Cynognathus, Seeley, 1895

Liu, Jun & Powell, Jaime, 2009, Osteology of Andescynodon (Cynodontia: Traversodontidae) from the Middle Triassic of Argentina, American Museum Novitates 3674, pp. 1-20 : 13-17

CLAVICLE: A pair of isolated clavicles is known in PVL 3894, and both clavicles are intact apart from slight damage to their ends ( fig. 12 ). The morphology is consistent with the general pattern in other cynodonts. The well developed as in Cynognathus , Pascualgnathus , and Luangwa , especially in PVL 4426.

ULNA: One right ulna is present in PVL 3890 ( fig. 13E ), measuring 40 mm in height and about 12 mm in proximal width. The shape is the same as in Pascualgnathus . The articular surface for the humerus is only a very slightly concave facet, which is even less developed than in Massetognathus . The ulnar crest on the anteromedial margin is poorly preserved.

medial ramus of each clavicle is much longer than the lateral ramus and intersects with it at an angle of about 130 °. The anterior edge of the medial end is not well preserved, but is clearly shorter than the posterior edge. The area of contact with interclavicle lies on the dorsal surface of the medial end, and this part of the clavicle bears visible striations. The clavicular flange retroverts slightly posteriorly from the lateral shaft, extending ventrad to the transitional place between the medial side and lateral side.

FORELIMB

HUMERUS: This bone is preserved in many specimens: PVL 3890 ( fig. 13A–D ), PVL 3894, PVL 4424–4427. Among these specimens, the length of the humerus ranges approximately from 30 mm to 43 mm. The humerus resembles that of Thrinaxodon and most other traversodontids in its overall shape. The maximum width across the epicondyles is around half the humeral length.

Although the humerus is small, the extremities including the head and capitulum are well ossified. The ectepicondyle is a broad swelling on the distolateral corner of the humerus, and is thicker than the entepicondyle. Both an ectepicondylar foramen and an entepicondylar foramen are present, although the margin of the former is usually broken. The trochlea is

PELVIC GIRDLE

A single ilium is known in PVL 4390, and another in PVL 3890 ( figs.10A , 14A ). Two incomplete pelvic girdles are also known ( fig. 14B, C ). PVL 3894 includes an ilium, a nearly complete ischium, and an incomplete pubis exposed from the left side, whereas PVL 6226 comprises an ilium, an incomplete ischium, and a pubis exposed from the right side.

ILIUM: The basic shape of this bone is similar to that seen in other traversodontids such as Luangwa ( Kemp, 1980) , Pascualgnathus ( Bonaparte, 1966) , and Massetognathus ( Jenkins, 1970) . The iliac blade is elongate anteroposteriorly, its length exceeding its height. The anterior process forms a convex flange, whose length anterior to the acetabulum is less than the acetabular height but greater than the anteroposterior diameter of the acetabulum. The height of the anterior process varies among specimens, PVL 3890 representing an extreme case in which the proportional height of the anterior process is greater than in other specimens and its anterior tip more obtuse. The ventral edge of the anterior process is relatively straight, and runs nearly parallel to the thin and sharp dorsal edge of the iliac plate. In lateral view, the anterior portion of the dorsal edge is convex in PVL 3890 and PVL 4390 but relatively straight in PVL 3894, whereas the posterior part is nearly horizontal ( fig. 10A ), slightly concave ( fig. 14B, C ) or distinctly concave ( fig. 14A ). The posterior process is low and short in PVL 4390 ( fig. 10A ), but much longer in other specimens. In PVL 3890 the length of the posterior process even exceeds that of the anterior process. The ventral edge of the posterior process is nearly straight, as in Massetognathus , and different from the concave shape seen in Luangwa and Pascualgnathus . The ventral edges of the anterior and posterior processes converge at an angle of 130 ° in most specimens, similar to the condition in Pascualgnathus , but this angle is wider (,140 °) in PVL 3894. The lateral aspect of the blade is concave, but only slightly so in PVL 3890. The base of the ilium is a narrow neck with a breadth of less than 1 cm in PVL 3890 and 3894, a distance corresponding to less than one-quarter the length of the blade.

Some of the differences in the available four specimens mentioned above could be due to incompleteness or postmortem distortion. The posterior process obviously is incomplete in PVL 3894 and may also be missing its tip in PVL 6226 ( fig. 14C ), as the preserved shape of the end of the posterior process in this specimen is a bluntly rounded point that differs substantially from the sharp point seen in traversodontids such as Pascualgnathus ( Bonaparte, 1966) , Massetognathus ( Jenkins, 1970) , and Luangwa ( Kemp, 1980) . If the missing part is restored to reflect the morphology of these taxa, the length of the posterior process is close to that of the anterior process.

