Aardonyx celestae

Otero, Alejandro, Krupandan, Emil, Pol, Diego, Chinsamy, Anusuya & Choiniere, Jonah, 2015, A new basal sauropodiform from South Africa and the phylogenetic relationships of basal sauropodomorphs, Zoological Journal of the Linnean Society 174 (3), pp. 589-634 : 612-614

AARDONYX CELESTAE

The morphology of Sefapanosaurus resembles in general terms that of other sauropodomorphs placed basally to Sauropoda. The diagnostic characters of Sefapanosaurus are discussed here and comparisons with other sauropod outgroups are conducted, with particular emphasis on the features that distinguish the new taxon from Aardonyx , which is also known from the Elliot Formation ( Yates et al., 2010) and to which the specimens described here were referred by McPhee et al. (2014).

Astragalus with triradiate cross-section

The ascending process of the astragalus in basal sauropodomorphs is quite distinctive in being mediolaterally broad. There is some variation within the group in the mediolateral width of the ascending process, and forms closely related to Sauropoda usually have a comparatively narrower process at its proximal end (e.g. Lessemsaurus , Blikanasaurus ). The ascending process of Sefapanosaurus becomes narrow proximally, resembling the condition in Blikanasaurus and the basal sauropods Vulcanodon and Tazoudasaurus . However, a unique feature of the new taxon amongst Sauropodomorpha is that this process has a triradiate cross-section, which extends from the base up to its distal end. When viewed proximally, the proximal tip of the ascending process displays a ‘T’-shape.

Tall ascending process of the astragalus

The ascending process of the astragalus amongst Sauropodomorpha is characterized by being mediolaterally wider than craniocaudally deep. However, its proximodistal height varies, being very low in some taxa (e.g. Massospondyus, Glacialisaurus ) to moderately tall in others (e.g. Coloradisaurus , Euskelosaurus , Blikanasaurus ). Sefapanosaurus has a proximodistally tall ascending process that exceeds in height the range of variation seen amongst basal sauropodomorphs. In this sense, the height of the ascending process is 35% of the mediolateral width of the main body of the astragalus. By contrast, other basal sauropodomorphs consistently have lower values, which range between 25–27% (e.g. Plateosaurus , Coloradisaurus , Adeopapposaurus , Massospondylus , Glacialisaurus , Mussaurus , Blikanasaurus ).

Ascending process of the astragalus framed medially and caudally by thick ridges

The ascending process of most basal sauropodomorphs presents thin medial and lateral margins (e.g. plateosaurids, massospondylids, Sarahsaurus , Mussaurus ). In Sefapanosaurus two noticeable ridges frame the medial and lateral margins of the ascending process of the astragalus. The lateral margin, placed caudally, rises from the caudal margin of the main body of the astragalus and continues through the proximal end of the ascending process. The medial margin rises near the medial end of the astragalar body and suddenly slopes towards the proximal tip of the ascending process. Both structures delimit a caudal fossa of the astragalus. This kind of framing of the ascending process is not reported in any other basal sauropodomorph.

In addition to the autapomorphies present in the astragalus (holotype), four autapomorphies are present in the appendicular material associated with the holotype, which are referred here to S. zastronensis . They are discussed below.

Long ridge extending from the craniodorsal margin

of the coracoid to the coracoid foramen

The coracoid of basal sauropodomorphs is quite conservative in terms of general shape and features, as corroborated by the fact that only two phylogenetically informative characters were used in recent phylogenetic analyses on basal sauropodomorphs (e.g. Upchurch et al., 2007; Yates, 2007a). The general shape of the basal sauropodomorph coracoid is traditionally subovoid, and such morphology is maintained without drastic modifications until Titanosauria, in which the proximodistally length exceeds the length of the scapular articulation and the cranioventral margins become rectangular ( Carballido et al., 2012). In terms of features, the coracoid tubercle, placed cranially to the glenoid region, on the lateral surface, is the only noticeable structure that can be distinguished in several basal sauropodomorph coracoids (i.e. Sarahsaurus , Adeopapposaurus , Coloradisaurus , and sauropods more derived than Tazoudasaurus ), apart from the coracoid foramen.

Sefapanosaurus presents a novel structure, a long ridge on the craniodorsal margin of the coracoid, which points toward the coracoid foramen although it does not reach it. This structure is not reported in other basal sauropodomorphs and is regarded here as an autapomorphy of this taxon. The long ridge of the coracoids is probably the osteological correlate of the muscle biceps brachii, as extant crocodilians display a similar ridge running along the long axis of the coracoids, on the craniolateral surface close to the coracoid foramen ( Meers, 2003).

Distal carpal I with proximally pointing tip on the palmar surface, giving a triangular shape in palmar or dorsal view

Distal carpal elements within basal sauropodomorphs share the same basic morphology: distal carpal I is the largest element (except in Mussaurus , which is subequal to distal carpal II), is subdiscoidal in shape, and covers totally or partially the proximal surface of the first metacarpal. Distal carpal I of Sefapanosaurus shares this general morphology, except for the presence of a notable pointed tip on the palmar margin of the bone, which gives the distal carpal a triangular shape when viewed in dorsal or palmar views. This contrast with the widespread condition amongst most basal sauropodomorphs in which the proximal surface of distal carpal I is uniformly convex to slightly flat (e.g. Plateosaurus , Massospondylus , Adeopapposaurus ). A similar pattern has been reported in Lufengosaurus ( Young, 1941) , although less pronounced than in Sefapanosaurus .

