Appendicular skeleton

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With the conspicuous exception of Holmes (1977, 1980), few since the early work of Romer (1922) have concentrated on both the osteological and muscular components of the basal tetrapod limb.

Figure 5. Left lateral views of the atlas-axis complex of taxa near the origin of amniotes. Except where noted, all illustrations are after Sumida et al. (1992). (A) The embolomerous anthracosaurian Proterogyrinus\ (B) the seymouriamorph Seymouria; (C) the diadectomorph Limnoscelis\ (D) the diadectomorph Diadectes; (E) the caseosaurian pelycosaur Cotylorhynchus; (F) the eupelycosaurian pelycosaur Ophiacodon-, (G) the millerettid Milleretta (partially after Gow, 1972); (H) the parieasaur Bradysaurus (partially after Boonstra, 1934); (I) the captorhinid reptile Captorhinus\ (J) the protorothyridid reptile Paleothyris; (K) the araeoscelidian reptile Petrolacosaurus. All scalebars are equivalent to 1 cm except G where it equals 0.5 cm. Abbreviations: atic, atlantal intercentrum; atna, atlantal neural arch; atpc, atlantal pleurocentrum; axic, axial intercentrum; axna, axial neural arch; axpc, axial pleurocentrum; pro, proatlas. *

Proterogyrinus Skeleton

(I) Captorhinus

(J) Paleothyris

(K) Petroiacosaurus

(I) Captorhinus

(J) Paleothyris

(K) Petroiacosaurus

That the appendicular elements supply fewer characters for phylogenetic analyses than do the cranium and lower jaws only compounds this bias. Of the few surveys of tetrapod limb structure that exist, most do not include the more distal antebrachial, crural, and pedal structures; those that do address these areas are generally limited to consideration of single taxa (Romer, 1957; Holmes, 1977, 1980; Sumida, 1989a).

Pectoral Girdle and Limb

Heaton's (1980) inability to produce cladistically useful features of the pectoral girdle in his attempt to distinguish among groups of primitive tetrapods highlights the conservative nature of the complex. The pectoral girdle is a multipartite structure incorporating dermal and endochondral elements (Fig. 6). Over time, the endochondral elements have increased in relative mass in the girdle at the expense of the dermal elements. A median, dermal interclavicle is the only unpaired element of the girdle. Paired dermal clavicles and cleithra form the anterior edge of the complex. On each side, a more robust endochondral scapulocoracoid dominates the remainder of the girdle. Primitively, the scapulocoracoid consisted of only dorsal scapular and ventral coracoid ossifications, with the glenoid cavity spanning their articulation. Two coracoid ossifications is a more derived condition. Unfortunately, the pectoral girdle in the potentially intermediate Westlothiana is extremely poorly preserved.

The more basal, distinctly anthrocosaurian condition of the pectoral girdle retains significant development of the dermal elements. Robust clavicular elements are retained in all the taxa surveyed here, and Romer (1922) reconstructed this region as a important area of origin for the clavicular head of the deltoid complex; however, its anterior orientation indicates that it probably provided attachment of

Figure 6. Left lateral views of the pectoral girdle in taxa near the origin of amniotes. (A) The anthracosaurian Proterogyrinus (partially after Holmes, 1984; Romer, 1970); (B) the seymouriamorph Seymouria (partially after White, 1939); (C) the diadectomorph Limnoscelis\ (D) the caseosaurian pelycosaur Varanops (partially after Langston and Reisz, 1981); (E) the captorhinid reptile Labidosaurus\ (F) the araeoscelidian reptile Araeoscelis (partially after Vaughn, 1955). Abbreviations: ant cor, anterior coracoid; cl, clavicle; cle, cleithrum; cor, coracoid; icl, interclavicle; post cor, posterior coracoid; s, scapula. All scalebars are equivalent to 1 cm. 4

ele ele

Proterogyrinus SkeletonChasmatosaurus Shoulder Girdle Skeleton

(C) Limnoscelis

Reptile Scapula

episternal and omohyoid musculature of the neck as well. The cleithrum is reduced to a splint in most amniote taxa. Primitively it is large, extending to the dorsal limit of the girdle in Proterogyrinus, Seymouria, and diadectomorphs. In amniotes other than synapsids the scapulodeltoid musculature probably originated exclusively from the scapular ossification, whereas in anthroacosaurians it likely spanned the scapula-cleithrum articulation. Pelycosaurs are transitional between these two conditions. Proterogyrinus, Seymouria, and diadectomorphs share the primitive condition of a broad scapular blade and single coracoid ossification. More derived taxa possess separate anterior and posterior coracoid ossifications.

