The sternum and interclavicle of Aelurognathus tigriceps (Broom & Haughton, 1913) (Therapsida: Gorgonopsia), with comments on sternal evolution in therapsids Author Sidor, Christian A. Department of Biology and Burke Museum, University of Washington, P. O. Box 351800, Seattle, WA 98195 - 1800 (United States of America) casidor @ uw. edu (corresponding author) casidor@uw.edu Author Mann, Arjan Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, MRC 121, P. O. Box 37012, Washington, D. C. 20013 - 7012 (United States of America) & Negaunee Integrative Research Center, Field Museum of Natural History, 1400 S Lake Shore Dr, Chicago, IL 60605 (United States of America) text Comptes Rendus Palevol 2024 2024-02-08 23 6 85 93 http://dx.doi.org/10.5852/cr-palevol2024v23a6 journal article 10.5852/cr-palevol2024v23a6 1777-571X 14204330 urn:lsid:zoobank.org:pub:EE66EC81-1BC3-4BA1-9E70-81D6F9FCAC59 Aelurognathus tigriceps ( Broom & Haughton, 1913 ) Scymnognathus tigriceps ( Broom & Haughton, 1913 ) : 26. REFERRED MATERIAL . — NHCC LB350 , a scattered partial skeleton of a large individual preserving the preorbital portion of the skull and lower jaws, both humeri, the proximal half of the right femur, the posterior portion of the interclavicle, and sternum ( Table 1 ). Referral to A. tigriceps is based on the presence of five to six tightly packed maxillary postcanine teeth, a tall and bulbous snout, massive dentary symphysis, and a lower level of pachyostosis around the antorbital and jugal compared to other rubidgeines ( Kammerer 2016 ) . HORIZON AND LOCALITY . — Collected from locality L1, upper Madumabisa Mudstone Formation, Luangwa Basin. This locality is approximately 13 kilometers south-southeast of Mulilo ( Muchinga Province ) and corresponds to locality 4 of Drysdall & Kitching (1963) . Vertebrate fossils from the upper Madumabisa Mudstone Formation generally imply a Lopingian age ( Angielczyk et al. 2014 ), but a more precise correlation for this locality will require the systematic review of historical specimens, given the updated biostratigraphic subdivisions recently proposed for the middle of the basin ( Peecook et al. 2020 ). Detailed locality information is available from the NHCC or by contacting C.A.S. FIG . 1. — The sternum and interclavicle of Aelurognathus tigriceps ( Broom & Haughton,1913 ) (NHCC LB350). Specimen in: A , ventral view; B , dorsal view; C , posterior view; D , left lateral view. Anterior is to the top of the page in A and B , and to the left in D . Dorsal is to the top of the page in C . Abbreviations: bm , possible bite mark; c1 , facet for articulation of first rib; c2 , facet for articulation of second rib; c3 , facet for articulation of third rib; fs , facet for articulation of first sternebra; ic , interclavicle; k , weathered ventral keel; st , sternum. Scale bar: 2 cm. DESCRIPTION Interclavicle Figure 1A shows the sternum and interclavicle of NHCC LB 350 in ventral view.The interclavicle lies superficial to the sternum, and has been displaced slightly anterolaterally as evidenced by a shallow impression on the ventral surface of the sternum that indicates their contact surface in life. The caudal portion of the interclavicle is relatively flat, with its lateral margins tapering posteriorly to form a blunted V-shape. Interestingly, this shape contrasts the condition depicted for Aelurognathus Haughton, 1924 byBroom (1930: fig. 2), where the posterior margin of the interclavicle was indicated as being broadly curved. Based on the impression on the sternum, we believe the posterior end of the interclavicle would have been more pointed in life, as shown in the reconstruction of this element ( Fig. 2A ). Anteriorly, the interclavicle in NHCC LB350 tapers towards its “neck”, which separates the anterior and posterior portions of the bone and features the base of an elongate ventral keel, unfortunately weathered off to a large extent in the current specimen. However, the outline of the keel shows that its lateral margins were subparallel and it had a slightly broader anterior end. The right flank of the interclavicle is dislocated and separated from the rest of the interclavicle by a long, matrix-filled longitudinal crack. Other damage includes several oval to irregularly shaped marks scattered across both sides of the interclavicle and sternum that we interpret as bite marks, likely due to scavenging. The ventral surface of the interclavicle shows a pattern of very fine radiating lines that diverge posteriorly. Its dorsal surface ( Fig. 1B ) is mostly obscured by matrix and a fragmentary right posterior coracoid, but based on what is seen in cf. Arctops sp. (NHCC LB396) and other gorgonopsians, one might expect more substantial striations to be present on the interclavicle where it contacted the sternum. TABLE 1. — Representative measurements from the associated skeleton of Aelurognathus tigriceps ( Broom & Haughton, 1913 ) (NHCC LB350). All measurements are in millimeters. Symbols: ‡ , estimated; * , measurement taken between proximal and distal articulation surfaces.

