view with anterior facing left. A. from VIMS 17716 (vial At10-8); 19.1 mm; 8 DPH. B. from VIMS 17716 (vial At10-14); 19.1 mm; 14 DPH. C. from VIMS 17716 (vial At10-18); 29.9 mm; 18 DPH. D. from VIMS 17716 (vial At10-21); 35.6 mm; 21 DPH. E. from VIMS 17716 (vial At10-23); 42.8 mm; 23 DPH. F. from VIMS 17716 (vial At10-62); 131.4 mm; 62 DPH. br, branchiostegal; cl, cleithrum; clv, clavicle; dr, distal radials; ds1, first dorsal scute; iclv, interclavicle; pcl, postcleithrum; pfs, pectoral-fin spine; prp, propterygial restraining process; pt, posttemporal; ra, radials; scc, scapulocoracoid cartilage; scl, supracleithrum; sop, subopercle.

sharp angle (Fig. 2B–E). The thickened lateral margin of the lateral arm is nearly flat early in development, but later curves to form the margin of the gill chamber (Fig. 2D–F). There is also a Journal of Morphology

thickening of the center of the posttemporal and the first indications of a spine-like process at its anterior most dorsal position. Although the position of the spine is variable among individuals, it

ANATOMY AND EARLY DEVELOPMENT

9

Fig. 7. Pectoral girdle and fin spine development of Acipenser transmontanus. All images in ventral view with anterior facing left. A. from VIMS 17716 (vial At10-8); 19.1 mm; 8 DPH. B. from VIMS 17716 (vial At10-14); 19.1 mm; 14 DPH. C. from VIMS 17716 (vial At10-18); 29.9 mm; 18 DPH. D. from VIMS 17716 (vial At10-21); 35.6 mm; 21 DPH. E. from VIMS 17716 (vial At10-23); 42.8 mm; 23 DPH. F. from VIMS 17716 (vial At10-62); 131.4 mm; 62 DPH. br, branchiostegal; cl, cleithrum; clv, clavicle; dr, distal radials; iclv, interclavicle; mtg, metapterygium; pfs, pectoral-fin spine; prp, propterygial restraining process; ptg, propterygium; ra, radials; sc, scapula; scc, scapulocoracoid cartilage; scl, supracleithrum; sop, subopercle.

Journal of Morphology

10

C. B. DILLMAN AND E. J. HILTON

Fig. 8. Pectoral girdle and fin spine development of Scaphirhynchus albus. All images in lateral view with anterior facing left. A. from VIMS 17714 (vial Sal11-11); 17.9 mm; 11 DPH. B. from VIMS 17714 (vial Sal11-18); 20.1 mm; 22 DPH. C. from UAIC 12581.01; 36.8 mm; Unknown DPH. D. from UAIC 12583.01; 45.0 mm; Unknown DPH. E. from UAIC 12582.01; 56.0 mm; Unknown DPH. F. from VIMS 12193; 79.0 mm; Unknown DPH. br, branchiostegal; cl, cleithrum; clv, clavicle; dr, distal radials; ds1, first dorsal scute; iclv, interclavicle; pcl, postcleithrum; pfs, pectoral-fin spine; prp, propterygial restraining process; pt, posttemporal; ra, radials; scc, scapulocoracoid cartilage; scl, supracleithrum; sop, subopercle.

generally occurs in the midportion of the posttemporal. Ornamentation of the posttemporal consists of spines and ridges of bone. The posttemporal of A. medirostris appears strongly by 27.0 mm TL (Fig. 4C) and is similar in Journal of Morphology

overall form to that of A. fulvescens. Both the medial and horizontal arms are present, although the horizontal arm is highly truncated anteriorly. Instead of meeting posteroventrally in a strong angle, the bone curves gradually along this edge.

ANATOMY AND EARLY DEVELOPMENT

11

Fig. 9. Pectoral girdle and fin spine development of Scaphirhynchus albus. All images in ventral view with anterior facing left. A. from VIMS 17714 (vial Sal11-11); 17.9 mm; 11 DPH. B. from VIMS 17714 (vial Sal11-18); 20.1 mm; 22 DPH. C. from UAIC 12581.01; 36.8 mm; Unknown DPH. D. from UAIC 12583.01; 45.0 mm; Unknown DPH. E. from UAIC 12582.01; 56.0 mm; Unknown DPH. F. from VIMS 12193; 79.0 mm; Unknown DPH. br, branchiostegal; cl, cleithrum; clv, clavicle; dr, distal radials; iclv, interclavicle; mtg, metapterygium; pfs, pectoral-fin spine; prp, propterygial restraining process; ptg, propterygium; ra, radials; sc, scapula; scc, scapulocoracoid cartilage; scl, supracleithrum; sop, subopercle.

By 35.0 mm TL (Fig. 4D), a strong dorsal enlargement with a rounded edge is present. At 41.5 mm TL (Fig. 4E), a second posterodorsally directed enlargement is present. This gives the bone a swollen appearance in its dorsalmost portion. A

single spine-like process, though weaker than in other species examined, occurs late in ontogeny (Fig. 4E) and is nearly absent in the largest specimen of our series (69.0 mm TL; Fig. 4F). The presence of spines, however, may be individually Journal of Morphology

12

C. B. DILLMAN AND E. J. HILTON

variable, as larger specimens (i.e., subadult) specimens of A. medirostris may possess well-developed cranial spines (Hilton, pers. obsv.). The posttemporal of A. transmontanus is well ossified by 19.1 mm TL (Fig. 6B), and at a smaller size than other species examined. At its first appearance, the posttemporal is roughly rectangular in shape. As in A. medirostris, there is a slight dorsomedial swelling of the bone. A strong posteriorly directed spine near the center grades into the strong ridge of the supracleithrum (Fig. 6B). By 29.9 mm TL (Fig. 6C), a prominent anterior extension is visible, with the lateral edge of the bone appearing thickened. At this size, the posterolateral corner of the posttemporal overlaps the supracleithrum. The overall shape at this stage is similar to that seen in A. fulvescens. There is slight variation in the shape of the posttemporal among individuals, but overall the bone is similar throughout the rest of the observed ontogenetic stages. In S. albus, the posttemporal, as in other species examined, is a flattened J-shape (Fig. 8C). By 36.8 mm TL, the margins of the bone are robust, although some interior portions are only thinly, if at all, ossified. The dorsomedial extension is rounded anteriorly and strongly tapers posteriorly. The lateral edge of the posttemporal is thickened along the opercular wall. At 56.0 mm TL, there are two ridges of ornamentation running along the anterior posterior axis, with spines on each ridge (Fig. 8E). While the number and strength of the spines is variable among individuals, they are present in all specimens of our series, and are not reduced even in the largest individuals studied here (79.0 mm TL; Fig. 8F). Supracleithrum. The supracleithrum is the first bone of the pectoral girdle to ossify in all species examined and carries the sensory canal from the trunk anteriorly to the skull. In A. fulvescens, the supracleithrum is fully formed by 34.8 mm TL (Fig. 2B). The supracleithrum is a very large lateral ossification, roughly rhomboid in shape, with an anterior edge that is slightly concave in some individuals (Fig. 2C–E). The dorsal portion of the supracleithrum broadly contacts the posttemporal. Across the center of the supracleithrum, there are spines in-line with those of the anteriormost lateral scutes. These spines are formed along the pathway of the lateral line sensory canal, which is housed within both the supracleithrum and lateral scutes. In A. medirostris, the supracleithrum appears by 23.5 mm TL (Fig. 4B) and by 27 mm TL, it is very large and has a ridge of bone in-line with the horizontal axis of the lateral scutes. By 41.5 mm TL, the supracleithrum is large, robust, and is curved anteriorly, forming the edge of the opercular chamber. The bone of this element is most dense in its midregion (Fig. 4E). The supracleithJournal of Morphology

