A tiny new Middle Triassic stem-lepidosauromorph from Germany: implications for the early evolution of lepidosauromorphs and the Vellberg fauna

The Middle Triassic was a time of major changes in tetrapod faunas worldwide, but the fossil record for this interval is largely obscure for terrestrial faunas. This poses a severe limitation to our understanding on the earliest stages of diversification of lineages representing some of the most diverse faunas in the world today, such as lepidosauromorphs (e.g., lizards and tuataras). Here, we report a tiny new lepidosauromorph from the Middle Triassic from Vellberg (Germany), which combines a mosaic of features from both early evolving squamates and rhynchocephalians, such as the simultaneous occurrence of a splenial bone and partial development of acrodonty. Phylogenetic analyses applying different optimality criteria, and combined morphological and molecular data, consistently recover the new taxon as a stem-lepidosauromorph, implying stem-lepidosauromorph species coinhabited areas comprising today’s central Europe at the same time as the earliest known rhynchocephalians and squamates. It further demonstrates a more complex evolutionary scenario for dental evolution in early lepidosauromorphs, with independent acquisitions of acrodonty early in their evolutionary history. The small size of most terrestrial vertebrates from Vellberg is conspicuous, contrasting to younger Triassic deposits worldwide, but comparable to Early Triassic faunas, suggesting a potential long-lasting Lilliput effect in this fauna.

from the maxilla, preserving two straight tooth and the cross-section of a third one. It remains unclear whether the premaxilla is straight or inclined anteroventrally, or whether a premaxillary nasal process was present and how far posteriorly it could have reached.
The left maxilla is preserved in lateral view and in articulation with the remaining of the skull. It is broken in two pieces by the prefrontal, and the dorsal (=facial, nasal) process is damaged. It is very long, reaching as far posteriorly as the posterior end of the orbit, and forming most of the ventral margin of the orbit laterally. The right maxilla was displaced to the left side of the skull, but it is still visible in medial view, with the supradental shelf visible on the exposed surface. It shows an incomplete premaxillary ramus and a low dorsal process. The displaced right maxilla has most of the dorsal process region preserved, indicating that this process was relatively low compared to that of most diapsid reptiles. The low height of the dorsal process can also be inferred based on the low angle between the dorsal margin of the premaxillary process of the maxilla and the preserved portion of the dorsal (= facial) process of the preserved portions of the left maxilla. Additionally, the dorsal process is located at the mid-length of the maxilla. This low dorsal process of the maxilla is similar to the condition observed in early rhynchocephalians, such as Gephyrosaurus, but different from the early lepidosaur Sophineta, and the stem squamate Megachirella (Simões et al. 2018;TRS, pers. obs.) The maxillae bear teeth up to the level of the main body of the jugal. It could have accommodated up to 35 teeth, and their morphology and size are rather uniform. The maxillary tooth crowns reach less than one third of the height of the maxillary ramus, and there is no maxillary lingual wall ( Fig. 2A). There are no visible interdental ridges separating the teeth, although we note that this particular region is of more difficult observation due to the low resolution of the CT scans. There is no detectable ankylosis of the dental tissues to the jaw bones, and the enamel is smooth and without any signs of plicidentine.
The left nasal is preserved as a slender, sinuous, L-shaped element that is longer than wide, and it is preserved in dorsal view. A ventrolateral process is present at its posterior end, as also 5 occurring in most diapsid reptiles, extending in a right angle from the main portion of the bone.
The contact with the frontal is straight. The nasal is about 30% shorter than the right frontal.
The prefrontal contributes to the anterior, as well as a small part of the dorsal, margins of the orbit (Fig. 1, S2a). It extended anteriorly, as indicated by its corresponding articulation facet on the frontals ( Fig. 1A; prf.af), but this extension has not been preserved. The preserved part of the prefrontal is a dorsoventrally deep and gently, posteriorly curved bar of bone (Fig. 3C, D). Its ventralmost part is hidden within the matrix, medial to the maxilla and extending ventrally relative to it (Fig. 3D). An unindentified canal extends dorsoventrally within most of the bone (Figs. 3D, S2b).
The paired frontals are slender elements at their mid-length, and they are almost twice as wide along the anterior margin between the anterolateral processes, and about 1.5 times wider at their posterior margins. The preserved right anterolateral process is short and rounded at its anterior end, resembling in overall morphology the anterolateral processes observed in early diapsid reptiles (Schoch & Sues 2018;Sobral et al. 2015). The posterolateral ends of the frontals diverge from each other at about 20° from the midline. Articular surfaces for the pre-and postfrontal can be found on the lateral margin of the left frontal ( Fig. 1b; prf.af, pof.af). The frontals formed the mid-dorsal border of the orbit. On the ventral side, the subolfactory processes (crista cranii) are more developed than in Fraxinisaura (Fig. S3).
The left postfrontal is well-preserved, but slightly displaced distally relative to the left frontal. It is an unusual element in that it is very stout and markedly T-shaped, with a ventral process that is twice as long as the other two. The angle formed between the anterior and ventral processes is slightly greater than 90°, and the angle between the ventral and posterior processes is ca. 90°. This triradiate postfrontal with a long ventral process resembles the stem-squamate Marmoretta (Evans 1991) and the hupehsuchian Nanchangosaurus (Chen et al. 2014).
The postorbital is mostly preserved within the matrix and not externally visible by the naked eye, thus its anatomical interpretation comes from the CT scans. It is similar to the postorbital of many other diapsids in which the ventral process is very long and thin, whereas the dorsal process is more robust and somewhat shorter (Fig. 2C). The extent of the posterior process cannot be assessed with certainty as it is incompletely preserved, but it is robust and at least slightly longer than the ventral one. Perhaps due to the methacrylate resin applied to the specimen during its preparation, under the microscope only the facet for the jugal is visible, but segmentation of this element using CT scan data also shows a faint semilunar orbital ornamentation on the main body of the bone.
The left jugal is completely preserved, except for the posteriormost tip of the posteroventral process. The exposed portion of the specimen gives the impression the jugal is a short and stout element, but CT scans show that the distalmost tips of the anterior and dorsal processes lie within the matrix (Fig. 3A, B), underneath the maxilla and the postorbital, respectively. They are thus longer than they appear on the exposed portion of the matrix. The dorsal process is inclined at about 120° degrees in relation to the anterior process and measures 2,95mm (Fig. 3D). The anterior process is slightly longer than the dorsal one, reaching at about the mid-length of the orbit anteriorly. The jugal of Vellbergia resembles the one in other diapsid reptiles with a relatively reduced posteroventral process, such as Pappochelys (Schoch & Sues 2018) and Marmoretta (TRS. pers. obs.).
The squamosal is preserved within the matrix (Fig. 2D), posteriorly and medially to the postorbital. Only the process for contact with the parietal is preserved, so it is unclear whether it contains a ventral process, how long it would be or if it contacted the dorsal process of the jugal.
Posterior to it, a rounded ridge indicates an elevated area which could have separated the quadrate facet from the anterior border of the bone. Posterior to this elevation, a small foramen is visible, and which we consider to not represent an artefact of preservation, as some lepidosauromorph squamosals have foramina in this area; in particular, the squamosal of Diphydontosaurus has a foramen in this exact position (Whiteside 1986).
In the posterior cheek region, there are two other elements that could be interpreted as being a single broken unit representing the quadrate and perhaps also the quadratojugal (Fig. 1). The most dorsal of these elements is comprised of an articulatory head that may be missing its anteriormost portion and a long, ventrally extending shaft, which faced laterally. The preserved portion of the ventral element contains a short shaft. The base of the ventral element has a visible and well-developed mandibular condyle. CT scans show no evidence for a quadrate foramen or a quadratojugal fenestra. Further, the medial (pterygoid) process cannot be observed in the current available material. It is impossible to safely delimit which portions of the preserved elements belong to the quadrate or to the quadratojugal separately.
The parietals are not entirely preserved, with only the posterior part of the right parietal being visible in ventral view and located dorsally to the squamosal. It appears T-shaped, with a modest and laterally directed supratemporal (=posterior) process. The posterior margin between the supratemporal processes is relatively straight, as also observed in the early lepidosaur Sophineta, many later evolving lepidosaurs, and at least some early diapsids, such as Wumengosaurus (Wu et al. 2011) and Pappochelys (Schoch & Sues 2018). The anterior end is relatively narrow and The lower jaw is approximately 12.5mm long, with the postdentary region somewhat short.
The left ramus is preserved in ventrolateral view, while the right ramus is more ventrally exposed.
The posterior third of the lower jaw is twice as tall as the anterior two thirds. Unfortunately, the posterior end of the left ramus conceals the right one, precluding the observation of the details of the anatomy of the adductor and glenoid fossae. The outline of the adductor fossa, however, can be seen in the CT scans (Fig. S4).