Craniomedial process of the ulna distally tapered and twice as long as the craniolateral process

One of the features that characterizes sauropodomorphs closely related to Sauropoda is the morphology of the proximal ulna, which has an incipient craniolateral process that allows the recognition of a shallow concavity on the ulna for the radius ( Yates & Kitching, 2003; Bonnan & Yates, 2007; Yates et al., 2010; Otero & Pol, 2013). The general morphology of the craniomedial and craniolateral processes amongst sauropod outgroups is relatively uniform, with the former much more developed with a wide proximal base and rounded distal tip, and the latter much less developed with a more acute tip. Although in Sefapanosaurus the craniomedial process is still much more developed than the craniolateral, the shape and development of the processes differ from those of other sauropod outgroups. In this sense, the proximal depth/maximum length ratio of the craniomedial process in Sefapanosaurus is 0.6, whereas this ratio is much higher in other sauropod outgroups (e.g. 1.1 in Mussaurus , 1.2 in Aardonyx , 1 in Melanorosaurus ) and basal sauropods (e.g. 1 in Lessemsaurus , 1.6 in Antetonitrus ). Additionally, the shape of the tip of the craniomedial process in Sefapanosaurus tapers distally, whereas in the rest of the taxa it terminates in a rounded surface. These proportions and shape show that the craniomedial process is long and gracile in Sefapanosaurus , in comparison with a shorter and stouter process in other taxa.

Prominent projection on the craniomedial surface of the distal end of the fibula

With the exception of the lateral tuberosity, present in most basal sauropodomorphs, and the anterior crest present in the fibula of Antetonitrus ( McPhee et al., 2014) , the fibula of basal sauropodomorphs is a gracile, almost featureless bone. The latter is reflected in the scarcity of phylogenetic characters that describe its morphology (only three in the most recent phylogenetic analyses, Otero & Pol, 2013; McPhee et al., 2014). The fibula of Sefapanosaurus displays a medial projection placed at the distal end, a unique feature amongst basal sauropodomorphs, which gives a stout appearance and might be related to the articulation to the distal end of the tibia.

Comparisons with Aardonyx celestae

Although the above-mentioned autapomorphic features allow the recognition of Sefapanosaurus as a distinct species of basal sauropodomorph, in a recent paper ( McPhee et al., 2014), the specimen BP/1/386 was tentatively referred to A. celestae , but without detailed jus- tification being provided. This referral, albeit tentative, prompts a clear justification of the distinction between A. celestae and S. zastronensis . In this regard, Sefapanosaurus can be distinguished from Aardonyx by several features of different skeletal elements, which are discussed below ( Fig. 18 ).

The presence of low, almost inexistent cervical diapophyses, with the concomitant absence of diapophyseal laminae, was regarded as an autapomorphy of Aardonyx ( Yates et al., 2010) . Comparing equivalent elements of both taxa (middle cervicals, considering the height of the neural arches), it can be seen that Sefapanosaurus has a welldeveloped cervical diapophysis, which extends ventrally and exceeds the limit of the neurocentral suture, as in most basal sauropodomorphs. Comparing the same middle elements, the medial wall of the postzygapophysis is almost vertical and extends caudally with a strong convex margin in Aardonyx but in Sefapanosaurus the medial wall is medially slanted and the caudal margin is slightly convex.

The limb bones of the two taxa can also be differentiated. The caudodistal tubercle of the radius (also present in other sauropod outgroups), is less developed in Sefapanosaurus than in Aardonyx , and is proximodistally elongated in the latter. Additionally, the distal articular surface of the radius of Sefapanosaurus is more rounded than in Aardonyx . The ulna is more gracile in Sefapanosaurus , with a proximal surface bearing a craniomedial process that is much longer than the craniolateral process, whereas in Aardonyx the latter is much shorter. The distal end of the ulna is more robust in Aardonyx when viewed in lateral or medial views. Finally, the medial margin of metacarpal I is markedly concave in Sefapanosaurus , whereas only a mild concavity is present in Aardonyx (as well as in Antetonitrus and Lessemsaurus ).

Figures

Figure 18. Comparisons between Sefapanosaurus zastronensis gen. et sp. nov. and Aardonyx celestae. A, middle cervical vertebrae of Sefapanosaurus (BP/1/7409) and Aardonyx (BP/1/6513) in right lateral view depicting diapophysis and postzygapophysis. B, right radius of Sefapanosaurus (BP/1/7435) and Aardonyx (BP/1/6321) depicting the caudodistal tubercle (top) and the distal articular surface (bottom). C, left ulna of Sefapanosaurus (BP/1/7437) and Aardonyx (BP/ 1/5379) depicting the proximal articular surface (top) and the distal end robusticity (bottom). D, left metacarpal I of Sefapanosaurus (BP/1/7438) and Aardonyx (BP/1/5379) depicting the ventromedial concavity. Sefapanosaurus is placed on the left in (A) to (D). Abbreviations: cdt, caudodistal tubercle; di, diapophysis; poz, postzygapophysis; vmc, ventromedial concavity. Not to scale.