Whether the coracoid is composed of one or two elements, the configuration of the glenoid fossa is remarkably conservative among the taxa surveyed. It is traditionally described as "screw shaped", wherein the articular surface approximates a partially helical strap; the anterior portion of the fossa faces ventrolaterally, the middle almost directly laterally, and the posterior portion dorsolateral^ (Fig. 6). It appears that the cartilaginous covering of the articular surfaces of the glenohumeral joint was rather thin (Holmes, 1977; Sumida, 1989a), allowing reasonable hypotheses of movement to be based on the manipulation of bony elements alone. The articular surfaces of the glenoid and humerus are closely congruent, a condition that likely dictated a highly constrained and stereotyped humeral excursion. In forms as disparate as Seymouria, the robust parieasaur Bradysaurus,

Figure 7. Reconstructions of left humeri of taxa near the origin of amniotes in distal, ventral aspect. (A) The anthracosaurian Proterogyrinus (from Holmes, 1984); (B) the seymouriamorph Seymouria; (C) Westlothiana (partially after Smithson et al., 1994); (D) the diadectomorph Limnoscelis\ (E) the diadectomorph Diadectes (partially after Romer, 1956); (F) the caseosaurian pelycosaur Aerosaurus (partially after Langston and Reisz, 1981); (G) the primitive eupelycosaur Ophiacodon\ (H) the parieasaur Bradysaurus (after Boonstra, 1934); (I) the captorhinid reptile Captorhinus; (J) the protorothyridid reptile Paleothyris (partially after Heaton and Reisz, 1986); (K) the araeoscelidian reptile Petrolacosaurus (after Reisz, 1981); and (L) the araeoscelidian reptile Araeoscelis (partially after Vaughn, 1955). All scalebars equal 1 cm. Abbreviations: delt, deltoid process; ect, ectepicondyle; ent, entepicondyle; ent for, entepicondylar foramen; sup, supinator process. 4

(A) Proterogyrinus

(B) Seymouria

(C) Westlothiana

(H) Bradysaurus
Proterogyrinus Skeleton

and the comparatively gracile araeoscelidian Petrolacosaurus, the screw shaped glenoid is retained.

Romer (1922, 1956) characterized the humerus of Paleozoic tetrapods as essentially tetrahedral, an organizational perspective that has been adopted by nearly all subsequent workers. In simplified terms, the axes of the proximal and distal articular surfaces are at nearly right angles to one another. Figure 7 demonstrates these features and significant structural landmarks. In most forms, the proximal articular surface has a high degree of congruence with the glenoid articulation. The anterior region of the proximal articular surface is concave, matching the ventrolaterally directed convexity of the leading surface of the glenoid. Similarly congruent relationships exist for the more posterior surfaces of the glenoid and humeral head. Jenkins (1971) and Holmes (1977) have shown that the complementary articular surfaces constrain a rotation of the humerus along its long axis as it is retracted. At maximal protraction of the humerus, the humeral concavity at the leading margin of the humeral articular head fits snugly into an anterior glenoid convexity, orienting the distal articular surface cranioventrally. Retraction of the humerus prescribes rotation of the humerus on its long axis, resulting in a ventralward orientation of the distal humeral articulation by the middle of the excursion arc and a posteroventral orientation at the end of the arc. Unfortunately, no extant taxa exhibit a gleno-humeral joint similar to the screw-shaped joint of late Paleozoic tetrapods. However, independent analyses of the forelimb in captorhinids and pelycosaurs (Jenkins, 1971; Holmes, 1977; Sumida, 1989a) have all converged on an interpretation similar to that summarized here.

Skeletal indicators of the muscular role critical in postural support dominate the proximal humerus. Although not clearly defined in Seymouria or Westlothiana, a well developed process for insertion of the latissimus dorsi is extremely well-developed in diadectomorphs, synapsids, and most basal sauropsids (Fig. 7). Its development in tetrapods with a broad range of body sizes is perhaps indicative of increased importance of the retractor musculature in these taxa generally. Extremely broad surfaces of attachment on the dorsal and ventral surfaces of the proximal humeral head are indicative of significant attachments of the scapulohumeralis and coracobrachialis

Reconstruction Posturale

Figure 8. Reconstructions of right antebrachial elements of taxa near the origin of amniotes. (A) The anthracosaurian Proterogyrinus (from Holmes, 1984); (B) the seymouriamorph Seymouria; (C) Westlothiana (partially after Smithson et al. 1994); (D) the diadectomorph Limnoscelis\ (E) the caseosaurian pelycosaur Casea (after Romer and Price, 1940); (F) the captorhinid reptile Captorhinus (from Holmes, 1977); and (G) the araeoscelidian reptile Petrolacosaurus (after Reisz, 1981). Elements are shown slightly disarticulated to facilitate viewing of both the radius (left) and ulna (right). All scale bars equal 1 cm except for C where it equals 0.5 cm. Abbreviations: ole, olecranon process; r, radius; u, ulna.