rum is broadly overlapped by the posterior half of the posttemporal. The supracleithrum is broad dorsally with a rounded anterior margin, and tapers ventrally after overlapping the cleithrum. The supracleithrum in A. transmontanus appears by 19.1 mm TL (Fig. 6A), but much earlier, at 8 DPH, than the other 19.1 mm TL specimen (14 DPH; Fig. 6B). The supracleithrum is a very large rhomboidal bone (Fig. 6B–F), the ventral portion of which curves anteroventrally and follows the posterior portion of the gill chamber. The ventralmost portion broadly overlaps the cleithrum but does not extend significantly past this area of overlap (Fig. 6D–F). The anterodorsal portion of the supracleithrum is narrowly overlapped by the posterior portion of the posttemporal. Ornamentation of the supracleithrum consists of a very large and strong posteriorly directed spine that is retained throughout all ontogenetic stages observed here (Fig. 6B–F). The supracleithrum in S. albus first appears at 20.1 mm TL (Fig. 8B). At 36.8 mm TL (Fig. 8C), the length of the anterior edge is thickened relative to the other portions of the bone. As seen in the posttemporal of this species, the margins of the supracleithrum are more densely ossified in early ontogeny than the middle portion, which appears thin and nearly transparent (Fig. 8C–F). Laterally, there are one or two rows of spinous ornamentation in line with the lateral scute row (Fig. 8C–E). Along the anterior edge of the supracleithrum, there is a strongly developed spine in smaller individuals (Fig. 8D and E). This spine becomes smaller during ontogeny and is only weakly developed in the largest individual examined here (Fig. 8F). In one individual (Fig. 8C), there is a cavity in this position instead of a spine. The supracleithrum of S. albus is very elongate compared to the other species studied here. Postcleithrum. The postcleithrum is irregularly shaped in A. fulvescens and not well developed in most individuals in this series. When present, it appears as a smooth (i.e., unornamented) wedge-shaped bone (Fig. 2D) that occasionally arches dorsally (Fig. 2B and C). In some larger individuals, it appears as a long thin bone well separate from the overlying bones (Fig. 2D and E), while in others, it appears to be almost fused to the cleithrum (Fig. 2B). The postcleithrum of A. medirostris is an irregularly shaped smooth bone that is only intermittently observed in this series (Fig. 4C–E). When present, it is a thin bone, with the ventral portion directed slightly posteriorly via an angular projection. Compared to that of A. fulvescens and A. medirostris, the postcleithrum of A. transmontanus is a rather large bone and is less variable in its overall shape, although it is also smooth and unornamented. In all individuals where the postcleithrum

ANATOMY AND EARLY DEVELOPMENT

was observed (Fig. 6B-F), its dorsalmost portion continues along the same axis as the posterodorsal portion of the supracleithrum. The anterodorsal portion of the postcleithrum lies slightly under the supracleithrum and terminates posteriorly near the posterior margin of the cleithrum. The postcleithrum in S. albus was observed in only two individuals in our series (Fig. 8D and F), in which it extends just past the end of the supracleithrum (Fig. 8D) or ends in line with the dorsalmost point of the supracleithrum (Fig. 8F). Posteriorly it is nearly flat, but it is arched anteriorly. As in the other species examined no ornamentation is apparent. Cleithrum. The cleithrum of A. fulvescens is well ossified by 34.8 mm TL (Fig. 2B), and is fully formed by 41.1 mm TL (Fig. 2C). At these early stages, the cleithrum lacks ornamentation along its lateral surface, but ventrally in larger individuals, there is a distinct posteriorly directed spine (Fig. 3E) or ridge (Fig. 3F) running the length of the ventral surface of the cleithrum. In A. medirostris, the cleithrum appears by 27.0 mm TL (Fig. 4C). Laterally the bone appears thin, but, as in all acipenserids, extends medially to form the posterior portion of the gill chamber (Fig. 4C). The dorsal portion of the vertical projection of the cleithrum appears thin along its lateral edge. By 35.0 mm TL (Figs. 4 and 5D), the area of the propterygial restraining process is thickened, and is well developed by 41.5 mm TL (Fig. 4E). On the ventral surface of the cleithrum of A. medirostris, there is a ridge of bone that is roughly in line with the ventral scutes (Fig. 5E and F). The cleithrum of A. transmontanus forms very early during ontogeny at 19.1 mm TL (Fig. 6A) along the anterior edge of the scapulocoracoid cartilage, and extends to near the midline of the ventral scutes (Fig. 6F). On its ventral surface, the cleithrum of A. transmontanus is marked by lines of ornamentation that radiate in a fan-shape from the propterygium restraining process posteriorly (Fig. 7F); there was not a distinct anteroposterior ridge or spine developed on the cleithrum in our specimens of A. transmontanus. The cleithrum of S. albus was first seen at 21.1 mm TL, and was strongly formed by 36.8 mm TL (Fig. 8C). The overall shape of the cleithrum of S. albus differs from those of Acipenser spp. in several distinct ways. In lateral view, the cleithrum has an anterior edge that arches in most individuals (Fig. 8C–E); this was not observed in the largest specimen examined (Fig. 8F). The main axes of the supracleithrum and the cleithrum form a much smaller angle than that of other species of sturgeons examined in this study (cf. Figs. 6 and 8). A sharply pointed propterygial restraining process is first apparent at 79 mm TL (Figs. 8 and 9F). The medial edge of the cleithrum is concave where it approaches the clavicle (Fig. 9C and D).

