VERTEBRAL DEVELOPMENT IN THE DEVONIAN SARCOPTERYGIAN FISH EUSTHENOPTERON FOORDI AND THE POLARITY OF VERTEBRAL EVOLUTION IN NON-AMNIOTE TETRAPODS

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1 Journal of Vertebrate Paleontology 22(3): , September by the Society of Vertebrate Paleontology VERTEBRAL DEVELOPMENT IN THE DEVONIAN SARCOPTERYGIAN FISH EUSTHENOPTERON FOORDI AND THE POLARITY OF VERTEBRAL EVOLUTION IN NON-AMNIOTE TETRAPODS S. COTE 1 *, R. CARROLL 1, R. CLOUTIER 2, and L. BAR-SAGI 1 1 Redpath Museum, McGill University, 859 Sherbrooke St. W., Montreal, Quebec, H3A 2K6, Canada; 2 Département de Biologie, Université de Québec à Rimouski, 310 allée des Ursulines, Rimouski, Quebec, G5L 3A1, Canada ABSTRACT Study of a growth series of twenty-seven specimens from the Upper Devonian of Escuminac Bay, Québec documents a complex pattern of vertebral development in the osteolepiform fish Eusthenopteron foordi. Ossification begins with elements associated with the caudal, anal, and second dorsal fins. Development of the haemal arches, caudal radials, and caudal neural arches continues anteriorly and posteriorly from near the level of the anterior margin of the caudal fin. Trunk neural arches ossify later than the caudal neural arches and as a separate sequence. Trunk intercentra most likely begin ossification posteriorly and continue forward after the ossification of haemal arches is complete. Comparisons of many different patterns of vertebral development within the modern actinopterygians demonstrates that the sequence of development in Eusthenopteron foordi is unique. The diverse patterns of vertebral development observed in fossil and modern fish presumably result from an interplay between the inherent anterior to posterior sequence of development controlled by the Hox genes, and varying selective forces imposed by the physical and biological environment in which the fish develop. Initiation of vertebral development in the caudal region of Eusthenopteron foordi can be attributed to selection for early function of the tail in propulsion. In contrast, vertebral development in Carboniferous amphibians typically proceeds from anterior to posterior. This may reflect development in the still water of ponds and lakes in contrast with the coastal environment inhabited by the hatchlings of Eusthenopteron foordi. The sequences of vertebral development seen in Carboniferous labyrinthodonts and lepospondyls are divergently derived from that observed in Eusthenopteron foordi. INTRODUCTION Amphibians are unique among tetrapods in commonly expressing a biphasic life history with fossilizable larval stages that document early ontogenetic development. The sequence of development of vertebral elements differs markedly among the major taxa of both Paleozoic and modern amphibians. Differences in developmental patterns provide a potential means of inferring phylogenetic relationships, but also reflect major difference in their ways of life that are significant in tracing their evolutionary history. Carroll et al. (1999) attempted to establish relationships between Paleozoic and modern amphibian orders on the basis of different patterns of vertebral development. They documented a consistent pattern in the timing and direction of ossification of the arches and centra in anurans and the larvae of labyrinthodonts, specifically temnospondyl branchiosaurs, in which the arches ossify before the multipartite centra in a clearly anterior to posterior sequence. They contrasted this pattern with that seen in lepospondyls (particularly microsaurs) and specific salamanders, in which cylindrical centra ossify at a very early ontogenetic stage, prior to the neural arches (Fig. 1). The early formation of cylindrical centra in many salamanders was used to suggest that they might share a common ancestry with lepospondyls (Carroll et al., 1999) since this pattern was certainly a derived character relative to the presence of multipartite centra in both labyrinthodonts and their putative sister-taxa, such as the osteolepiform Eusthenopteron foordi. However, a sister-group relationship between frogs and labyrinthodonts could not be supported by the common pattern of ver- *Current address: Harvard University, Department of Anthropology, Peabody Museum, 11 Divinity Avenue, Cambridge, Massachusetts, Current address: Angell Memorial Animal Hospital, 350 S. Huntington Ave., Boston, Massachusetts, tebral development if the sequence of development seen in Carboniferous labyrinthodonts were primitive for tetrapods. Knowledge of the early history of Chondrichthyes, Osteichthyes, and Placodermi indicates that neural arches evolved long before centra (Goodrich, 1930; Remane, 1936; Carroll, 1988). This suggests that the pattern of vertebral development seen in Carboniferous labyrinthodonts, in which the arches ossify before the centra, and in an anterior to posterior direction, is probably primitive for tetrapods. However, the sequence and direction of vertebral development has never been described in the closest sister-group of tetrapods, the osteolepiform sarcopterygians. Large numbers of immature specimens of the best known of osteolepiforms, Eusthenopteron foordi Whiteaves, are present in numerous collections and have been used for study of the pattern of development of both the body proportions (Thomson and Hahn, 1968) and the skull (Schultze, 1984). However, vertebral development has been largely ignored. Andrews and Westoll s (1970) description of the skeleton of Eusthenopteron foordi remains the most comprehensive and widely accepted (Fig. 2), however it deals only with mature specimens. The current study documents the sequence and direction of vertebral ossification in Eusthenopteron foordi and compares this data with the pattern of development seen in modern fish, amphibians, and Carboniferous labyrinthodonts and lepospondyls. VERTEBRAL DEVELOPMENT IN EUSTHENOPTERON Extensive collections of Eusthenopteron foordi from the Upper Devonian (middle Frasnian) locality of Miguasha in Québec were examined from the parc de Miguasha, Québec (MHNM) (approximately 800 specimens), the Natural History Museum, London (BM(NH)), and the Museum of Comparative Zoology, Harvard (MCZ). Study was concentrated on 27 specimens ranging from less than 3 cm to 29.5 cm in length. The smallest showed no trace of ossificiation of the internal skeleton, but in the largest, all elements of the endochondral skeleton had be- 487