Figure 8. Reconstructions of right antebrachial elements of taxa near the origin of amniotes. (A) The anthracosaurian Proterogyrinus (from Holmes, 1984); (B) the seymouriamorph Seymouria; (C) Westlothiana (partially after Smithson et al. 1994); (D) the diadectomorph Limnoscelis\ (E) the caseosaurian pelycosaur Casea (after Romer and Price, 1940); (F) the captorhinid reptile Captorhinus (from Holmes, 1977); and (G) the araeoscelidian reptile Petrolacosaurus (after Reisz, 1981). Elements are shown slightly disarticulated to facilitate viewing of both the radius (left) and ulna (right). All scale bars equal 1 cm except for C where it equals 0.5 cm. Abbreviations: ole, olecranon process; r, radius; u, ulna.

musculature respectively. The deltopectoral crest is robust in all taxa surveyed.

Whereas the proximal and distal articular heads of the humerus are similarly well developed in all taxa considered, the proportions of the intervening humeral shaft reflect the individual body proportions of each taxon. The humeral shaft is extremely short and almost indistinct in the heavy-bodied Proterogyrinus, Seymouria, diadectomorphs, parieasaurs, large pelycosaurs, and larger captorhinids. The shaft is slimmer and more elongate in the more gracile Westlothiana, protorothyridids, millerettids, protorothyridids, smaller pelycosaurs, small captorhinids, and araeoscelidians. In conjunction with a short humeral shaft, the supinator, entepicondylar, and ectepicondylar processes are robust. The extreme development of these processes provided enormous levers for supinator, flexor, and extensor musculature of the forearm, respectively. Holmes (1977) has suggested that the antebrachium must have been very nearly horizontal at the beginning of the forelimb power stroke, requiring extensive flexor musculature originating from the entepicondyle to maintain postural support during this part of the step cycle. All of the taxa examined here appear to have had such well developed flexor musculature. It is not until diadectomorphs and more derived taxa that the ectepicondyle is better developed, indicating that the contribution of muscular extensors of the antebrachium to propulsion and postural support evolved somewhat later.

Between the entepicondyle and ectepicondyle are an extremely well-developed capitulum and trochlea for radial and ulnar articulations, respectively. Congruence between the distal humerus and antebrachial elements was high, further stereotyping potential movements. The trochlear notch of the ulna is easily recognizable, but the development of a distinct olecranon process that extends well above it is not clearly evident in Proterogyrinus or Seymouria. An olecranon is present in Westlothiana but is not well developed, perhaps because the forelimbs are extremely small size (Smithson et al, 1994). In association with a well-developed olecranon process, the ulnar trochlea is deep and directed anteromedially in diadectomorphs and the Amniota. Manipulation of the components of the elbow joint demonstrates that the anteromedially directed trochlea produces a posterolaterally directed extension of the elbow joint and propulsion. A more efficient lever accommodated the propulsive force delivered by the triceps musculature as the olecranon became more prominent.

In their analysis of the shoulder musculature of varanid lizards Jenkins and Goslow (1983) demonstrated that all three heads of the triceps musculature fired during the middle of propulsive phase. As the firing patterns seen in varanids were broadly similar to those of mammals they characterized as "primitive", they concluded that the activity patterns must have been inherited from some common ancestor of extant reptiles and mammals. Interpolation of this suggestion into the scenario described previously would suggest that in at least some groups of Paleozoic tetrapods similar firing patterns of the triceps musculature may well have occurred. Jenkins and Goslow (1983) further indicated that the vertebral column of varanids exhibited a lateral convexity toward the support phase at the midpoint of the propulsive phase. Similar orientations of the vertebral column may have existed in captorhinid reptiles (Heaton and Reisz, 1980; Sumida, 1989a) and other late Paleozoic tetrapods (Sumida, 1991). Anteromedial force provided by the triceps and directed by the anteromedially oriented trochlear notch of the ulna may have compensated at least partially for the lateral component of vertebral movement near the propulsive forelimb (Figs. 4 and 9).