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The dentary bears at least 26 teeth that are similar in morphology to the maxillary ones, albeit shorter and more peg-like. Tooth implantation of the anterior dentary teeth is the same as the one observed on the maxillary dentition, with the teeth located lingually to the lateral wall of the jaw bone, with no traces of interdental ridges and no lingual wall ( Fig. 2A, S4). In contrast, the posterior teeth have their bases located partially dorsally to the apex of the lateral wall of the jaw, and partially lingually to it-apicolingually located relative to the apex of the labial wall of the jaws ("pleuroacrodonty"; see main text for discussion). The dentary symphysis is strongly curved medially, a very unusual condition among early diapsid reptiles, but similar to the dentaries of some early evolving lepidosaurs, such as the rhynchocephalians Gephyrosaurus The splenial is present. It is partially exposed on the right side of the skull, missing its anteriormost part, and also visible in the CT scans ( Fig. 1, 2B, S4). It tapers at the tip and extends quite far anteriorly, approaching the mandibular symphysis and closing the Meckelian canal lingually. However, it cannot be determined if the splenial contributed to the mandibular symphysis.

Phylogenetic Analyses
Strict consensus tree from the equal weights maximum parsimony analysis (Fig. S7).
Best fit tree from the implied weighting maximum parsimony analysis (Fig. S8).
Majority rule consensus tree from the combined morphological and molecular data under Bayesian inference analysis (Fig. S9).
Supplementary Data: the nexus file with the full data matrix, including the scorings for Vellbergia.      .   Table 1. List of taxa for leaf stability index (lsDif) analysis depicted in Fig. 5