2 488 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 3, 2002 FIGURE 1. Vertebral development in Paleozoic and modern amphibians. A, dorsal view of a larva of the temnospondyl labyrinthodont Branchiosaurus salamandroides from the Westphalian D of Nýřany, Czech Republic. Neural arches ossify from anterior to posterior; they are just beginning to form at the base of the tail. Centra ossify later, from paired, crescentic intercentra and pleurocentra. B, dorsal view of a late larval stage of the modern anuran Rana pipiens. The neural arches ossify from anterior to posterior in the trunk region, cylindrical centra form only at metamorphosis. Neither centra or arches form in the tail. C, ventral view of a juvenile specimen of the lepospondyl microsaur Hyloplesion longicostatum, from the Westphalian D of Nýřany, Czech Republic. Even the smallest known specimens of this species have cylindrical centra, but loosely attached neural arches. The poor resolution of the last preserved caudal vertebrae indicate that the centra ossify in an anterior to posterior direction. D, the hynobiid salamander Salamandrella keyserlingii. Both arches and centra develop from anterior to posterior, but in contrast with the frog, the centra form first, and extend to the end of the tail. The most posterior centra initially chondrify as small paired elements. Only a few paired arches can be seen just behind the skull. Reproduced from Carroll et al., Larval temnospondyl labyrinthodonts resemble anurans in that the arches form prior to the centra, and chondrification and ossification of both arches and centra proceed from anterior to posterior. Microsaurs resemble some salamanders in having cylindrical centra that form as early or earlier than the arches. Vertebral development in all these groups is derived relative to that of Eusthenopteron foordi. come ossified and resembled the shape of bones in previously described adults (Figs. 3A H, 4A E). Isolated bones of Eusthenopteron suggest adults reached a size of approximately 1.5 m (Schultze, 1984), although the largest complete specimen, on display at parc de Miguasha, is only 1.06 m long. As others have done in the past, it was assumed that a series of different sized specimens, belonging to a single species from a single locality, represent differences in age. It has previously been shown that the changes accompanying size increase in Eusthenopteron foordi are similar to those seen in growth and maturation studies in modern fish (Schultze, 1984). This series allows comparison of juvenile to mature specimens and indicates the order in which different areas and elements of the vertebral column ossify. The specimens vary greatly in degree of completeness and quality of preservation. Thirty cm was the size limit of those specimens of this genus designated as juvenile by Thomson and Hahn (1968). They based their recognization of a juvenile stage on observations suggesting that most pronounced modifications of body shape had occurred before the 30 cm stage, but admitted that this was an artibrary decision. Study of additional specimens by Schultze (1984) failed to show statistical support for changes in the limb positions for which Thomson and Hahn had argued. However, Schultze did recognize that changes in skull proportions, specifically the relative length of the orbit and the postorbital region of the skull, were characteristic of an early stage in growth that he also referred to as juvenile, although he did not indicate a specific size range for juvenile individuals. The current study suggests that the time at which all elements of the endochondral skeleton have become ossified may be a nonarbitrary means of differentiating between juvenile and adult individuals. This occurs at a total body length between 27.4 and 29.5 cm. Not even the smallest specimens of Eusthenopteron, less than 3 cm in length, show any features typically associated with larvae, nor can any changes seen within the series be associated with a definable metamorphosis (Moser, 1984). On the basis of current evidence, development can be considered direct. The size series (Table 1) shows a fairly even distribution from 3 cm to 29.5 cm, with some clustering of specimens at