The presumed attachment of the radius to the ulna via an interosseous membrane and the potentially free rotation of the radius on the capitulum likely indicates that the movement of the radius was primarily limited somewhat by the movement of the ulna on the humerus. A comparison of the potential movements at the distal end of the antebrachium is more difficult, as knowledge of carpal elements is extremely limited in Proterogyrinus, Seymouria, Westlothiana, and diadectomorphs. Without knowledge of the manus in these taxa, an understanding of the possible transformations across the transiton to the amniote condition is not possible. However, a brief description of the condition in representative amniotes (Fig. 10) can lend some insight into certain of their locomotory capabilities. The radiale is essentially a distal extension of the radius, whereas the intermedium and ulnare function together as an extension of the ulna. The distal articular surfaces of these two complexes are in separate planes of the manus, precluding a transverse joint at either of their ends. The most reasonable suggestion remains that of Holmes (1977; supported by Sumida, 1989a): The flat articular surfaces of the bones of the manus were all capable of minimal movements at each joint; summed, this allowed for a moderately flexible forefoot. Although the origin of the pisiform is unclear, diadectids, Tseajaia, pelycosaurs, captorhinids, protorothyridids, and araeoscelidians all have a pisiform on the ulnar side of the carpus. It is particularly well developed in captorhinids and

Captorhinid Skeleton

Figure 9. Dorsal view of the anterior portion of the vertebral column and the right forelimb near the middle of the support phase in a captorhinid reptile (based partially on Heaton and Reisz, 1980; Holmes, 1977) to aid in demonstrating the direction of force constrained by the anteromedially directed trochlea of the ulna toward the convexity of the laterally bent vertebral column. Arrow represents the direction of force only and is not meant to imply magnitude.

Figure 9. Dorsal view of the anterior portion of the vertebral column and the right forelimb near the middle of the support phase in a captorhinid reptile (based partially on Heaton and Reisz, 1980; Holmes, 1977) to aid in demonstrating the direction of force constrained by the anteromedially directed trochlea of the ulna toward the convexity of the laterally bent vertebral column. Arrow represents the direction of force only and is not meant to imply magnitude.

pelycosaurs, providing a stout lever for the flexor carpi ulnaris and extensor carpi ulnaris. This condition is taken to an extreme in the large captorhinid Labidosaurus, which also extends the radiale medially in an analogous fashion on the radial side of the carpus.

In most of the taxa surveyed, the fourth digit of the manus is the largest (Fig. 10), however only protorothyridids (Fig. 10C) and araeoscelidians (Fig. 10D) exhibit a dramatically longer fourth digit (Vaughn, 1955; Carroll, 1969b; Reisz, 1981; Heaton and Reisz, 1986). Thus, models dependent on rotation of the manus and pushoff via the elongate fourth digit (e.g. Rewcastle, 1981) are not applicable to basal amniotes and their presumptive sistergroups. More realistically, the model is applicable only to protorothyridids, araeoscelidians and their derivatives.

Pelvic Girdle and Limb

The structure of the pelvic girdle (Fig. 11) is more conservative than that of the pectoral girdle: Dorsal ilia articulate with sacral

Paleothyris SkeletonPaleothyris Skeleton

Figure 10. Reconstructions in dorsal view of left manus of basal amniotes. (A) The basal eupelyocosaurian pelycosaur Ophiacodon (partially after Romer and Price, 1940); (B) the captorhinid reptile Labidosaurus; (C) the protorothyridid reptile Paleothyris (after Carroll, 1969b); and (D) the araeoscelidian reptile Petrolacosaurus (after Reisz, 1981). In some cases, reconstructions presented here are based on the right manus, but all are presented in left view to facilitate comparison. All scalebars equal 1 cm. Abbreviations: i, intermedium; lc, lateral centrale; mc, medial centrale; p, pisiform; rade, radiale; ule, ulnare; 1-5, first to fifth digits.

(A )Ophiacodon

(C) Paleothyris

(B) Labidosaurus

(D) Petrolacosaurus

Figure 10. Reconstructions in dorsal view of left manus of basal amniotes. (A) The basal eupelyocosaurian pelycosaur Ophiacodon (partially after Romer and Price, 1940); (B) the captorhinid reptile Labidosaurus; (C) the protorothyridid reptile Paleothyris (after Carroll, 1969b); and (D) the araeoscelidian reptile Petrolacosaurus (after Reisz, 1981). In some cases, reconstructions presented here are based on the right manus, but all are presented in left view to facilitate comparison. All scalebars equal 1 cm. Abbreviations: i, intermedium; lc, lateral centrale; mc, medial centrale; p, pisiform; rade, radiale; ule, ulnare; 1-5, first to fifth digits.

vertebrae in varying numbers, often depending on the size of the animal, and quadrangular, plate-like pubic and ischial elements meet one another in the ventral midline. Substantive differences are found primarily in the degree of development of processes of the ilium.