Skeleton	Mesurements
Skull preorbital length	167‡
Humerus (left)
length*	211
width across distal epicondyles	97
minimum midshaft diameter	22
Humerus (right)
length*	214
width across distal epicondyles	112
minimum midshaft diameter	23
Femur
midshaft diameter, transverse	37
midshaft diameter, anteroposterior	22
head transverse width	57

Sternum The sternum of NHCC LB350 measures 103 mm in maximum anterior-posterior length and 82 mm transversely.Its outline is best observed in dorsal view ( Fig. 1B ), where it is shield-like, with a broad anterior portion that tapers, albeit irregularly, posteriorly. Most of the dorsal surface of the sternum forms a relatively shallow concave, or dish-shaped, surface. In lateral view ( Fig. 1D ), the ventral surface of its rib-bearing portion is angled slightly ventrally, which gives the sternum a slightly concave ventral surface when viewed from the side. As preserved, the element is not perfectly symmetrical, with most asymmetry concentrated about the outline of the costal facets. The anterior margin of the sternum is oriented transversely and is slightly thickened at the midline. This thickening continues onto the dorsal surface of the sternum for a short distance, but then flattens out both posteriorly and laterally. Overall, the element is thin dorsoventrally near its center ( c. 6-8 mm ), but is thickened to varying degrees around its perimeter, with the thickest area occurring posteriorly ( 16 mm ). In dorsal view, the anterolateral corners of the sternum are slightly raised and lack finished bone surface. Several authors have suggested that these corners likely contacted the coracoids in life ( Broom 1930 ; Colbert 1948 ; Sigogneau-Russell 1989 ). Three well-defined costal articulations are present on each side of the caudal half of the sternum, with those on the left side being better preserved ( Fig. 1D ). The first costal facet is the smallest and is positioned at the boundary between the broad anterior portion of the sternum and where it begins to taper. On both sides, the second costal facet has a pocket of unfinished bone adjacent to it, suggesting this is a natural feature. The third costal facet is the best ossified and saddleshaped, with a parallelogram-like outline. The anatomy preserved suggests that all three were mobile joints, not synarthroses. In dorsal or ventral views ( Fig. 1A, B ), a semicircular notch is present just posterior to each costal facet, with the anterior two having a sloping surface of finished bone, such that their ventral margin is lateral to their dorsal margin. The third notch is a deep, roughly triangular fossa of unfinished bone with a raised rim ( Fig. 1C ), unlike the preceding two notches that blend into the dorsal surface of the sternum. Bendel et al. (2022) suggested that the third rib articulated on a facet shared between the sternum and the first sternebra in Gorgonops Owen, 1876 , which is the arrangement seen in modern mammals and would make sense based on developmental studies ( Chen 1953 ). However, a corresponding inference in NHCC LB350 seems difficult given the wide separation of the third costal facet from the relatively narrow and posteriorly-facing sternebral articulation surface. In other words, the first sternebra would need to be very wide in order for it to share the third rib articulation with the sternum. At the caudal end of the sternum, the articulation for the anteriormost sternebra has a complex shape. In ventral view ( Fig. 1A ), it can be seen to be formed by two faces that meet at a very high angle. In posterior view ( Fig. 1C ), the articulation surface as a whole is subrectangular in outline, with a rounded ventral margin, but each face is gently convex. There is no indication of a distinct articular surface for a costal cartilage, as implied by the reconstruction of Gorgonops by Bendel et al. (2022 : fig. 2B). Broom (1930) inferred the presence of an additional element posterior to the sternum in his reconstruction of the pectoral girdle of Aelurognathus , and the prominent thickness of this region in NHCC LB350 supports his deduction. The sternum of non-mammalian therapsids is often shown in ventral view, as it represents the external surface of the element, but this view affords relatively less anatomical detail than the dorsal view because the rib articulation surfaces generally point posterodorsolaterally. However, the semicircular notches posterior to each facet can remain prominent in ventral view. In NHCC LB350, the ventral surface of the sternum bears fine lines that radiate out from the center of the element. In addition, a striking feature is the well-developed anteroventral depression for the articulation of the interclavicle, which seems to have escaped notice in the gorgonopsian literature, if it is indeed a widespread feature. The surface of this depression is smooth, at least in the area where it is not covered by the interclavicle. On the left side, from the first costal articulation to the anterolateral corner, the sternum is fractured and displaced slightly dorsally. A gentle longitudinal ridge connects the depression for the interclavicle to the sternebral articulation along the midline. To either side of this ridge, the ventral surface of the sternum is relatively flat, but forms a thickened rim adjacent to the costal facets.