Differences in the curvature of the upper margin could also be explained in part by incomplete preservation. Furthermore, as Jenkins (1970) mentioned with respect to African cynodonts, a cartilaginous suprailiac blade may have been present, so that variations in the shape of the fossilized blade may at least partly reflect differences in ossification. This is particularly likely in the subadult specimen PVL 4390, in which the iliac blade appears complete, but the posterior process is nevertheless very short.

The shape of the ilium of PVL 3890 ( fig. 10A ) is strongly distinguished by the high anterior process, which is more similar to Cynognathus than other traversodontids. Because the morphology of this bone is so different from the condition in other available specimens, it is reasonable to ask whether the ilium of PVL 3890 actually pertains to a different species. However, the only known another cynodont species from the same formation is a trithelodontid, Cromptodon mamiferoides , and the ilium of trithelodontids is characterized by the complete lack of a posterior process. Accordingly, neither the ilium of PVL 3890 nor any of the other, more typical ilia described in this study can possibly be assigned to Cromptodon . Furthermore, both morphotypes are associated with definite skulls of Andescynodon . Accordingly, we tentatively accept both iliac morphotypes as pertaining to A. mendozensis , and attribute the difference between the morphotypes to intraspecific variation.

PUBIS: The distal end of the pubis is not preserved ( fig. 14B, C ). The preserved part of the pubis has the same basic shape as in early cynodonts, and looks very similar to the pubis of Pascualgnathus . The constrained neck is distinct below the pubic head, while the pubic plate is less anteroposteriorly expanded. The anterior edge of the pubis is reflected laterally, achieving an anterolateral orientation. The proximal part of the pubic plate forms a concave lateral surface. The pubis extends ventrally and slightly medially from the acetabular region.

The exact orientation of the pelvic girdle is difficult to establish from the isolated bones. If the ventral edge of the posterior process of the iliac blade is assumed to be nearly horizontal, the pubis lies entirely anterior to the acetabulum and is directed slightly anteriorly.

In Luangwa, Kemp (1980) interpreted an unossified area between the pubis and ilium, on the anterior edge of the pelvis, as indicative of the presence of an epipubic element. This part of the pubis is completely ossified in the specimens described in the present study, so there is no evidence for an epipubic element in Andescynodon .

ISCHIUM: The ischium is nearly complete in PVL 3894 ( fig. 14B ). This bone comprises a head, which contributes to the acetabulum, and a ventromedially and posteriorly directed plate. The ischial ridge is not well developed on the dorsal side of the plate, but a groove formed by the concavity of the dorsal side extends from the head and terminates on the ischial tuberosity ( fig. 14B ). The mediolateral width of the dorsal side of the plate decreases from the head toward the tuberosity. The anterior part of the ischium, which meets the posterior edge of the pubic plate, is only partly preserved. The part of the anterior edge that contributes to the obturator foramen is smooth and concave. The posterior edge is continuous with the ischial symphysis as a convex border.

The sutural relationships among the three pelvic bones are unclear in the acetabulum, where these bones form a unified hemispherical socket.

HINDLIMB

FEMUR: Only two left femora are known from PVL 3890 and PVL 3894 . The femur of the latter specimen is better preserved, and is described below ( fig. 15 ). The femur is 43 mm in height. The narrowest point on the slender shaft is less than 5 mm in diameter. The proximal end is expanded .

The femoral head is deflected medially and anterodorsally, and is offset conspicuously from the axis of the shaft. The trochanter major is separated from the head by a shallow groove. A round intertrochanteric fossa (adductor fossa of Kemp, 1980) lies between the head and the trochanter major on the ventral surface of the femur. The trochanter minor is a prominent, medially deflected flange that can be clearly seen even in dorsal view, as is also the case in PVL 3890. The distal end of the femur of PVL 3890 expands gradually, with a width of 11 mm. The lateral and medial condyles are not preserved.

Figures

Fig. 12. PVL 3894, right clavicle in A, anterodorsal; and B, posterior and slightly ventral views; left clavicle in C, anterodorsal; and D, posterior and slightly ventral views. Abbreviations as in text.

Fig. 13. PVL 3894, left humerus in A, dorsal; B, ventral; C, anterior; and D, posterior views; PVL 3890, E, right ulna in lateral view. Abbreviations as in text.

Fig. 10. PVL 4390. A, sacral region in dorsal view; third sacral vertebra and associated ribs in B, dorsal, C, ventral, D anterior, and E, posterior views. Abbreviations as in text.

Fig. 14. Pelvis in lateral view. A, PVL 3890, left ilium; B, PVL 3894, left pelvis; C, PVL 6226, right pelvis. Abbreviations as in text.

Fig. 15. PVL 3894, left femur in A, anterior; and B, posterior views. Abbreviations as in text.