Primitively, as in Proterogyrinus and (presumably) Westlothiana, the ilium has distinct dorsal and posterior processes. The dorsal process articulated medially with the sacral ribs and laterally served as the origin of the iliofemoralis. Romer (1922, 1956) suggested that the posterior process was a point of origin for tail musculature useful in aquatic locomotion. However, in his detailed analysis of Proterogyrinus, Holmes (1984) took issue with this interpretation, pointing out that extant, terrestrial taxa retain similar processes. It is likely that the iliofibularis muscle arose from the posterior process, whereas the iliotibialis probably arose from a ridge running ventral to the junction between the two processes. The processes remain distinguishable and both are robust in Seymouria. In diadectomorphs, basal synapsids, parieasaurs, and other reptiles the two processes have essentially consolidated into a single iliac blade. Although this probably did not affect the origins of the iliofemoralis and iliofibularis muscles significantly, it effectively moved the origin of the iliotibialis away from the hip joint, providing it a somewhat greater mechanical advantage in extension of the knee joint. Parieasaurs are unique in having a prodigious anterior extension of the iliac blade, one that is probably associated with the development of as many as four sacral ribs for support in this heavy-bodied group.

Some debate has centered around the function of an externally directed "shelf' of the ilium in diadectomorphs, a feature Heaton (1980) recognized as a synapomorphy of the group. Romer (1922,

Figure 11. Reconstructions of the pelvic girdle in taxa near the origin of amniotes. (A) The anthracosaurian Proterogyrinus (after Holmes, 1984); (B) the seymouriamorph amphbian Seymouria (partially after White, 1939); (C) Westlothiana (after Smithson et al., 1994); (D) the diadectomorph Limnoscelis (after Berman and Sumida, 1991); (E) the diadectomorph Diadectes (after Romer 1922, 1956); (F) the captorhinid reptile Labidosaurus (after Sumida, 1989a); and (G) the araeoscelidian reptile Araeoscelis (after Vaughn, 1955). All scalebars equal 1 cm. Abbreviations: act, acetabulum; il, ilium; ildpr, dorsal process of ilium; ilpopr, posterior process of ilium; isc, ischium; pu, pubis.

Limnoscelis Fossil

(A) Protemgyrinus

(B) Seymouria

(A) Protemgyrinus

(B) Seymouria

(D) Limnoscelis

(E) Oiadectes

(F) Labidosaurus

(G) Araeoscelis

(D) Limnoscelis

(E) Oiadectes

(F) Labidosaurus

(G) Araeoscelis

Cotylorhynchus Femur

Figure 12. Reconstructions of left femur of the captorhinid reptile Labidosaurus hamatus illustrating important processes and landmarks. (A) Ventral view; (B) anterior view; (C) dorsal view; and (D) posterior view. Proximal end of the femur is toward the top of the page in all illustrations. Abbreviations: act art, acetabular articulation; add, adductor crest; ant cond, anterior condyle; intcon fos, intercondylar fossa; int fos, intertrochanteric fossa; int tr, intrnal trochanter; post cond, posterior condyle.

Figure 12. Reconstructions of left femur of the captorhinid reptile Labidosaurus hamatus illustrating important processes and landmarks. (A) Ventral view; (B) anterior view; (C) dorsal view; and (D) posterior view. Proximal end of the femur is toward the top of the page in all illustrations. Abbreviations: act art, acetabular articulation; add, adductor crest; ant cond, anterior condyle; intcon fos, intercondylar fossa; int fos, intertrochanteric fossa; int tr, intrnal trochanter; post cond, posterior condyle.

1956) and Olson (1936b) suggested that this represented a "rebuilding" of the iliac blade in diadectomorphs and a transitional stage between amphibians and reptiles in which the epaxial muscles were segregated to the dorsal aspect of the sacroiliac articulation. This implies that the lateral iliac shelf of diadectomorphs is homologous to the iliac blade of more derived tetrapods. Epaxial musculature appears to have been restricted to the medial side of the iliac blade in reptiles (Sumida, 1989a), but the potential form of their insertion into the iliac shelf of diadectomorphs is not clear. In pointing out the existence of a similar shelf in the temnospondyl amphibian Eryops Olson (1936b) speculated that it may have served as an expansion of the area of origin for the iliofemoralis muscle (Olson, 1936b).

The acetabulum is markedly conservative in its construction in all taxa surveyed (Fig. 11). It is unremarkable for the most part, smoothly concave and almost circular in outline, with a slight pinching of the recess near its dorsal limit in some taxa. Apparently, it allowed significant potential movement of the femoral head. The proximal head of the femur (Figs. 12 and 13) itself is not nearly as complex as that of the humerus. It is broadly curved and smoothly convex in all taxa examined here. The articular surface is roughly pitted in Proterogyrinus and Seymouria. Smithson et al. (1994) described the proximal articular surface of the femur in Westlothiana as more similar to that of protorothyridids than that of Proterogyrinus. Smithson et al. (1994) illustrate it as more poorly ossified as those of basal reptiles, however this could be an artifact of preservation. In fact, Westlothiana is more similar to diadectomorphs and primitive amniotes in the possession of a well developed internal trochanter. The internal trochanter is relatively very large in Limnoscelis and Diadectes, and distinct in synapsids and basal reptiles. Even the more gracile araeoscelidians display clear evidence of the internal trochanter (Vaughn, 1955; Reisz, 1981).