3 COTE ET AL. EUSTHENOPTERON VERTEBRAL DEVELOPMENT 489 FIGURE 2. Eusthenopteron foordi Whiteaves. Reconstruction of the skeleton. Reproduced from Andrews and Westoll (1970). In the specimens used for our study the intercentra appeared to be closer together than is indicated in this reconstruction. Abbreviations (used in this and all following figures): af, anal fin; cf, caudal fin.; cl, cleithrum; cna, caudal neural arch and associated spine; dd1, distal support (radial) of first dorsal fin; df1, first (anterior) dorsal fin; df2 second (posterior) dorsal fin; f, femur; ha, haemal arch and associated spine; hu, humerus; ic, intercentrum; na, neural arch and associated spine; pc, pleurocentrum; pcf, pectoral fin; pd1, proximal support (basal plate) of first dorsal fin; pd2, proximal support (basal plate) of second dorsal fin; pra, proximal support (basal plate) of anal fin; pvf, pelvic fin; pvg, pelvic girdle; ran, radials (3) of anal fin; rd2, radials (3) of second dorsal fin; sa, sacral vertebra; vcr, ventral caudal radial; v1, 1 st vertebra (trunk vertebrae are numbered from 1 to 31 from anterior to posterior). approximately 15 cm and 24 cm (see also Parent and Cloutier, 1996:fig. 11). This may indicate that overall body growth is slowed at these time, or more likely, it could be an artifact of sampling. Description of Specimens The four smallest specimens (e.g., Figs. 3A, 5A), ranging from 2.9 to 4.2 cm in total length, show body scales, clearly defined fin structure, generally unjointed lepidotrichia, and the dermal bones of the skull, but no trace of vertebral elements or endochondral fin supports. These specimens are all preserved in dorsal or ventral view, with a broadly flattened head and a narrow body. The beginning of internal ossification can first be noted in the 5.0 cm specimen MHNM (Figs. 3B, 5B), which shows the entire body exposed in lateral view, with well defined dermal fin elements and the initiation of ossification of the posterior endochondral fin supports. These include three neural arches with spines, extending posteriorly from the anterior margin of the epichordal lobe of the caudal fin and four radials extending posteriorly from the anterior margin of the hypochordal lobe, but no haemal arches. It cannot be determined whether or not the radials of the anal fin were ossified, but two radials were present in the second dorsal fin. No endochondral supports were evident in the more anterior dorsal fin or the paired fins. Specimen MHNM (Figs. 3C, 5C; 6.4 cm in length) had ossified all three radials of the second dorsal, as well as two of the radials of the anal fin. The outline of caudal neural arches and ventral radials can be recognized through the overlying scales, but their specific number is difficult to establish. The haemal arches were apparently not yet ossified. An increasing number of elements can be seen in the caudal region of the poorly preserved MHNM (Fig. 6A), estimated to be 9.0 cm in length. These include the first appearance of haemal arches, in addition to the neural arches and radials of the caudal fin. Approximately six neural arches with spines and four caudal radials are ossified, spanning the anterior portion of the caudal fin, in addition to three caudal haemal arches. All three radials of the anal fin are ossified. Two of the three radials in the second dorsal and both bones (one radial and the basal plate) supporting the first dorsal are ossified, but no bones are visible in the pelvic fin. In MHNM (Fig. 6B; 12.0 cm), the caudal neural arches and radials have ossified further posteriorly and additional haemal arches have been added anteriorly. The large proximal support (basal plate) for the anal fin has begun to ossify, but that for the second dorsal fin has not. In MHNM (Fig. 7; 16.8 cm), almost the entire body is well preserved, but endochondral ossification is limited to the posterior end of the animal. Neither pectoral nor pelvic fins show endochondral bones. Supports for the first dorsal are well formed, although the elements have been displaced and appear side by side. Anterior haemal arches are evident only to the level of the distal end of the proximal support (basal plate) for the anal fin. The proximal support for the second dorsal fin is not ossified. BM(NH) P (not illustrated) is only 15.5 cm in length, but has an ossified proximal support for the second dorsal. In MHNM (Fig. 8; estimated total length 18.2 cm), in which the endochondral skeleton is very well exposed, all but the most posterior caudal neural arches and ventral radials are ossified. However, the last several posterior haemal arches remain unossified, showing that all of these elements ossified initially in association with support for the anterior margin of the caudal fin, with subsequent ossification extending from the front to the back within the caudal skeleton, and slightly later from the anterior margin of the caudal fin forward toward the trunk. In this specimen, 11 neural arches are ossified, but they do not extend forward of the base of the second dorsal fin. In contrast, the haemal arches extend to a level just behind the pelvis where they continue as intercentra, the main central elements in Eusthenopteron. Eleven trunk intercentra, including the sacral vertebra (as identified by Andrews and Westoll (1970)), are present anterior to the pelvis, at which point the block ends. These would be vertebrae 22 through 32 in a fully developed adult. Traces of ribs, the bases of the trunk neural arches, and possibly also pleurocentra are ossified in the posterior trunk. By this stage, the supports for the dorsal and anal fins have reached almost the adult form. In a marked advanced over the next smaller specimens, the paired elements of the pelvic girdle are fully formed, together with the proximal elements of the pelvic fins. Both the proximal and distal endochondral supports for the first dorsal fin are clearly defined. MCZ 5810 (Fig. 9; 19.3 cm) is a critical specimen, possessing most of the vertebral column, although in some places the vertebrae are covered by scales that could not be removed without damaging the underlying bone. The dermal bones of the head are crushed and a break runs vertically through the skull between the parietals and postparietals. However, no bones are

4 490 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 3, 2002 FIGURE 3. Reconstructions of a series of specimens of Eusthenopteron foordi showing changes in degree of ossification of the endochondral skeleton in relation to increasing size. These changes are assumed to represent modifications during ontogeny. Body outline indicated in solid lines based on the fossil as preserved, with some amount of reconstruction. Dotted lines indicate missing portions of the fossil. Endochondral elements are identified in subsequent illustrations. A, MHNM A/B; B, MHNM ; C, MHNM A/B; D, MHNM A/B; E, MHNM A/B; F, MHNM A/B; G, MHNM A/B; H, MCZ entirely missing and it is clear that the full set of dermal elements was ossified. On the other hand, it is evident that the braincase and palatoquadrate were unossified between the welldefined bones of both sides of the dermal skull. The supports for the anal and second dorsal fin are complete and the humerus and two other elements of the pectoral fin are ossified. The pelvic girdle is missing, as is the distal element of the first dorsal fin support, most likely the result of local breakage of the specimen. Sixteen intercentra, which begin behind the opercular series and proceed posteriorly along the body are visible through the scales. This spans the positions of vertebrae 7 through 22 or 23 in an adult. There is a large gap between the most posterior intercentra and the haemal arches. No centra or arches are present anterior to the back of the operculum, although approximately six would be present in this area of an adult (Andrews and Westoll, 1970; Hitchcock, 1995). Neural arches are visible above what would be the seventh, eleventh, and fourteenth intercentra in an adult. In the caudal region, nine neural arches with spines and twelve haemal arches are present, as well as eight ventral radials. Although generally poorly preserved, with areas at the base of the tail and in the region of the operculum obliterated by the formation of pyrite, MHNM (Fig. 10; 21.0 cm) includes most of the skeleton. Impressions of intercentra and neural