13

Clavicle. The clavicle of A. fulvescens is well developed by 34.8 mm TL and initially develops as a straight bone with a slight ridge along its lateral edge (Fig. 3B). After initial ossification, the anterior margin bears lateral and medial projections that become larger during ontogeny (Fig. 3B–F). Early in ontogeny, the lateral projection extends farther anteriorly than the medial projection (Fig. 3B–D) but by 47.0 mm TL (Fig. 3E), the medial projection is farther anterior (Fig. 3E and F). The posterior edge of the clavicle is variable and appears from nearly smooth (Fig. 3D and F) to ridged (Fig. 3B, C, and E). The clavicle in A. medirostris first appears by 27.0 mm TL (Fig. 5C) and by 35.0 mm TL (Fig. 5D), the lateral and medial projections are present. At 41.5 mm TL (Fig. 5E), the anterior margin of the clavicle (i.e., from the lateral projection to the contact with the cleithrum) is thickened and by 69.0 mm TL (Fig. 5F), the medial arm of the clavicle is also thickened. The lateral anterior projection is always farther anterior than the medial arm (Fig. 5D and E). The margins of the medial projections are nearly parallel one another in the largest specimens of A. medirostris examined here (Fig. 5F). The clavicle of A. transmontanus is a deep red by 19.1 mm TL (Fig. 7B) indicating a strong ossification, and the anterior edge is maintained as an area of thickened bone or ornamentation throughout development (Fig. 7C–E). The lateral arm of the clavicle is always at the distal end of this concavity (Fig. 7D and E). The medial arm of the clavicle is thickened and sits at approximately 90 to the lateral arm (Fig. 7D and E). The medial arm reaches the interclavicle (Fig. 7C and D) and slightly overlaps it (Fig. 7E). Laterally the clavicle contacts the medial edge of the cleithrum and appears as wedge-shaped with a narrow point facing anteriorly (Fig. 7C–F). The clavicle of S. albus is fully present by 36.8 mm TL (Fig. 9C) at which point it extends from the laterally projecting arm of the scapulocoracoid from the edge of the cleithrum toward the interclavicle. The clavicle is thickened along the ventral midline of the bone (Fig. 9C–F); this thickened ridge continues toward the anteriormost portion of the lateral arm (Fig. 9C–F). The lateral arm of the clavicle of S. albus is always farther anterior than the medial arm. The lateral edge of the clavicle abuts the cleithrum along its entire length. Interclavicle. The interclavicle of A. fulvescens is individually variable (Fig. 3B–E), but is present by 34.8 mm TL. It is highly variable and here is seen as either five or seven sided (Fig. 3C and D, respectively). In a 47.0 mm TL specimen (Fig. 3E), the interclavicle is highly irregular in its shape and almost appears as two separate elements. The interclavicle lies under the medial Journal of Morphology

14

C. B. DILLMAN AND E. J. HILTON

projections of the clavicle (Fig. 3D). Its ornamentation is variable, ranging from ornamentation around its periphery (Fig. 3C), bearing enlarged ridges near its margins (Fig. 3D) or multiple scattered ridges of ornamentation (Fig. 3E). The interclavicle in A. medirostris appears by 35.0 mm TL (Fig. 5D). In general, it is variable in shape, as in other species, but is rounded in two individuals (Fig. 5D and E), or arching anteriorly in another (Fig. 5F). The interclavicle of A. transmontanus is variable among individuals, but does appear to possess some broadly similar features during ontogeny. Anteriorly, the interclavicle varies from nearly straight (Fig. 7C and D) to slightly rounded anteriorly (Fig. 7E). There are several posteriorly directed projections in all individuals, with the median projection extending the farthest posteriorly. Typically the lateral edge of the interclavicle also possesses projections (Fig. 7C and D). The interclavicle of S. albus is variable in shape, from nearly triangular (Fig. 9C), pentagonal (Fig. 9E), or rounded (Fig. 9D and F). In some individuals, the edges of the interclavicle are thickened (Fig. 9C–E). The interclavicle lies under the medial projections of the clavicle early in ontogeny (Fig. 9D), and is completely covered anteriorly by the medial projections of the clavicles in the largest individuals examined here (Fig. 9F). Scapulocoracoid cartilage. In all species examined here, the scapulocoracoid cartilage is a large cartilaginous block extending dorsal to the supracleithrum and ventral to the clavicle (Fig. 1). Medially it extends to the midline of the body and terminates anteriorly near the interclavicle. Laterally, the scapulocoracoid articulates with the propterygium, metapterygium, and proximal radials at the glenoid ridge. No ossification was seen on the scapulocoracoid, but as noted by Hilton et al. (2011), there is variation in individuals with respect to the ossifications of the structure (see also Hilton and Bemis, 1999). Just dorsal to the glenoid ridge, there is an opening, the medial upper muscle channel (Jessen, 1972). The posterior portion of the scapulocoracoid in A. fulvescens is unique in this study, in that there are two discrete processes directed posteriorly (Fig. 2B–E). In A. medirostris, there is a large bulbous convex flaring directed posteriorly (Fig. 4C– F). This occurs on the dorsal half of the cartilage, that is, dorsal to the pectoral fin, and no ossifications were seen in this series. The scapulocoracoid of A. transmontanus is unique among the taxa examined here. The dorsal portion of the cartilage is typically the same width as the ventral portion (Fig. 6C and D, but see Fig. 6E). Even when it is broader (as in Fig. 6E), it is not as pronounced as that of A. fulvescens or A. medirostris. The scapulocoracoid in S. albus is a robust cartilage, as in other acipenserids, but differs by typically having Journal of Morphology

