Sphenocondor gracilis, Apesteguía & Gómez & Rougier, 2012

Apesteguía, Sebastián, Gómez, Raúl O. & Rougier, Guillermo W., 2012, A basal sphenodontian (Lepidosauria) from the Jurassic of Patagonia: new insights on the phylogeny and biogeography of Gondwanan rhynchocephalians, Zoological Journal of the Linnean Society 166 (2), pp. 342-360 : 345-349

publication ID	https://doi.org/ 10.1111/j.1096-3642.2012.00837.x
persistent identifier	https://treatment.plazi.org/id/B03C87F3-5122-462F-FF7E-6FA2722B0CD1
treatment provided by	Marcus
scientific name	Sphenocondor gracilis
status	sp. nov.

Treatment

SPHENOCONDOR GRACILIS SP. NOV.

Etymology: The specific epithet ‘gracilis’ refers to the slenderness of the jaw.

Holotype: MPEF-PV 2358 View Materials , a single dentary preserved in two slabs as part (A) and counterpart (B).

Locality and horizon: Queso Rallado locality, about 2.3 km north-west of Cerro Cóndor village, 360 km west from Trelew, Chubut Province, Argentina ( Fig. 1 View Figure 1 ). The bearing beds consist of silicified mudstones within series of mudstones and limestones that are part of the Lower Member of the Cañadón Asfalto Formation ( Rougier et al., 2007b) and that were deposited in a lacustrine carbonate environment ( Figari, Courtade & Constantini, 1996). The age of this unit has usually been considered as Callovian ( Proserpio, 1987) or Callovian–Oxfordian (e.g. Tasch & Volkheimer, 1970; Musacchio, Beros & Pujana, 1990; Figari & Courtade, 1993; Page et al., 1999; Silva Nieto et al., 2002; Cabaleri et al., 2010). However, recent radiometric data ( Cuneo & Bowring, 2010) suggest that it might be older (beginning its deposition in the Toarcian). Despite some difficulty in establishing precise correlations between the Queso Rallado Quarry and the dated beds, the age of the fossils is almost certainly Middle Jurassic.

Diagnosis: Small sphenodontian rhynchocephalian differing from all other rhynchocephalians in having the following combination of features: slender lower jaw with a low and squared coronoid process; dentary posterior process as long as the base of the coronoid process; at least two strongly recurved and profusely striated successional teeth with no separation between them (differing from Theretairus and Sphenovipera ) and with an anterior flange marked by a wide canal; unadorned additional teeth; two dentine types organized in a radiating pattern; symphysis anterodorsally projected; adductor fossa centred under the coronoid process; and alternate hatchling dentition organized in three groups of different size and showing a slight basal constriction. These last three characters constitute autapomorphic features.

DESCRIPTION AND TAXONOMIC COMPARISONS

Dentary

The lower jaw is very slender and, as in basal forms (e.g. Diphydontosaurus avonis Whiteside, 1986 ), the dentary represents about 90% of the total length of the jaw. The height of the dentary varies along the jaw ( Table 1), from 2 mm in the precoronoid region to 3.72 mm at the level of the coronoid process. The dental margin of the dentary is about 15.9 mm from the symphysis to the anterior part of the coronoid process, and can be divided into successional, hatchling, and additional regions, based on the dentition ( Fig. 2 View Figure 2 ). The presence of a deep and wide Meckelian groove on the medial side of the mandible is only evident in the posterior part of slab B ( Fig. 2B View Figure 2 ). The lateral surface of the dentary, as preserved in slab A ( Fig. 2A View Figure 2 ), shows development of secondary bone (secondary dentine of Fraser, 1986) below the tooth row (at least on the anterior portion of the dentary). This feature has been regarded as a character of derived rhynchocephalians, being absent in basal taxa such Measurements Value Total length of dentary 24.96 Total length of jaw (estimated) 26.4 Maximum height of jaw at symphysis 2.5 Minimum height of jaw (anteriorly) 1.42 Height of jaw at coronoid process 3.72 Length of coronoid process at base 4.53 Length of coronoid process at top 3.07 Height of coronoid process 1.73 Length of posterior process 5.29 Height of last successional tooth 0.54 Length of adductor fossa 5.82 Height of adductor fossa 1.18 as Gephyrosaurus and Diphydontosaurus ( Fraser, 1988; Jones, 2006; Jones et al., 2009).