5 COTE ET AL. EUSTHENOPTERON VERTEBRAL DEVELOPMENT 491 FIGURE 4. Continuation of Figure 3. A, MHNM ; B, MHNM A/B; C, MHNM A; D, BM(NH) P. 6803; E, MHNM arches extend anteriorly to at least the front of the shoulder girdle, and may pass beneath the operculum. Unfortunately, the impressions of the vertebrae are not preserved in the matrix, but against the medial surface of the dermal scales, and so show almost no surface detail. Therefore, it is not possible to determine whether or not pleurocentra were present. Endochondral supports for both the pectoral and the pelvic fins are visible. As in MCZ 5810, no endochondral bone of the skull is preserved. Ossification in the caudal region can next be seen in MHNM (Fig. 11; estimated total length 24 cm), an especially well-preserved specimen. The caudal fin is supported by seven to eight neural arches with spines, four to five haemal arches, and 11 to 12 radials. This compares, respectively, with 12, 10,

6 492 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 3, 2002 TABLE 1. Reconstructed length of Eusthenopteron foordi specimens. Abbreviations: MHNM, specimens from the collection of the Parc de Miguasha; BM(NM) P, specimens from the collection of the British Museum of Natural History; MCZ, specimens from the Museum of Comparative Zoology at Harvard. Total length is measured from the tip of the snout to the end of the medial portion of the caudal fin in small specimens in which it extends beyond the dorsal and ventral lobes, and by the ends of the dorsal and ventral lobes of the caudal fins in large specimens, where they exceed the length of the medial portion. For incomplete specimens, total length was estimated on the basis of the proportions of the body that was preserved, accepting Schultze s (1984) finding that the position of the fins did not change significantly during growth. Rank Specimen number MHNM MHNM A/B MHNM MHNM MHNM MHNM A/B MHNM A/B MHNM A/B BM(NH) P BM(NH) P MHNM A/B MHNM A/B MCZ 6518 A/B MCZ 5810 MCZ 9159 BM(NH) P.7074 MHNM MHNM B MCZ 9265 BM(NH) P MHNM A/B MHNM A BM(NH) P BM(NH) P BM(NH) P BM(NH) P MHNM Approximate length (cm) and 14 in the restoration of an adult specimen illustrated by Andrews and Westoll (1970). Based on the latter specimen, the longest of each element in MHNM is identified as the most anterior support for the caudal fin. The length of the neural arches and the radials is reduced more anteriorly. Unfortunately, this specimen does not extend anterior to the fin supports for the second dorsal and anal fins. MHNM (Fig. 12; estimated total length 25 cm) is uniquely preserved, showing the entire vertebral column to the base of the tail as well as the palatoquadrate and the braincase, but without any dermal bones or any trace of fins. The cranial elements are preserved in dorsal view, to judge by the smooth, rather then tooth covered surface of the palatoquadrate. This is the smallest specimen in which there is definite evidence that the braincase and palatoquadrate are ossified. The first four vertebrae are disarticulated, but all 31 trunk intercentra, and a putative sacral vertebra, can be accounted for. The intercentra at the anterior end of the column are visible in ventral view (in contrast with the skull). The first 16 are extensive crescents, continuous at the ventral midline. The next few are either broken ventrally or were originally paired, while those more posterior (exposed in lateral view) were certainly paired. The last three elements presumably represent the most proximal portion of the tail, but the area of the haemal arch is not preserved. Small, paired, circular pleurocentra are visible in association with intercentra 23 to 33. Pleurocentra may have been present more anteriorly, but would not be exposed since only the ventral surface of the intercentra can be seen. In specimens BM(NH) P (Fig. 13; 26.4 cm) and BM(NH) P (not illustrated; 26.7 cm), the vertebral column is represented primarily by impressions showing the external surface of the scales closely overlying the arches and centra. This mode of preservation shows that they were relatively large and well ossified, but reveals no surface detail. BM(NH) P shows a number of clearly defined distal radials of the pectoral fin. More proximal elements are present, but reveal little detail. Neural arches are associated with many of the the intercentra. In the largest specimen studied, MHNM (Fig. 14), estimated at 29.5 cm in total length, the entire column had achieved an essentially adult condition, with probable ossification of all elements from the skull to the end of the tail, including the ribs. Unfortunately, due to the manner of initial preparation, little surface detail remains. The elements of the pelvic fin distal to the femur are well exposed and fully ossified. Summary of Ossification Sequences Numerous, well-preserved specimens document the developmental sequence of endochondral bones in the posterior portion of the vertebral column in Eusthenopteron foordi (Table 2; Figs. 3, 4, 15A). Ossification of caudal neural arches, haemal arches, and ventral radials begins at the anterior margin of the caudal fin and proceeds both posteriorly and anteriorly. Anterior to the caudal fin, there is a gap in the ossification of neural arches in MHNM until roughly the level of the sacral vertebra but the haemal arches and intercentra form a continuous series, ossifying from posterior to anterior, into the posterior trunk. The gap in the sequence of neural arches below and just anterior and posterior to the second dorsal fin may be related to the need for flexion in this area for effective swimming. The neural arches in this area are reduced in size relative to those anterior and posterior to them in the adult stage as well (Fig. 2). Pleurocentra are only seen with certainty in the posterior trunk region after all the other elements are formed. The distal radials of both the anal and second dorsal fins ossify before the large proximal supports; in contrast, the proximal and distal elements of the first dorsal fin appear to ossify simultaneously. Few specimens clearly show the pattern of development in the anterior portion of the column. Specimens MHNM and MHNM show a complete covering of scales, but the endochondral elements of the caudal, dorsal, and haemal fins protrude through them. No such protrusions are evident in the area of the paired fins or the vertebral column anterior to the level of the base of the anal fin. MCZ 5810 (Fig. 9) shows intercentra in the area between the pectoral and pelvic fins, corresponding with vertebrae 7 through 22 or 23. This specimen is most simply interpreted as indicating that ossificiation of the anterior intercentra begins just behind the operculum and proceeds both anteriorly and posteriorly. This appears to occur as a separate event from the ossification of the haemal arches. However, MHNM (Fig. 8), which is one centimeter shorter than MCZ 5810, shows fully developed intercentra beginning with the sacral vertebra and extending anteriorly to the 22 nd intercentrum, where the block ends. This specimen may be interpreted as demonstrating that the intercentra develop as a continuation of the series of haemal arches, beginning at the end of the trunk and proceeding anteriorly. However, we cannot exclude the possibility that intercentra in this specimen began ossification near the anterior end of the trunk and continued in a posterior direction. These two specimens seem to provide contradictory information regarding the sequence of development of the intercentra. However, further study of MCZ 5810 reveals that the gap seen between the haemal arches and the last preserved inter-