a nearly vertical posterior edge (Fig. 9C–E, but see Fig. 9F) that extends posteriorly to the third proximal radial. This extends the cartilage farther posteriorly than in any other species examined. The medial upper muscle channel is also much larger in S. albus than in other species. Fin supporting elements. No evidence of ossification in any of the chondral skeletal elements was observed for any species examined in this study. This is likely due to the truncation of our growth series prior to onset of ossification, as ossifications are present in larger individuals (i.e., adult specimens) of these species; no first time of ossification was attempted to be determined in this study. The propterygium in A. fulvescens develops as two independent cartilages that fuse during ontogeny, with the anterior cartilage developing by 34.8 mm TL (Fig. 3B), after the more posterior element is already present (Fig. 3A). Although these cartilages fuse, the distal tip of the propterygium appears forked (Fig. 3C–E). The anterior portion of the propterygium supports the developing pectoral-fin spine (Fig. 3C–E). The metapterygium is directed along the anterior–posterior axis, ending anteriorly in line with the fourth proximal radial in A. fulvescens (Fig. 3B–E). The proximal radials decrease in length posteriorly along the length of the metapterygium (Fig. 3B–D). The proximal radials are rounded and tapered, such that the distal tips are broader than the proximal ends. The number of proximal radials anterior to the metapterygium (i.e., supported directly by the scapulocoracoid cartilage) is variable among individuals, but A. fulvescens typically has four (Fig. 3B, C, and E, but see Fig. 3D). The more posterior proximal radials (i.e., those supported directly by the metapterygium) first appear at 22.0 mm TL (Fig. 3A). These metapterygial radials are nearly always paired or fused proximally (i.e., bifurcated; Fig. 3C–E). The distal radials are small and present by 41.1 mm TL (Fig. 3C). They either contact (Fig. 3E) or are positioned slightly posterior to the distal end of the proximal radials (Fig. 3C and D). The fin support elements in A. medirostris are typical of acipenserids, with three or four rounded proximal radials occurring between the propterygium and metapterygium. There are between two (Fig. 5B) and three radials (Fig. 5C and D) along the length of the metapterygium. The anterior cartilage of the propterygium in A. medirostris forms by 27.0 mm TL (Fig. 5C) and tapers distally. This distal tapering remains distinct throughout ontogeny (Fig. 5C–F). The distal radials are visible by 27.0 mm TL (Fig. 5C) and are situated slightly posteriorly to the proximal radials and eventually fill the space in between the distal portion of the proximal radials (Fig. 5E and F). The distal radials form anteriorly to posteriorly and vary in

ANATOMY AND EARLY DEVELOPMENT

shape from rounded (Fig. 5E) to triangular (Fig. 5F). As in other species examined here, the propterygium of A. transmontanus develops as two independent chondrifications, the posterior of which forms earlier. The anterior propterygial cartilage is present by 29.9 mm TL (Fig. 7C). The anterior portion of the propterygium is much broader than the posterior cartilage (Fig. 7C–E). The number of proximal radials supported by the scapulocoracoid cartilage varies, but is typically three in A. transmontanus (but see Fig. 7D). The metapterygium supports two or three radials, which decrease in length posteriorly along the metapterygium. The proximal radials are generally wider distally. The distal radials are primarily rounded and develop anterior to posterior, with the first appearing at 29.9 mm TL (Fig. 7C). The distal radials are positioned just posterior to the distal end of its associated proximal radial. Similar to the species of Acipenser examined here, the propterygium of S. albus initially develops as two independent chondrifications that fuse later in ontogeny. The anterior element is visible by 20.1 mm TL (Fig. 9B). When fused these elements form a strong wedge-shaped cartilage (Fig. 9C–F). There are typically four premetapterygial proximal radials in S. albus (Fig. 9A–C, E–F; but see Fig. 9D). The proximal radials are rounded distally, but are not much narrower proximally in larger individuals (Fig. 9C–F). The proximal radials associated with the metapterygium decrease in length anterior to posterior. There are typically three radials along the metapterygium; the posteriormost of which appear paired (Fig. 9C and D) and then fused in the largest individual (Fig. 9F). The distal radials appear by 36.8 mm TL (Fig. 9C). They are broad, nearly flat, and variable in size, although distinct in shape from that of Acipenser spp. (cf. Figs. 3, 5, 7 with Fig. 9). The distal radials are positioned posterior to each proximal radial, except for the largest one, which lies between the propterygium and the first proximal radial (Fig. 9C–F). The more posteriorly positioned distal radials, associated with the proximal radials of the metapterygium, are adjacent to the proximal radials (Fig. 9E and F). The anterior four distal radials span the space in the fin between each of the proximal radials (Fig. 9E and F). Fin rays. The fin rays were first seen at 41.1 mm TL (Figs. 2 and 3C) in A. fulvescens. At this stage, ossification is restricted to the proximal portion of the fin and ends near the mid-portion of the fin web. During ontogeny, ossification proceeds medially and the ossified fin rays eventually overlap the distal portion of the proximal radials. The ossification also continues distally along the fin rays (Figs. 2 and 3D-F). In the available specimens, the proximal end of the fin rays overlap the distal tips of the proximal radials (Fig. 3D and E),

15

but even in the largest specimens in this series (170 mm TL), the ossification of the fin rays do not reach the distal margin of the fin. The fin rays of A. medirostris are first ossified by 35.0 mm TL (Figs. 4 and 5D). Ossification of the fin rays proceeds anterior to posterior, and proximally to distally trailing just behind the formation of fin rays (visualized by the actinotrichia of the fin ray staining slightly with Alcian Blue). The proximal end of ossification of the fin rays is at the distal end of the proximal radials (Fig. 5F). The ossified portions of the fin rays do not reach the distal fin margin in our series. In A. transmontanus, the fin rays first ossify at 29.9 mm TL (Fig. 7C). Ossification occurs quite early and at nearly the same time as the fin rays form, and proceeds from anterior to posterior, though the portions of the fin rays associated with the propterygium and proximal radials appear to ossify nearly all at once. The ossification of each fin ray initially occurs proximally, and then extends distally along the fin ray. The fin rays associated with the proximal radials supported by the metapterygium ossify last (42.8 mm TL; Fig. 7E and F). In the largest specimens of our series of A. transmontanus, the proximal ends of the fin rays covers various proportions of the proximal radials (Fig. 7E) and the fin rays do not reach the distal fin margin. The fin rays are nearly all ossified by 36.8 mm TL (Fig. 9C) in S. albus, and continue ossifying posteriorly during development. By 45.0 mm TL (Fig. 9D–F), nearly all of the fin rays are ossified across the entire fin. In the specimens from our series, the proximal ends of the fin rays cover approximately 25–33% of the proximal radials (Fig. 9D–F), and the fin rays do not reach the distal fin margin. The number of fin rays in the pectoral fin is variable across species, and in the latest stages of development for each species examined, we observed 38 fin rays in A. fulvescens, 36 in A. medirostris, 42 in A. transmontanus, and 56 in S. albus. The number of fin rays that are found to overlay the cartilaginous propterygium, including those not incorporated into the fin spine in our series, is 4 in A. fulvescens, 3 in A. medirostris, 6 in A. transmontanus, and 7 in S. albus. There does not appear to be much variation in the number of fin rays incorporated into the fin spine across the species examined here with 2 in A. fulvescens, 2 in A. medirostris, 2 in A. transmontanus, and 1 or 2 in S. albus. Pectoral-Fin Spine Development of pectoral-fin spine. The pectoral-fin spine in A. fulvescens first appears by 33.7 mm TL and is robust by 41.1 mm TL (Fig. 3C), and surrounds the anterior portion of the propterygium. By 45.2 mm TL (Figs. 2 and 3D), the first fin ray is being incorporated into the fin Journal of Morphology

16

C. B. DILLMAN AND E. J. HILTON

edge of the fin spine stops short of the medial edge of the propterygium (Fig. 9C–F). The fin spine of S. albus in the terminal condition is much finer than in any species of Acipenser examined.