The ventral margin of the dentary is rather straight as in basal rhynchocephalians and some derived forms such as Cynosphenodon huizachalensis Reynoso, 1996 and ‘Sphenodontid B’ from the Early Cretaceous beds of Anoual, Morocco ( Evans & Sigogneau-Russell, 1997).

The symphyseal region is badly damaged, but a large and pointed fragment is evident in slab A ( Fig. 2A View Figure 2 ). This fragment might represent part of a large successional tooth or a symphyseal spur (crowned or not by a symphyseal tooth), although its poor preservation prevents us from confirming its identity. A symphyseal spur occurs in Sphenodon , Sphenovipera , Cynosphenodon , Rebbanasaurus ( Evans et al., 2001; Reynoso, 2005; Fig. 4F View Figure 4 ), the Kirtlington sphenodontid, and the sphenodontians LACM 135616 and LACM 1335531 from the Morrison Formation (S. A., pers. observ.). The region that often bears a mental process is not preserved.

The coronoid process is low and squared, more developed than in Diphydontosaurus and similar in height to those of Planocephalosaurus ( Fraser, 1982) , Sphenodon , and eilenodontines (including Kaikaifilusaurus ; Apesteguía & Novas, 2003; Apesteguía, 2008). In this aspect, it clearly differs from Clevosaurus ( Fraser, 1988) and Ankylosphenodon ( Reynoso, 2000) . The coronoid bone is not preserved, but the posterior margin of the dentary bears the notch that represents the anterior margin of the mandibular foramen, commonly shared with the surangular.

In Sphenocondor the adductor fossa is longer than high, eye-shaped, and centred ventral to the coronoid process ( Figs 2 View Figure 2 , 3A View Figure 3 ); it differs from that of most sphenodontians, which is located posterior to the level of the coronoid tip. In Sphenovipera ( Reynoso, 2005) the adductor fossa is also located ventral to the coronoid process, but differs from that of Sphenocondor in being oblique rather than horizontal.

The posterior process of the dentary is broken and slightly out of place, but it is rather well preserved and the missing part left a clear mould indicating its size and shape. The process is very long, as much as the long coronoid base. No postdentary bones have been preserved.

Teeth

There are around 20 preserved teeth in the holotype of Sphenocondor ; all of them are acrodont and more or less conical in shape ( Fig. 2 View Figure 2 ). As in most sphenodontians, there are represented several dental generations arranged in an anterior-to-posterior sequence, comprising: (1) a successional dentition encompassing at least three teeth (probably four); (2) an alternating hatchling series composed of 15–16 teeth; (3) an additional generation composed of three large teeth uniform in size and shape ( Fig. 2 View Figure 2 ).

The anterior-most region is not well preserved. There is a large separated fragment of bone that could be considered as part of the anteromedial symphyseal spur ( Figs 2 View Figure 2 , 4A View Figure 4 ), or part of a large successional tooth. However, the preservation does not allow certain identification. Behind it follow two strongly recurved and profusely striated successional teeth, the second much taller than the first. They are labiolingually compressed and bear on the labial side a groove or canal that delimits an anterior flange. This trait is also present in juvenile specimens of Godavarisaurus ( Evans et al., 2001; Fig. 4B View Figure 4 ), which also share the general shape and relative position of the pieces. More than one successional tooth in adults is also present in other sphenodontians ( Table 2), such as Theretairus ( Simpson, 1926) and Sphenovipera ( Reynoso, 2005) . However, they differ from Sphenocondor in having ‘caniniforms’ widely separated and rounded in crosssection. The presence in Sphenocondor of anterior flanges in the successional teeth probably represents a derived feature shared with Godavarisaurus .