7 COTE ET AL. EUSTHENOPTERON VERTEBRAL DEVELOPMENT 493 FIGURE 5. Three of the smallest, but fairly well preserved specimens of Eusthenopteron foordi. A, MHNM A, preserved in ventral view, showing the impression of the dorsal surface of the skull. Long, essentially unjointed lepidotrichia of the pectoral and caudal fins are visible. No endochondral fin supports or vertebral elements are ossified at this stage; B, MHNM , complete skeleton in lateral view. Endochondral supports for the caudal, anal, and second dorsal fin are visible through the scales. All fins are supported by unjointed lepidotrichia; C, MHNM B. Ventral view of head region, showing the underside of the skull roof and cheeks. The body is twisted so that the lateral surface of the caudal region is exposed. Endochondral supports for the caudal, anal, and both dorsal fins can be seen through or between the scales. No endochondral elements can be seen more anteriorly. Image reversed left to right for comparison with the other two specimens.

8 494 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 3, 2002 FIGURE 6. Eusthenopteron foordi. A, MHNM A/B, showing the support for the first dorsal fin and the first haemal arches in addition to the elements seen in smaller specimens; B, MHNM A/B, also shows the proximal fin support for the anal fin. centra may be an artifact of preservation, since there is damage to the specimen in that area. It is therefore possible that intercentra may have extended back to the haemal arches. If this were the case, it would suggest that intercentra begin ossification posteriorly as part of the sequence of haemal arches and proceed anteriorly. This may have occurred quite rapidly, since MHNM (Fig. 7), which shows no intercentra and an incomplete set of haemal arches, is less than two centimeters shorter than MHNM which possessed all the haemal arches and at least the last 11 intercentra. Trunk neural arches are one of the last elements to begin ossification. This certainly occurred as a separate event from the ossification of the caudal neural arches. However, the direction in which they ossified is uncertain. The earliest appearance of trunk neural arches is in MHNM (Fig. 8), however they appear irregularly over a few intercentra and no pattern can be established. This is also the case in MCZ Therefore, ossification of trunk neural arches could have occurred either from front to back or from back to front. Pleurocentra are first clearly seen at the posterior end of the trunk and anterior caudal region in MHNM , but they may have been present more anteriorly in this specimen and yet not be visible. They too might have begun ossification from either the anterior or posteriorly end of the body. Based on the nearly universal anterior to posterior direction of expression of the Hox genes (which control major aspects of development of the body axis; Shubin et al., 1997; Coates and Cohn, 1999), and the geologically earlier appearance of chondrification and/or ossification of the neural and haemal arches relative to the centra in both Chondrichthyes and Osteichthyes, the pattern of development in Eusthenopteron foordi appears highly derived relative to more primitive aquatic vertebrates. The sequence also appears very different from that seen in both Carboniferous and modern amphibians, which is commonly anterior to posterior. One immediate question is whether the pattern seen in Eusthenopteron foordi is common among modern bony fish or is a specialization of sarcopterygians or of the lineage leading to tetrapods. Among sarcopterygians, neither fossil nor living coelacanths have ossified centra, and so provide only a limited basis for comparison (Andrews, 1977). Centra as well as neural and haemal arches are ossified in Devonian lungfish (Denison, 1968), but the relative sequence of their ossification has not been established. Recent work by Arratia (2001) will provide a basis for establishing the pattern of vertebral development in the modern species. The primary basis for comparison thus lies with the phylogenetically divergent living actinopterygian fish. ACTINOPTERYGIAN FISH Surprisingly, there is relatively little published information regarding vertebral development in modern actinopterygian fish, and there has been no recent synthesis of the available data. Dunn (1984) listed work that had been published up to that date and states that the pattern of ossification varies considerably among taxa. More recent studies further document this variability. In the following section, observations on vertebral development taken from the available literature are re- FIGURE 7. Eusthenopteron foordi. MHNM A. Nearly complete skeleton with continuous scale cover. Endochondral supports are clearly evident posteriorly (darkened for emphasis), and can not be seen in association with the pelvic or pectoral fins.