Fig. 10. Allometric relationships between pectoral-fin spine length and total length in the four species of Acipenseridae examined in this study.

spine. By 47.0 mm TL (Figs. 2 and 3E), a second fin ray is incorporated into the spine at the base, although its distal portion remains separate (Figs. 2 and 3E). At 170 mm TL, the number of fin rays incorporated into the fin spine remains at two (Figs. 2 and 3F), but fusion along the length of the incorporated fin rays is complete. The fin spine never reaches the distal-most tip of the pectoral fin, but rather terminates at the point where the fin begins to curve posteriorly (Fig. 3F). The base of the pectoral-fin spine completely overlaps the propterygium (Fig. 3C–E). In A. medirostris, at its first formation (27 mm TL; Figs. 4 and 5D), the incipient pectoral-fin spine appears similar to the other developing fin rays. By 41.5 mm TL (Figs. 4 and 5E), the spine has grown and wrapped around the anterior cartilage of the propterygium. At 69 mm TL (Figs. 4 and 5F), two fin rays have been partially incorporated into the spine. The proximal base of the fin spine ends in line with the medial edge of the propterygium (Fig. 5E and F). The pectoral-fin spine also first occurs very early in A. transmontanus at 29.9 mm TL (Fig. 7C). The first fin ray is incorporated into the fin spine by 35.6 mm TL (Fig. 7D), and by 42.8 mm TL (Fig. 7E) a second fin ray is becoming incorporated. In the largest specimens in this series, only two fin rays are incorporated into a very robust pectoralfin spine (Fig. 7F). The fin spine continues to ossify distally (Fig. 7D–F). The base of the fin spine is quite broad and reaches past the proximal edge of the propterygium. In S. albus, the pectoral-fin spine is visible along the anterior edge of the propterygium by 36.8 mm TL (Fig. 9C). The first fin ray appears branched distally (Fig. 9C and E), and is first incorporated into the pectoral-fin spine proximally (Fig. 9D and E). The fin spine is fused to the first fin ray by 45.0 mm TL (Fig. 9D) and in this series no other fin rays are incorporated into the spine. The medial Journal of Morphology

Pectoral-fin spine measurements. The same general growth trend is found in each species, although the rate of fin spine growth is different for each species (Fig. 10). The earliest evidence of a fin spine is at 25.3 mm TL in A. transmontanus. Until about 35 mm TL, all species have similarly sized pectoral-fin spines. At TL greater than 40 mm, each species tends to diverge from the others. A. medirostris and A. transmontanus both experience a quicker growth of the fin spine, while S. albus tends to have the slowest fin spine growth. At larger sizes A. fulvescens appears to be intermediate between A. medirostris and A. transmontanus with respect to relative fin-spine length, but there are a large amount of data missing in our series for A. fulvescens between 50 mm TL and 100 mm TL. However, each of the species included here occupies a unique place in morphospace with respect to the largest sizes available. Sequence of Ossification and Chondrification We plotted the body size at development for each of the bones and cartilages discussed above

Fig. 11. Timing and development of the dermal bones and chondral structures of the pectoral girdle in species of sturgeon examined in this study. Elements are listed from dorsal to ventral. Species are coded by color where A. fulvescens, black; A. medirostris, red; A. transmontanus, blue; and S. albus, green

ANATOMY AND EARLY DEVELOPMENT

for all individuals in each ontogenetic series to determine if there is a general pattern of ossification sequence across species (Fig. 11). This excludes the primary supporting elements of the fin, that is, propterygium, proximal radials, and metapterygium. The supracleithrum is the first bone to ossify in all species, and it is quickly followed by ossification of the posttemporal. In our series, these two bones develop first in S. albus, followed by A. transmontanus, A. medirostris, and A. fulvescens. In contrast, A. medirostris and A. transmontanus were first to have the postcleithrum, cleithrum, and clavicle ossify in our series. These two species are very similar to each other in body size at development for all elements, with the exception of the clavicle, for which A. transmontanus is slightly in advance of A. medirostris, and the interclavicle, for which A. transmontanus is substantially earlier than all other species examined and A. medirostris is the last to have an interclavicle ossified (Fig. 11). Cartilaginous elements of the pectoral girdle show an interesting pattern during development. Scaphirhynchus develops the propterygium anterior extension, distal radials, and the fin rays earlier than any species of Acipenser examined here (Fig. 11). These elements are also the largest in S. albus when compared to other sturgeon at the end of the ontogenetic stages investigated. Contrariwise, the pectoral-fin spine develops much later in S. albus than in Acipenser (Fig. 11).

DISCUSSION Morphological Variation of the Pectoral Girdle in Acipenseridae The pectoral girdle is situated at the division between the head and trunk regions, and is used in different functions specific to each region (Gudo and Homberger, 2002). As in all fishes, the pectoral girdle of sturgeon serves as the origin for muscles associated with feeding (Carroll and Wainwright, 2003), is a powerful structure used for movement (Liem et al., 2001, McGonnell, 2001), and station holding (Wilga and Lauder, 1999, Kane and Higham, 2012); see Drucker and Lauder (2002) for a review of the evolution of pectoral girdle function in actinopterygians, specifically with regard to its role in locomotion. The pectoral fins in sturgeons are held in a horizontal plane, that is, in-line with the substrate and originating from the ventral portion of the body (Rosen, 1982, Wilga and Lauder, 1999). Unlike more derived actinopterygian fishes, the pectoral fins of sturgeon have portions of the pectoral endoskeleton in the fin web itself (Wilga and Lauder, 1999). In spite of broad similarities of the pectoral girdle and fin across the species examined clear differences exist that will be important to incorporate into