Posterior to these successional teeth, after a short diastema, begins the hatchling series. The latter teeth are alternated in size, as characteristic for hatchling dentition ( Robinson, 1976), and can be divided into three groups according to size ( Fig. 2 View Figure 2 ). The anterior group comprises four smooth, small teeth that are almost completely worn down and might have occluded with the maxillary successionals. The second group is composed of five or six well-preserved, conical teeth that are intermediate in size, about twofold the anterior hatchling pieces, and bear sharp cusps relatively unworn. These hatchling teeth might have matched the anterior-most hatchling teeth on the maxilla, explaining the difference in wear with ht, hatchling teeth.

respect to the preceding hatchling teeth. The third and most posterior hatchling group comprises six high, conical teeth with rounded walls and devoid of striations. The latter are clearly larger and less worn than the other hatchling teeth, possibly indicating that these larger teeth belong to a younger hatchling generation that developed as the dentary bone grew during early ontogeny. The relative height of these posterior hatchling teeth is similar to those of Sphenodontian B ( Evans & Sigogneau-Russell, 1997), with respect to the dentary height below them. The aforementioned size variation in the alternating hatchling series with teeth becoming larger posteriorly is also present in other sphenodontian taxa, such as Planocephalosaurus ( Fraser, 1982) .

As in other sphenodontians, which are typified by the addition of acrodont teeth at the rear of the jaw that are not replaced during ontogeny ( Robinson, 1976), at the posterior end of the tooth line of Sphenocondor there is an additional series made up of three badly preserved teeth exposed in labial view; this preservation prevents us from identifying flanges on the lingual surface of these teeth. These additional teeth are conical and sharp, with their bases being about twice as long as the bases of the posterior hatchling teeth. At least on the labial surface, Sphenocondor lacks well-developed flanges on the additional teeth, thus differing from additional teeth in many derived sphenodontians (e.g. Tingitana , Clevosaurus , Homoeosaurus , Kallimodon , Sphenodon ). They resemble some of the teeth of Planocephalosaurus , and some of the additional teeth of Godavarisaurus .

The natural breakage in two slabs running diagonally along the jaw allowed us to study the lateral side of the anterior region, the medial side of the posterior region, and the sagittal inner structure of the mid region ( Figs 2 View Figure 2 , 3 View Figure 3 ). The posterior hatchling teeth allow the inspection of their respective pulpar cavities ( Fig. 3B, D View Figure 3 ). As the more anterior tooth is not sagittally broken, only part of the pulpar cavity is visible, differing from the following, which sustained a sagittal break. The pulpar cavities are wide in both cases, expanded in the interior of every tooth and narrowing downwards, but remaining open. The pulpar cavities join each other before reaching the upper margin of the Meckelian canal, establishing an oblique ventral connection.

The fifth posterior hatchling tooth is the best to study the inner structure of the teeth ( Fig. 6B View Figure 6 ) because the pulpar cavity of this bulbous tooth is fully exposed by a perfect sagittal section and devoid of further damage. The outer layer of the tooth is formed by a uniform thick layer of enamel that covers the entire dental piece. Under the enamel there is a thick cover of dentine composed by two morphological types. The upper part of the dentine is represented by a hyaline dentine that is thicker in the top of the tooth crown. The lower part of the dentine is thick and whitish. A conspicuous odontoblastic line runs from the enamel base to the upper part of the pulp across the dentine layers. Additionally, numerous radiating lines, the odontoblast prolongations, run across the dentine layer. The pulp cavity is rhomboid in shape and increases its width downwards to reach a maximum at the dorsal margin of the dentary, where it narrows abruptly, separating the main pulp cavity from the dorsal margin of the Meckelian groove.