Although basal anthracosaurs and batrachosaurs surely possessed the puboischiofemoralis internus muscle, the bony markers of its attachment are best seen in Westlothiana and more derived taxa. Romer (1922) proposed that the puboischiofemoralis internus in reptiles retains the ancestral functions of protraction and elevation of the femur. All taxa surveyed have an extremely deep intertrochanteric fossa, evidence of the continuing importance of the insertion of the puboischiofemoralis externus as an adductor of the hip joint for postural support. Seymouria and heavy-bodied parieasaurs show the most extreme development of the intertrochanteric fossa.

All the taxa surveyed possess a well developed adductor crest on the ventral surface of the femur (Fig. 13). Those with a prominent internal trochanter have refined the crest into a prominent, sharp ridge with a pronounced rugosity for attachment of the caudofemoralis muscle. Contrary to the interpretation of Romer (1922), it appears that the caudofemoralis was a muscle of significant mass in basal amniotes, and possibly their sister groups. Numerous studies of extant reptiles (Snyder, 1954; Rewcastle, 1981; Gatesy, 1990) have confirmed the importance of the caudofemoralis in femoral retraction; given the osteological evidence for its robust presence, the suggestion that it was similarly important in basal amniotes is quite reasonable. In taxa with extremely short femoral shafts the adductor crest runs in a markedly oblique angle from proximal to distal, probably to allow as long an insertion of the adductor musculature as possible. As the shaft becomes more distinct in more derived taxa the adductor crest nearly parallels the long axis of the femur.

The femur in Araeoscelis (Fig. 13J) (Vaughn, 1955; Reisz et al., 1984) is distinctly sigmoid, a condition found in other taxa (tentatively) assigned to the family Araeoscelidae [Kadaliosaurus (Credner, 1889); Zarcasaurus (Brinkman et al., 1984)]. In their studies on extant mammals, Bertram and Biewener (1988) suggested that although bone curvature may augment stress during loading, it serves to greatly increase loading predictability. This requirement may have been met in the strongly sigmoid curvature of the femur in araeoselids, a more lightly built and presumably agile group.

The distal end of the femur is divided into distinct anterior and posterior condyles in all taxa surveyed (Figs. 12 andl3). The anterior femoral condyle reaches further distally in strict dorsal or ventral view. The majority of the distal femoral articulation accommodates the tibia (Fig. 15). Holmes (1984) noted that in Proterogyrinus there is little ventral exposure of the femoral articulation, whereas the proximal tibial articulation is rather flat, indicative of the need to keep the knee joint somewhat below the level of the acetabulum to allow the crus to be directed toward the substrate. The articular surfaces of the knee joint are not well ossified in Seymouria or Westlothiana, precluding speculation about its orientation. In diadectomorphs and basal amniotes, a prominent ridge may be seen traversing the proximal head

Figure 13. Reconstructions of the left femur in taxa near the origin of amniotes. (A) the anthracosaurian Proterogyrinus (after Holmes, 1984); (B) the seymouriamorph Seymouria (partially after White, 1939 and Romer, 1956); (C) Westlothiana (partially after Smithson et al., 1994); (D) the diadectomorph Limnoscelis (partially after Romer, 1956; Berman and Sumida, 1991); (E) the diadectomorph Diadectes (after Romer, 1956); (F) the caseosaurian pelycosaur Varanops\ (G) the pareiasaurian Pareiasaurus (after Romer, 1956); (H) the protorothyridid reptile Paleothyris (partially after Carroll, 1969c); (I) the araeoscelidian reptile Petrolacosaurus (from Reisz, 1981); and (J) the araeoscelidian reptile Araeoscelis, anterior view (after Vaughn, 1955 and Romer, 1956). All scalebars equal 1 cm. ^

(A) Proterogyrinus (B) Seymouria

(C) Westlothiana

(F)Varanops (G) Pareiasaurus (H) Paleothyris

(D) Limnoscelis (E) Diadectes

(I) Petrolacosaurus (J) Araeoscelis (anterior view)

(B) Seymouha

(C) Westlothiana

(A) Prvtemgyrinus

(B) Seymouha

(C) Westlothiana

(A) Prvtemgyrinus

(D) Limnoscelis

{E)Ophiacodon

Figure 14. Reconstructions of left crural elements of taxa near the origin of amniotes in dorsal (preaxial) view. (A) The anthracosauroidian Proterogyrinus (after Holmes, 1984); (B) the seymouriamorph Seymouria; (C) Westlothiana (partially after Smithson et al. 1994); (D) the diadectomorph Limnoscelis; (E) the eupelycosaurian pelycosaur Ophiacodon (after Romer and Price, 1940); (F) the captorhinid reptile Labidosaurus (after Sumida, 1989a); and (G) the araeoscelidian reptile Petrolacosaurus (after Reisz, 1981). Elements are shown slightly disarticulated to facilitate viewing of both the tibia and fibula. All scalebars equal 1 cm. Abbreviations: fi, fibula; t, tibia.