17

new analyses of morphological variation and phylogenetic relationships across the Acipenseridae. Growth of the pectoral-fin spine starts similarly across the species investigated, but diverged during ontogeny. The number of incorporated fin rays has been proposed to explain the variation in the robustness of the pectoral-fin spine seen in adults (Findeis, 1997). Each of three species of Acipenser investigated here had two fin rays incorporated into the fin spine. Findeis (1997) noted that S. platorynchus incorporated only a single fin ray; we observed variation in the number of fin rays (one or two) that are incorporated into the fin spine of S. albus. Further study of the diversity of the number of fin rays incorporated into the fin spine of all species of Acipenseridae, including observations on the morphology of adult specimens, should be made. Still, the robustness of the fin spine, despite similar numbers of incorporated fin rays, varies across species of Acipenseridae. Variation in the relationship between the pectoralfin spine and the propterygium will be interesting to explore among a broader set of sturgeon species. Findeis (1997) noted that the anterior extension of the propterygium (his enlarged propterygium) was diagnostic of acipenserids and is a part of his fin spine character. However, the proportion of the propterygium covered by the ossified fin spine varies among species from completely to incompletely covered proximally. The morphology of the pectoral-fin spine has been used to differentially diagnose closely related species of sturgeon (Bakhshalizadeh et al., 2012). Further morphometric investigation, including an ontogenetic perspective, across the diversity of sturgeons would be an intriguing area of research. Determining the degree of overlap of the fin spine and the fin rays incorporated with the propterygium in all sturgeon species may better inform our understanding of this character, its distribution, and its bearing on our understanding of phylogeny. There are two proximal radials between the propterygium and metapterygium in the two extant members of the sister-group to Acipenseridae, the polyodontids Polyodon spathula and Psephurus gladius (Grande and Bemis, 1991), which is fewer than in any of the sturgeon species examined here. While there is some variation between species in this study (i.e., three or four proximal radials), the difference between the polyodontids and acipenserids should be investigated further to determine if the number of radials can be described as a character for the family Acipenseridae. We observed that the number of radials associated with the metapterygium is variable. Davis et al. (2004) termed these the metapterygial radials, and the configuration of these radials (i.e., branched or unbranched) and any variation (taxonomic or intraspecific) that may exist also needs to be documented across the diversity of Acipenseridae. Journal of Morphology

18

C. B. DILLMAN AND E. J. HILTON

The distal radials observed here are either rounded or wedge-shaped, and positioned either laterally or posterolaterally to the distal portion of the proximal radials. The distal radials in Scaphirhynchus are the most distinct of the four species examined, particularly the anteriormost element, which is greatly elongated. This elongation creates a large space between the propterygium and the anteriormost proximal radial in S. albus. Observations on specimens of S. platorynchus confirm this condition in that species as well, suggesting this may be a character of the genus Scaphirhynchus. Further examination of Huso and Pseudoscaphirhynchus and other Acipenser could prove useful for future studies. While counts and measures can be difficult to work into a phylogenetic coding system, the number of pectoral-fin rays across species of sturgeons is quite variable and it will be important to look at this across the diversity of Acipenseridae. Further documenting the number and relationship of the ossifications of the fin rays with the propterygium and the proximal radials may be particularly insightful. Similarly, and while not the subject of specific study here, the scapulocoracoid cartilage may provide information regarding relationships among sturgeon based on its shape and size. Findeis (1997) proposed as a synapomorphy for Scaphirhynchus the cleithral wall of the scapulocoracoid cartilage, and distinct from the cleithral arch seen in other genera. Findeis (1997) also proposed that the flat shelf of cartilage extending from the coracoid wall was a synapomorphy of acipenserids, with a lateral flaring unique to Scaphirhynchus. In our study, the scapulocoracoid cartilage was found to be variable among species, and a full documentation and description of this cartilage across these and other species will likely prove useful for this complex and highly structured skeletal element. Findeis (1997) described the clavicle process as a synapomorphy of acipenserids. This process was described as a small wedge on the clavicle that “interdigitates with the cleithrum.” Hilton et al. (2011) described the tight suturing between the clavicle and the cleithrum as a character supporting acipenserids. In addition to the interaction between these dermal elements, the clavicle also possesses anteriorly directed lateral and medial processes. During early ontogeny, these processes remain separate distally. In the species examined, the distal portion of lateral process is nearly always further anterior than the medial process. In A. fulvescens, there is a switch during ontogeny from the lateral process to the medial process terminating farther anteriorly. Findeis (1997) showed that these lateral processes are farther anterior in Huso huso, but that the medial processes are much farther anterior in Pseudoscaphirhynchus. In Scaphirhynchus, the anterolateral process is distinct, and differs from that of Acipenser spp., in Journal of Morphology

which it is not clearly separate from the medial extension that approaches the midline, the interclavicle, and its antimere. The cleithrum is similar in shape across the species of Acipenser examined here, and in A. brevirostrum (Hilton et al., 2011). Several characteristics of the cleithrum have been described as synapomorphies of acipenserids (Findeis, 1997, Hilton et al., 2011). In addition to those characters observed in previous studies, it was observed that in A. medirostris where the cleithrum narrows dorsally, it is exaggerated with a cephalically projected notch that is overlapped by the supracleithrum. In A. fulvescens, the dorsal narrowing also has an anteriorly projected notch, but the supracleithrum does not overlap into this area. In Scaphirhynchus, the dorsal portion of the cleithrum reaches strongly caudally. The supracleithrum is the largest dermal bone of the pectoral girdle in all species and there is a broad region of overlap between the supracleithrum and the cleithrum that is also seen in A. brevirostrum (Hilton et al., 2011). During ontogeny, the supracleithrum of A. medirostris extends ventrally far past the overlap with the cleithrum (except in the largest individual examined here), whereas the supracleithrum ends abruptly at the point where it contacts the cleithrum in the other species and in A. brevirostrum (Hilton et al., 2011). The shape of the supracleithrum is variable as well. In S. albus, it is roughly triangular, whereas it is more quadrilateral in the species of Acipenser examined. The supracleithrum in A. brevirostrum is also roughly triangular (Hilton et al., 2011). Anteriorly the supracleithrum broadly underlies the posttemporal (Findeis, 1997), a proposed synapomorphy for Acipenseridae. Hilton et al. (2011) clarified the synapomorphic condition by defining the area of overlap as the anterior shelf reaching the extrascapulars. The orientation of the posttemporal is similar across all species examined. The biggest difference noted here is the posteriorly flared posttemporal in A. medirostris where it overlaps the first dorsal scute. In A. brevirostrum, there is a slight overlap of the posttemporal and first dorsal scute that does not extend posteriorly beyond the widest point of the scute (Hilton et al., 2011: fig. 12). This is also the condition in A. medirostris, A. transmontanus, and S. albus. In A. fulvescens, the posttemporal extends posteriorly to the widest point of the first dorsal scute. The first dorsal scute of S. albus is much wider than in any species of Acipenser examined in this study, as evidenced by the modest area between this scute and the posttemporal. Form of the Pectoral Fin of Sturgeons During Ontogeny The pectoral-fin spine was proposed as a synapomorphy for Acipenseridae (Findeis, 1997), but is