9 COTE ET AL. EUSTHENOPTERON VERTEBRAL DEVELOPMENT 495 FIGURE 8. Eusthenopteron foordi. MHNM A/B. Caudal region and posterior trunk showing a gap in the neural arches between the caudal fin and the area of the sacral vertebra and the first evidence of the pectoral girdle and proximal support for the second dorsal fin. Neural arches begin to appear over some intercentra. viewed in a phylogenetic sequence beginning with the most primitive living actinopterygians. Specific comparison of vertebral elements between sarcopterygians and actinopterygians is complicated by the fact that the central elements are not homologous, and different patterns of development of both the arches and centra are indicated by a distinct terminology. Patterns of development among actinopterygians, emphasising the caudal region, are documented in detail by Arratia and Schultze (1992) and by Schultze and Arratia (1986). Where the terminology differs, the sarcopterygian term is used, followed by the designation for the actinopterygian analogue in parentheses. Polypterus (Polypteridae), considered the primitive sister-taxon of all other living actinopterygians, provides an informative contrast with Eusthenopteron foordi. Overall, the neural arches, vertebral centra, and ribs differentiate from anterior to posterior, while the lepidotrichia and endoskeletal supports of the dorsal fin form from posterior to anterior. Chondrification and ossification of the hypaxial caudal skeleton also occur in a posterior to anterior direction (Bartsch and Gemballa, 1992). Chondrification begins when the fish is 11.0 mm long with the occipital arches and the bases of the next three neural arches. By 14.5 mm, the first and second pair of neural arches and the posterior fin rays of the dorsal fin are ossified. Body scales begin to form in specimens approximately 30 mm in length, in marked contrast to their much earlier appearance relative to the centra and fin supports in Eusthenopteron foordi. Bartsch and Gemballa (1992:519) state that the development of the vertebral column in Polypterus is ruled by strict functional demand rather than by the ballast of evolutionary history. They interpret the early completion of the caudal and posterior dorsal fin skeleton as being necessary adaptations for locomotion. Development of the primitive chondrostean Polyodon spathula (Acipenseriformes: Polyodontidae), was recently described by Bemis and Grande (1999). The medial fins become distinguished from one another in an anterior to posterior sequence- dorsal, anal, then caudal. However, as in Eusthenopteron foordi, fin supports are initially elaborated in the caudal region. The first visible skeletal elements are the hypurals and middle radials of the dorsal and anal fin, which are initially all FIGURE 9. Eusthenopteron foordi. MCZ Complete skeleton covered with scales. Shows the humerus and several small bones of the pectoral support. Pelvic girdle is absent, most likely due to breakage of the specimen in this area. Intercentra can be seen from the area of the pectoral girdle back to the mid-trunk region. There are no intercentra in the opercular region, or in the area of the pelvic fin. The dermal skull is fully ossified, however there is no evidence of an ossified braincase or palatoquadrate.

10 496 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 3, 2002 FIGURE 10. Eusthenopteron foordi. MHNM Smallest specimen in which the vertebral column extends to the back of the skull, and the proximal elements of the pectoral fin are well ossified. Most of the endochondral bone is preserved as an impression on the inside surface of the scales, which shows little structural detail. Dashed outline indicates pyrite. Head omitted. FIGURE 11. Eusthenopteron foordi. MHNM A/B. Posterior portion of the skeleton showing the endochondral supports for the caudal, anal, and second dorsal fin in nearly their adult configuration. Lepidotrichia conspicuously jointed in contrast with smaller specimens.

11 COTE ET AL. EUSTHENOPTERON VERTEBRAL DEVELOPMENT 497 FIGURE 12. Eusthenopteron foordi. MHNM A. Endochondral bones of axial skeleton, stripped of all elements of the dermal skeleton. Palatoquadrate seems to be preserved in dorsal view, anterior portion of column in ventral view, and posterior portion of column in lateral view. This is the smallest specimen to show pleurocentra unequivocally. Braincase is extensively ossified. cartilaginous. Hypurals, radials, and dermal fin rays all begin development in the middle portion of the fin and spread both posteriorly and anteriorly. Polyodon does not form vertebral centra. The full adult complement of all caudal, anal, and dorsal fin structures are present by the time the fish is 47 mm. However, only the dermal fin rays are ossified. Bemis and Grande believe that fish at this stage have completed metamorphosis, which indicates that ossification of endochondral structures must occur sometime in the juvenile stage. Development of Amia calva has recently been described by Grande and Bemis (1998). As shown in their illustrations (one of which is redrawn as Figure 15B), the abdominal centra (those running from the occiput to the first centrum bearing a haemal canal) mineralize in an anterior to posterior sequence. Ossification of the ural centra occurs separately and begins before the ossification of even the most anterior of the diplospondylous centra (part of the preural caudal region). Chondrification and ossification of the neural arches extends into the caudal region FIGURE 13. Eusthenopteron foordi. BM(NH) P. 6803, with full pectoral fin support and more visible neural arches over the intercentra.