(D) Limnoscelis

{E)Ophiacodon

(F) Labidosaurus (G) Petmlacosaurus

Figure 14. Reconstructions of left crural elements of taxa near the origin of amniotes in dorsal (preaxial) view. (A) The anthracosauroidian Proterogyrinus (after Holmes, 1984); (B) the seymouriamorph Seymouria; (C) Westlothiana (partially after Smithson et al. 1994); (D) the diadectomorph Limnoscelis; (E) the eupelycosaurian pelycosaur Ophiacodon (after Romer and Price, 1940); (F) the captorhinid reptile Labidosaurus (after Sumida, 1989a); and (G) the araeoscelidian reptile Petrolacosaurus (after Reisz, 1981). Elements are shown slightly disarticulated to facilitate viewing of both the tibia and fibula. All scalebars equal 1 cm. Abbreviations: fi, fibula; t, tibia.

of the tibia, dividing it into a larger anteromedial surface and a somewhat smaller posterolateral surface. Articulation of well preserved materials represented by pelycosaurs and the captorhinids Labidosaurus and Captorhinus indicates that the ridge lay against the inner edge of the posterolateral femoral condyle and in line with the intercondylar fossa (Fig. 15). Based on this scheme of articulation, the crus is estimated to have laid at an angle of approximately 75 to 80° to the femoral axis (Holmes, 1977; Sumida, 1989a). Because the acetabular articulation was relatively free, it is not possible to determine whether the femur angled slightly ventrally (instead of

Limnoscelis Femur

Figure 15. Articular surfaces of the left knee joint of the captorhinid reptile Labidosaurus hamatus. (A) Proximal surface of tibia; (B) proximal surface of fibula; (C) distal surface of femur; and (D) reconstruction of hyperflexed knee joint to demonstrate position of reconstructed cruciate ligaments. Large arrows indicate the dorsal direction. Abbreviations as in Figs. 12 and 14.

Figure 15. Articular surfaces of the left knee joint of the captorhinid reptile Labidosaurus hamatus. (A) Proximal surface of tibia; (B) proximal surface of fibula; (C) distal surface of femur; and (D) reconstruction of hyperflexed knee joint to demonstrate position of reconstructed cruciate ligaments. Large arrows indicate the dorsal direction. Abbreviations as in Figs. 12 and 14.

directly lateral), if the crus was positioned at an angle other than 90° to the substrate, or if there was a combination of the two.

Tibial length is approximately half that of the femur in Proterogyrinus and Westlothiana. This measure approaches 65 to 75% in Seymouria and other more derived taxa except for araeoscelidians, where the measures are approximately subequal. Distinguishing muscle scars on crural elements with confidence is extremely difficult. Some taxa have a well developed cnemial crest on the tibia, but its presence may be a function of size of the animal. In most cases, the tibia is more robust than the fibula. Among diadectomorphs plus amniotes, the tibia is conspicuously more robust and longer than the fibula. However, in clearly anamniote batrachosaurs the fibula approaches the tibia in size and is, in fact, longer than the tibia in Proterogyrinus and Westlothiana.

The differential lengths of the tibia and fibula (Fig. 14) have clear implications for the cruro-pedal articulation. The tarsus in Seymouria is poorly known; however, reconstructions for those in

Paleothyris SkeletonProterogyrinus Skeleton

Figure 16. Reconstructions of the left pes in taxa near the origin of amniotes. All are in dorsal view except for B which is in ventral view. (A) The anthracosaurian Proterogyrinus (after Holmes, 1984); (B) Westlothiana (after Smithson et al., 1994); (C) the diadectomorph Diadectes (after Romer and Byrne, 1931; Romer, 1944); (D) the caseosaurian pelycosaur Varanops\ and (E) the protorothyridid reptile Paleothyris (after Carroll, 1969b). All scalebars equal one centimeter. Abbreviations: ast, astragalus; ast-cal, fused astragalus-calcaneus; c, centrale; cal, calcaneus; c,.5; first to fifth centralia; fe, fibulare; i, intermedium; te, tibiale; 1-5, first to fifth digits.