ANATOMY AND EARLY DEVELOPMENT

now recognized as a synapomorphy for Acipenseriformes (Grande et al., 2002, Hilton et al., 2011) with subsequent loss of the spine within Polyodontidae and †Peipiaosteidae (Hilton et al., 2011). The presence of a large cartilaginous propterygium was part of the fin-spine character for the family as described by Findeis (1997). As in acipenserids, a large propterygium is present in both Polyodon and Psephurus (Grande and Bemis, 1991: Figs. 21 and 43, respectively). In early stages of development of P. spathula, the propterygium is similar to that described above for acipenserids (Dillman, pers. obsv.). Species of the genus Scaphirhynchus generally have the weakest fin spine of extant sturgeons (typically incorporating only one fin ray; this study, see also Findeis, 1997), but among the largest pectoral fins in terms of surface area. Their fins are also of a distinctive circular shape, similar to those of Pseudoscaphirhynchus spp. and different from the more triangular pectoral fins in Acipenser and Huso. The three species of Acipenser examined exhibited similar fin spines, and the differences observed in the development and morphology of the fin spine and of the fin among these species should be characterized further using Eurasian species of Acipenser and Huso, given existing hypotheses of phylogeny (Birstein et al., 2002). The ontogeny of the fin spine is similar across the North American species examined in this study, and other North American sturgeons including A. brevirostrum (Hilton et al., 2011) and S. platorynchus (Findeis, 1993). In spite of broad similarities, there are observed allometric differences between each of the four species examined here. A. medirostris and A. transmontanus overlap early in ontogeny with respect to fin spine growth, as do A. fulvescens and S. albus, and it is worth noting that these two pairs exhibit similar lifestyles and geographic distributions. Both A. medirostris and A. transmontanus are anadromous and occur sympatrically in a portion of their range. The divergence between the growth rates of the fin spines between A. medirostris and A. transmontanus are unexplained but may be related to early life history characteristics, where variation is seen between populations in A. transmontanus (Kynard and Parker, 2005, 2010) and between A. transmontanus and A. medirostris (Kynard and Parker, 2005, 2010, Kynard et al., 2005). The timing of development for structures of the pectoral girdle and fin of these two species shows that A. transmontanus develops most bones at a smaller size, despite the overall similarity in the relative timing of development of each. A. fulvescens and S. albus are both potadromous. They are restricted to large rivers in the Mississippi River drainage, and the former also occurs in the Great Lakes and other northern drainages including the St. Lawrence River and

19

Hudson Bay drainages, while the latter occurs only in the main-stem portions of the Mississippi River and Missouri River. Comparable ontogenetic behavioral data is not available for these two species, but each has been investigated for early life history (LaHaye et al., 1992; Kynard et al., 2002). Scaphirhynchus is the only genus of sturgeon never to be rejected as monophyletic, and likely has had a long history of evolution independent of other North American sturgeons (i.e., it is possibly the sister genus to all other extant members of Acipenseridae; Birstein et al., 2002; Hilton et al., 2011). Species identification in Scaphirhynchus is difficult for early life history stages (Kuhajda et al., 2007; Hartfield et al., 2013), although behavioral differences between S. albus and S. platorynchus have been demonstrated (Kynard et al., 2002). A comparison of the osteological development of this sympatric pair should serve as an important next step in understanding species specific differences. CONCLUSIONS It is perhaps not surprising that striking morphological differences are not seen among the species of sturgeon included in this study, given the evolutionary (phylogenetic and functional) constraints in place on the pectoral girdle, as a portion of the skeletal system fundamental to both feeding and locomotion. However, the morphological variation documented in this study, while subtle in some instances, does provide an outline of characters, including ontogenetically variable characters that should be investigated in the context of the entire diversity of sturgeons, and ultimately may be useful in hypothesizing relationships among the species of sturgeon. ACKNOWLEDGMENTS Numerous people helped contribute to the specimens required for this study by making daily collections of individuals of each species for the developmental series. The authors are particularly grateful to Peter Struffenegger of Sterling Caviar for A. transmontanus. Scaphirhynchus albus was graciously provided by Craig Bockholt and Marc Jackson at Gavin’s Point National Fish Hatchery, with permission from recovery team leader George Jordan. Carlos Echevarrıa and Jaclyn Zelko at the Warm Springs National Fish Hatchery, Ron Bruch of the Wisconsin Department of Natural Resources, and Anna George and Kathlina Alford of the Tennessee Aquarium Conservation Institute aided collection of A. fulvescens. Joel Van Eenennaam at UC-Davis kindly provided individuals of A. medirostris. The authors thank Bernie Kuhajda (UAIC) for access to specimens, and Sarah K. Huber (VIMS) for curatorial assistance. Peter Journal of Morphology