12 498 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 3, 2002 FIGURE 14. Eusthenopteron foordi. MHNM cm in length. Skeleton is fully ossified. prior to the centra. The supports for the caudal fin ossify before those of the paired or dorsal fins. More details of vertebral development are provided in Schultze and Arratia (1986). Among teleost fish, the Catostomidae have been reported to ossify their vertebral centra in an anterior to posterior direction, but in the guppy (Poeciliidae), ossification begins in the middle of the vertebral column and proceeds both anteriorly and posteriorly (Weisel, 1967). Crane (1966) describes yet another pattern in the viperfishes (Chauliodontidae). Ossification of the vertebrae occurs first in the caudal region and proceeds anteriorly. In some specimens, the most anterior vertebrae remain unossified throughout life, which Crane suggests could be an adaptation for increased flexion required for efficient feeding. Therefore, in a phylogenetically and functionally diverse group of teleost fishes, a variety of very different patterns of vertebral ossification can be found. Within the order Perciformes, we see a more consistent pattern of vertebral ossification. Most perciforms for which the TABLE 2. Sequence of first appearance of endochondral bones of the skeleton of Eusthenopteron foordi. Skeletal element Caudal neural arches Ventral caudal radials First two radials of second dorsal fin Third radial of second dorsal fin First two radials of anal fin Third radial of anal fin Haemal arches of caudal fin Supports of first dorsal fin Proximal support for anal fin Proximal support of second dorsal fin Pelvic girdle and femur Posterior intercentra Anterior intercentra Pectoral support (humerus) Endochondral skull (braincase and palatoquadrate) Length (in cm) of smallest specimen expressing this element direction of vertebral ossification has been reported show an anterior to posterior direction of differentiation in both the arches and centra. For example, the swordfish, Xiphidae (Potthoff and Kelley, 1982), the closely related Scombridae (Wollom, 1970), and the Anarhichadidae (Pavlov and Moksness, 1997). Anisotremus virginicus (Haemulidae), known as the porkfish, shows a slightly different pattern (Potthoff et al., 1984). Cartilaginous neural arches (basidorsals) appear first both just behind the skull and separately in the mid-trunk region, while cartilaginous haemal arches (basiventrals) appear at the center and posterior end of the vertebral column. However, ossification of both neural and haemal arches occurs after the ossification of all vertebral centra in an anterior to posterior direction. The posterior portion of the dorsal fin, as well as the anal and caudal fins are the first to develop rays, while the pelvic and pectoral fins are the last to do so. The elements of the caudal fin support, including the hypurals, normally ossify after the centra of the trunk. Potthoff et al. (1984) cited several additional studies of various perciforms in which vertebral elements ossify from anterior to posterior. The perciform Archosargus probatocephalus or sheepshead (Sparidae), also ossifies all its vertebral elements in an anterior to posterior direction (Mook, 1977). Here, neural and haemal arches begin ossification soon after the centra, so that the anterior neural and haemal arches are ossified before the posterior centra. Ossification of the caudal skeleton begins with the lepidotrichia in 3.5 mm larvae, followed by the urostyle, hypurals, and finally the epurals- all in an anterior to posterior sequence. The hypurals are the last elements to complete ossification, when the fish is 25 mm in length. In her comprehensive study of centrarchid fishes (Centrarchidae), Mabee (1993) reported that all 30 species ossify their vertebrae following the same pattern, although the number of elements can vary among species. Vertebral centra ossify from anterior to posterior in a continuous sequence, like other perciforms (Fig. 15C). However, the two urostylar centra ossify earlier than the centra anterior to them, although the anterior urostylar centrum does ossify first. Another important difference from other perciforms is that ossification of the haemal arches begins in the middle of the series and then proceeds simultaneously in both anterior and posterior directions. This

13 COTE ET AL. EUSTHENOPTERON VERTEBRAL DEVELOPMENT 499 FIGURE 15. Comparison of the sequence of vertebral development in Eusthenopteron foordi with two modern fish. A, vertebral development in Eusthenopteron foordi with arrows indicating the direction in which elements ossify; B, larval specimen of Amia calva, which illustrates vertebral centra and neural arches ossifying in an anterior to posterior direction. Bone is shown in solid black and thick lines, thin lines and white indicate cartilage. Dermal fin rays ossify first in the caudal region; (modified from Grande and Bemis, 1998:129); C, direction of ossification of vertebral and fin elements in the Centrarchidae; (reproduced from Mabee, 1993). pattern is also shared by the fin rays of the dorsal, anal, and caudal fins, as well as the hypurals. The fin rays of the pectoral fin ossify in a dorsal to ventral direction. The earliest neural arches to ossify are those at the very anterior part of the vertebral column, with ossification continuing in a posterior direction. However, a second set of neural arches ossify in the midtrunk region, from which ossification proceeds in both anterior and posterior directions. The sequence of development begins with the anterior neural arches and hypurals, followed by the haemal arches and mid-trunk neural arches. Although centra are the last elements to appear, they form initially as ossified structures (autocentra) and so would be visible in fossilized specimens before the arches. Mabee states that this pattern of ossification has also been reported in Trachurus symmetricus (Carangidae) (Ahlstrom and Ball, 1954). In contrast with the uniform axial development seen in all centrarchids, Moser and Ahlstrom s (1970) study of the lanternfish (Myctophidae) showed that there can be variability in the sequence of ossification within groups as well. Dermal fin rays are always the first postcranial structures to ossify, but the order in which the fins ossify is variable. The endochondral supports of the caudal fin ossify before those of the other fins, but this occurs after the complete set of caudal dermal fin rays has formed. Vertebral centra, neural arches, and haemal arches all ossify in an anterior to posterior direction. The timing of ossification can vary between species; however, in most the centra, neural arches, and haemal arches ossify simultaneously. Generally, the ossification of the vertebral column occurs late in development relative to the ossification of other structures, however it is usually completed before the time of metamorphosis, which varies with species. Although this general pattern of anterior to posterior vertebral column development is common to most lanternfishes, two species follow the opposite sequence. The vertebral centra of