Figure 16. Reconstructions of the left pes in taxa near the origin of amniotes. All are in dorsal view except for B which is in ventral view. (A) The anthracosaurian Proterogyrinus (after Holmes, 1984); (B) Westlothiana (after Smithson et al., 1994); (C) the diadectomorph Diadectes (after Romer and Byrne, 1931; Romer, 1944); (D) the caseosaurian pelycosaur Varanops\ and (E) the protorothyridid reptile Paleothyris (after Carroll, 1969b). All scalebars equal one centimeter. Abbreviations: ast, astragalus; ast-cal, fused astragalus-calcaneus; c, centrale; cal, calcaneus; c,.5; first to fifth centralia; fe, fibulare; i, intermedium; te, tibiale; 1-5, first to fifth digits.

Proterogyrinus and Westlothiana are available (Holmes, 1984; Smithson, et al., 1994). More primitively, the fibula articulates with a distinct fibulare and intermedium. It appears that this might be the case in Seymouria, but poor preservation makes accurate determination difficult. In Proterogyrinus the tibia articulates with the intermedium and an element that Holmes (1984) restored as a fused tibiale plus fourth centrale. Smithson et al. (1994) restored the distal articulation of the tibia in Westlothiana with the intermedium and tibiale only. In no instance is there an articular plane that completely traverses the foot either at the proximal end of the tarsus or within it. The only complete transverse "joint" of the foot is between distal tarsal elements and the metatarsals.

Considerable argument has surrounded the homologies of the "astragalus" in amniotes. Rieppel (1993) suggested that the amniote astragalus is a neomorph that resulted from ontogenetic repatterning and that it is not homologous to the tibiale plus intermedium (Peabody, 1951). As a group very near the origin of amniotes, the Diadectomorpha would be a logical choice for information regarding this question. However, the group provides only confusing or autapomorphic features. The tarsus is poorly known in Limnoscelis. Independent tibiale, intermedium, and fibulare are known but poorly preserved in Tseajaia. In some (Romer, 1944; Rieppel, 1993) but not all (Romer and Byrne, 1931) specimens of Diadectes (Fig. 16) all three elements appear to be fused into a single "astragalo-calcaneal" element, apparently unique to the genus. Difficulties in understanding the homologies of tarsal elements make a diagnosis of the transformations of the ankle joint difficult, allowing a comparison of potential movements but not encouraging speculation on the homologies of joint articulations.

In amniotes in which these elements can be identified with confidence , the proximal tarsus (Figs. 16 and 17) has consolidated as an L-shaped astragalus and an approximately oval calcaneum. The fibula articulates with both the astragulus and the calcaneum, whereas the tibia articulates with the astragalus only and at a level distinct from that of the fibula. Most araeoscelidians have a unique articulation between the astragalus and tibia in which a distinct ridge of the tibia is set firmly into a trough of the astragalus, essentially creating a "locked" tibiotarsal joint (Vaughn, 1955; Reisz, 1981) (Fig. 17). More generally, the interposition of the fourth distal tarsal prevents the distal astragalo-calcaneal surfaces from defining a midtarsal joint in basal amniotes. As in the anamniotes surveyed in this study, the only

Mesotarsal

Figure 17. Articular surfaces of the left pes in (A) the captorhinid reptile Labidosaurus and (B) the araeoscelidian reptile Petrolacosaurus. Both are accompanied by a medial view of the astragalus to demonstrate the unique tongue and groove surface for the locking tibio-talar joint in Petrolacosaurus (after Reisz, 1981). Scalebars equal 1 cm. Abbreviations as in Fig. 16.

Figure 17. Articular surfaces of the left pes in (A) the captorhinid reptile Labidosaurus and (B) the araeoscelidian reptile Petrolacosaurus. Both are accompanied by a medial view of the astragalus to demonstrate the unique tongue and groove surface for the locking tibio-talar joint in Petrolacosaurus (after Reisz, 1981). Scalebars equal 1 cm. Abbreviations as in Fig. 16.

mesotarsal joint appears to be between distal tarsals and metatarsal elements. Although amniotes retain an intrapedal joint, the proximal tarsus may have been a less flexible mosaic of bones. This is taken to an extreme in araeoscelidians.

Digits of the pes are extensions of the flexible foot. As in the manus, the fourth digit is usually longer than the others in most of the taxa analyzed here but not remarkably so. Schaeffer (1941) suggested that the flexible mosaic of the tarsus in basal reptiles was similar to that in salamanders in which the foot is oriented anteriorly and the entire flexible mosaic was used in a progressive propulsion. Brinkman (1980, 1981) reiterated this view in his examinations of extant iguanid lizards. Rewcastle (1981) proposed that the condition of an extremely elongate fourth digit and the resulting mode of locomotion in iguanid lizards was a reasonable model for "primitive reptilian locomotion". It appears that only the more advanced protorothyridids and araeoscelidians exhibit a phalangeal morphology consistent with Rewcastle's (1981) hypothesis. Thus, Rewcastle's model may be correct, but perhaps at a position within the Reptilia as opposed to a condition characteristic of all amniotes.

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