20

C. B. DILLMAN AND E. J. HILTON

Konstantinidis, William E. Bemis, and an anonymous reviewer read and greatly improved a draft of this manuscript. This is contribution number 3404 of the Virginia Institute of Marine Science, College of William & Mary. LIETERATURE CITED Bakhshalizadeh S, Bani A, Abdolmalaki S. 2012. Comparative morphology of the pectoral fin spine of the Persian sturgeon Acipenser persicus, the Russian sturgeon Acipenser gueldenstaedtii, and the Starry sturgeon Acipenser stellatus in Iranian waters of the Caspian Sea. Acta Zool 94:471–477. Bemis WE, Findeis EK, Grande L. 1997. An overview of Acipenseriformes. Environ Biol Fishes 48:25–72. Birstein VJ, Doukakis P, DeSalle R. 2002. Molecular phylogeny of Acipenseridae: Nonmonophyly of Scaphirhynchinae. Copeia 2002:287–301. Carroll AM, Wainwright PC. 2003. Functional morphology of prey capture in the sturgeon, Scaphirhynchus albus. J Morphol 256:270–284. Davis MC, Shubin NH, Force A. 2004. Pectoral fin and girdle development in the basal actinopterygians Polyodon spathula and Acipenser transmontanus. J Morphol 262:608–628. Dingerkus G, Uhler LD. 1977. Enzyme clearing of alcian blue stained whole small vertebrates for demonstration of cartilage. J Stain Technol 52:229–232. Drucker EG, Lauder GV. 2002. Wake dynamics and locomotor function in fishes: Interpreting evolutionary patterns in pectoral fin design. Integr Comp Biol 42:997–1008. Fan X-G, Wei Q-W, Chang J, Rosenthal H, HE J-X, Chen D-Q, Shen L, DU H, Yang D-G. 2006. A review on conservation issues in the upper Yangtze River—A last chance for a big challenge: Can Chinese paddlefish (Psephurus gladius), Dabry’s sturgeon, (Acipenser dabryanus) and other fish species still be saved? J Appl Ichthyol 22(suppl 1):32–39. Findeis EK. 1993. Skeletal anatomy of the North American shovelnose sturgeon Scaphirhynchus platorynchus (Rafinesque, 1820) with comparisons to other Acipenseriformes. PhD dissertation, University of Massachusetts, Amherst. Findeis EK. 1997. Osteology and phylogenetic interrelationships of sturgeons (Acipenseridae). Environ Biol Fishes 48: 73–126. Grande L, Bemis WE. 1991. Osteology and phylogenetic relationships of fossil and recent paddlefishes (Polyodontidae) with comments on the interrelationships of Acipenseriformes. Soc Vertebrate Paleontol Mem, 1:i–vii, 1–121. Grande L, Bemis WE. 1996. Interrelationships of Acipenseriformes, with comments on “Chondrostei.” In: Stiassny MLJ, Parenti LR, and Johnson GD, editors. Interrelationships of Fishes. San Diego, CA: Academic Press. pp. 85–115. Grande L, Hilton EJ. 2006. An exquisitely preserved skeleton representing a primitive sturgeon from the Upper Cretaceous Judith River Formation of Montana (Acipenseriformes: Acipenseridae: n. gen. and sp). J Paleontol Mem 65:1–39. Grande L, Jin F, Yabumoto Y, Bemis WE. 2002. Protopsephurus liui, a well-preserved primitive paddlefish (Acipenseriformes : polyodontidae) from the lower cretaceous of China. J Vertebrate Paleontol 22:209–237. Gudo M, Homberger DG. 2002. The functional morphology of the pectoral fin girdle of the Spiny Dogfish (Squalus acanthias): implications for the evolutionary history of the pectoral girdle of vertebrates. Concepts Functional, Engineering Constructional Morphol 82:241–252. Hartfield P, Kuntz NM, Schramm HL. 2013. Observations on the Identification of Larval and Juvenile Scaphirhynchus spp. in the Lower Mississippi River. Southeastern Nat 12:251–266. Hilton EJ. 2005. Observations on the skulls of sturgeons (Acipenseridae): shared similarities of Pseudoscaphirhynchus

Journal of Morphology

kaufmanni and juvenile specimens of Acipenser stellatus. Environ Biol Fishes 72:135–144. Hilton EJ, Bemis WE. 1999. Skeletal variation in Shortnose sturgeon (Acipenser brevirostrum) from the Connecticut River: Implications for comparative osteological studies of fossil and living fishes. In: Arratia G, and Schultze H-P, editors. Mesozoic Fishes 2—Systematics and Fossil Record. Munich: Dr. Friedrich Pfeil, pp. 69–94. Hilton EJ, Forey PL. 2009. Redescription of Chondrosteus acipenseroides Egerton, 1858 (Acipenseriformes, Chondrosteidae) from the lower Lias of Lyme Eegis (Dorset, England, with comments on the early evolution of sturgeons and paddlefishes. J Syst Palaeontol 7:427–453. Hilton EJ, Grande L. 2006. Review of the fossil record of sturgeons, family Acipenseridae (Actinopterygii: Acipenseriformes), from North America. J Paleontol 80:672–683. Hilton EJ, Grande L, Bemis WE. 2011. Skeletal anatomy of the shortnose sturgeon, Acipenser brevirostrum Lesueur, 1818, and the systematics of sturgeons (Acipenseriformes, Acipenseridae). Fieldiana: Life Earth Sci 3:1–168. Jessen H. 1972. Schulterg€ urtel und Pectoralflosse bei Actinopterygiern. Fossils Strata 1: 1–101. Jin F. 1999. Middle and Late Mesozoic Acipenseriformes from northern Hebei and western Liaoning, China. In: Chen P-J and Jin F, editors. Palaeoworld No. 11. Hefei: Jehol Biota. Press of University of Science and Technology of China, pp. 188–260. Kane EA, Higham TE. 2012. Life in the flow lane: Differences in pectoral fin morphology suggest transitions in stationholding demand across species of marine sculpin. Zoology 115:223–232. Kuhajda BR, Mayden RL, Wood RM. 2007. Morphologic comparisons of hatchery-reared specimens of Scaphirhynchus albus, Scaphirhynchus platorynchus, and S. albus x S. platorynchus hybrids (Acipenseriformes : Acipenseridae). J Appl Ichthyol 23:324–347. Kynard B, Parker E. 2005. Ontogenetic behavior and dispersal of Sacramento River White Sturgeon, Acipenser transmontanus, with a note on body color. Environ Biol Fishes 74: 19–30. Kynard B, Parker E. 2010. Ontogenetic behavior of Kootenai River White Sturgeon, Acipenser transmontanus, with a note on body color: A laboratory study. Environ Biol Fishes 88:65– 77. Kynard B, Henyey E, Horgan M. 2002. Ontogenetic behavior, migration, and social behavior of pallid sturgeon, Scaphirhynchus albus, and shovelnose sturgeon, S. platorynchus, with notes on the adaptive significance of body color. Environ Biol Fishes 63:389–403. Kynard B, Parker E, Parker T. 2005. Behavior of early life intervals of Klamath River green sturgeon, Acipenser medirostris, with a note on body color. Environ Biol Fishes 72:85–97. LaHaye M, Branchaud A, Gendron M, Verdon R, Fortin R. 1992. Reproduction, early life-history, and characteristics of the spawning grounds of the lake sturgeon (Acipenser fulvescens) in Des-Prairies and Lassomption rivers, near Montreal, Quebec. Can J Zool 70:1681–1689. Liem KF, Bemis W, Walker WF, Grande L. 2000. Functional anatomy of the vertebrates: An evolutionary perspective, 3rd ed. Belmont, CA: Thomson/Brooks Cole. Mayden RL, Kuhajda BR. 1996. Systematics, taxonomy, and conservation status of the endangered Alabama sturgeon, Scaphirhynchus suttkusi Williams and Clemmer (Actinopterygii, Acipenseridae). Copeia 1996:241–273. McGonnell IM. 2001. The evolution of the pectoral girdle. J Anat 199:189–194. Rosen DE. 1982. Teleostean interrelationships, morphological function, and evolutionary inference. Am Zool 22:261–273. Wilga CD, Lauder GV. 1999. Locomotion in sturgeon: Function of the pectoral fins. J Exp Biol 202:2413–2432.