14 500 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 22, NO. 3, 2002 early ossification of the caudal region in Eusthenopteron foordi may be an adaptation necessary for early swimming in a fastcurrent marine environment. VERTEBRAL DEVELOPMENT IN DEVONIAN AND CARBONIFEROUS TETRAPODS FIGURE 16. Comparison between the body outlines of Eusthenopteron foordi and Esox lucius showing the similarity in positioning of the dorsal and anal fins and overall appearance, which has been used as evidence to suggest their similar behavior as adults. Both species reach similar size as adults. A, Eusthenopteron foordi outline adapted from Schultze (1984); B, Esox lucius outline adapted from Bry (1996). Hygophum atratum and Hygophum reinhardti ossify from posterior to anterior, as do their neural arches (Moser and Ahlstrom, 1970). This pattern is unlike that seen in any of the other perciforms. In addition, ossification begins very late in these two species, normally just before metamorphosis. Therefore, the lanternfish show variation both in the sequence of ossification between elements and in the direction of ossification within specific elements. From this scattered sampling of bony fish, it is obvious that the sequence and direction of chondrification and ossification of vertebral elements are extremely variable and are not obviously tied to the degree of taxonomic affinity. The most common pattern is for most elements to ossify in a predominantly anterior to posterior direction, but in some fish a posterior to anterior sequence is seen. Frequently, some elements of the caudal region will ossify prior to those in the posterior part of the trunk. The absence of a phylogenetically consistent pattern of vertebral development among bony fish strongly suggests the importance of functional controls, such as the feeding and locomotive needs of hatchlings and juveniles (Crane, 1966; Mook, 1977; Bartsch and Gemballa, 1992). Similarity of patterns of development must be supplemented by other data to provide a reliable means of establishing relationships. Ontogeny of the vertebrae does not necessarily reflect phylogeny. In fact, very divergent patterns of vertebral development may give rise to very similar adult forms. The pike and muskellunge, Esox (Esocidae), have long been recognized as similar to Eusthenopteron foordi in body form and probable behavior (Andrews and Westoll, 1970; Arsenault, 1982). This is based on their large size and the far posterior position of the dorsal and anal fins that give a powerful thrust in lurk and lunge feeding (Fig. 16). One might expect that they would also share the sequence of development of the vertebrae and fin supports seen in Eusthenopteron foordi. In fact, recent X-ray studies carried out by Alison Murray show that the vertebral column in the four North American species of Esox develops from anterior to posterior. This marked difference in development suggests that factors other than adult feeding habits may be important in the selective control of development. Reproduction in pike generally occurs in shallow, thickly vegetated habitats on submerged flood plains (Bry, 1996). This would indicate that there would be little wave or current activity in the environment in which young Esox were undergoing early development. In contrast, most specimens of Eusthenopteron foordi are found in tidally influenced estuarine environments (Chidiac, 1996; Cloutier et al., 1996). It is probable that the No fossils are yet known of larval or juvenile individuals of either Panderichthys, which is thought to be the closest known sister-taxon to tetrapods, or of any of the Upper Devonian amphibians. However, the high degree of development of the caudal fin in these genera strongly supports its importance in swimming, which Clack and Coates (1995) and Clack (2000) argue to be the primary, if not sole means of locomotion in Acanthostega and Ichthyostega. This suggests that the pattern of vertebral development described in Eusthenopteron foordi, with the early ossification of the tail region, may have been retained in Late Devonian amphibians. This is in strong contrast to both the adult structure and the patterns of development seen in most members of the major groups of primitive Carboniferous amphibians, the labyrinthodonts and lepospondyls. With the exception of some embolomeres and the nectrideans, most members of these groups, from the Visean on, had long slender tails with little evidence of adaptation for swimming. Some 20 to 30 million years separate Eusthenopteron foordi and the Upper Devonian tetrapods from the earliest adequately known members of the amphibian lineages that dominated the mid- to late Carboniferous and include the ancestors of the modern orders. During this time, the reproductive environment changed from wave and current influenced marginal marine conditions to quiet inland bodies of fresh water, including coal swamps, oxbow lakes, and larger, semipermanent lakes. The different environments in which Eusthenopteron foordi and Carboniferous amphibians reproduced may have strongly influenced their patterns of vertebral development. These quiet waters apparently did not require the early development of the tail as a major swimming structure, and their partial isolation may have limited the access of large aquatic predators. Reduction in these selective pressures may have allowed the axial skeleton in Carboniferous amphibians to develop in direct accordance with the anterior to posterior direction of expression of the Hox genes. The relatively large adult size of most labyrinthodonts may have required a fairly long period of aquatic development, during which first the neural arches and later the multipartite centra chondrified and then ossified. Lepospondyls had a much different growth strategy, with precocial ossification greatly limiting adult size, and resulting in formation of fully cylindrical centra at a stage when vertebrae in labyrinthodonts had barely begun to ossify (Fig. 1). With the description of vertebral development in Eusthenopteron foordi, it is obvious that the patterns of development in both labyrinthodonts and lepospondyls are derived relative to more primitive choanates. Hence, the pattern seen consistently in larval temnospondyls, in which development of the arches long precedes that of the centra, is a putative synapomorphy linking them with anurans, which follow a comparable sequence in all families in which vertebral development has been described. Comparison with salamanders is more difficult. It has long been assumed that the pattern of vertebral development commonly seen in advanced salamanders, including plethodontids, ambystomatids, and salamandrids, was typical for urodeles. This seemed to be borne out by the very early formation of cylindrical centra, and later appearance of arches in the hynobiid Salamandrella (Carroll et al., 1999). However, further study of a cleared and stained specimen of another hynobiid,

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