Abronia cuyama, Scarpetta & Ledesma, 2023

Scarpetta, Simon G. & Ledesma, David T., 2023, A strikingly ornamented fossil alligator lizard (Squamata: Abronia) from the Miocene of California, Zoological Journal of the Linnean Society 197, pp. 752-767 : 755-761

publication ID	https://doi.org/ 10.1093/zoolinnean/zlac024
publication LSID	lsid:zoobank.org:pub:8C786137-0AAF-40C5-ABDC-51477AFC4023
DOI	https://doi.org/10.5281/zenodo.7695195
persistent identifier	https://treatment.plazi.org/id/03FE87CF-FFA8-933B-D4BD-F961FAEFE939
treatment provided by	Plazi
scientific name	Abronia cuyama
status	sp. nov.

Treatment

ABRONIA CUYAMA SP. NOV.

Z o o b a n k r e g i s t r a t i o n: u r n: l s i d: z o o b a n k. org:pub: 8C786137-0AAF-40C5-ABDC-51477AFC4023

Holotype: The holotype UCMP 54560 View Materials is housed in the UCMP and was collected by Gideon T. James and a party of UCMP palaeontologists in 1957 or 1958. The specimen consists of a partial cranium including the anterior portion of the skull ( Figs 2A–F View Figure 2 , 3A–E, G–I View Figure 3 , 4A View Figure 4 ), an associated block of mudstone matrix containing a few posterior cranial elements ( Figs 2G–I View Figure 2 , 3F View Figure 3 ) and two vials of associated material containing osteoderms in matrix, a probable epipterygoid and unidentified bones.

Etymology: The species is named for the Cuyama Valley Badlands where UCMP 54560 was collected. ‘Cuyama’ is derived from the Chumash word kuyam (noun) that means ‘a place to come together’ or ‘clam’; the name of the new taxon honours the Chumash people on whose land the fossil was collected.

Diagnosis: Abronia cuyama is a squamate because it has a single premaxilla and pleurodont tooth implantation and an anguimorph because the Meckelian groove is directed ventrally ( Estes et al., 1988; Gauthier et al., 1988). The taxon is an anguid because there are rectangular and laterally imbricating osteoderms, a free margin of the intramandibular septum ( Gauthier, 1982) and a squamosal that lacks a posteromedial expansion ( Bhullar, 2011). The taxon is assigned to Gerrhonotinae because the frontal is fused and lacks suture marks ( Klembara et al., 2010; Scarpetta et al., 2021), there is a raised dorsal ossification on each side of the body of the premaxilla ( Scarpetta, 2018), and there are no separate palatal processes of the premaxilla ( Evans, 2008; Scarpetta et al., 2021). The new taxon shares with Abronia excluding species previously placed in Mesaspis ( Scarpetta et al., 2021) a relatively wide frontal, vermiculate, heavily sculptured osteoderms across the skull, contact of the posterior internasal and prefrontal osteoderms, and the presence of a frontonasal osteoderm ( Figs 2A–F View Figure 2 , 4A View Figure 4 ). The osteoderm keels on and anterior to the frontal and the tall osteoderm keels or ‘sails’ on the posterior cranial osteoderms are autapomorphies of the new taxon ( Figs 2H, I View Figure 2 , 4A View Figure 4 ). We performed phylogenetic analyses (see below) to place the new taxon systematically with respect to extant species of Abronia and other gerrhonotines.

Description: The specimen is comparable in size to skeletally mature extant gerrhonotines ( Fig. 4 View Figure 4 ). Several morphological features previously used to determine skeletal maturity in anguimorphs were not preserved on the fossil (e.g. terminal fusions of the braincase, osteodermal crust on the parietal; Bhullar, 2012), but the robust osteoderms, well-developed osteodermal crust on the frontal and relatively high tooth count ( Ledesma et al., 2021) imply a mature individual.

Portions of the alveolar plate, nasal process and incisive process of the premaxilla are preserved ( Fig. 3A, B View Figure 3 ). There are three teeth and several replacement teeth. The nasal process is narrow relative to the width of the alveolar plate and tapers distally almost to a point, and the anterior surface of the process is relatively flat. The alveolar plate lacks discrete palatal processes. The ventral portion of the nasal process is broken, hence the presence of an anterior foramen or an ossified bridge between the nasal process and the alveolar plate cannot be determined. The nasals are present but were difficult to segment from the matrix, and osteoderms are fused to the dorsal surface of the nasals. Some morphological features of the nasals could be observed from the CT slices. Specifically, it was possible to ascertain that the nasals have an anteromedial process that articulates with the nasal process of the premaxilla (xy slice 239; Supporting Information, Fig. S1A View Figure 1 ) and that the nasals are broadly separated for most of their length, especially anteriorly (xy slice 272; Supporting Information, Fig. S1B View Figure 1 ) and near their anterior–posterior midpoint (xy slice 448 XY; Supporting Information, Fig. S1C View Figure 1 ).

Both septomaxillae are present, but only the left septomaxilla was segmented from the surrounding matrix ( Fig. 3E View Figure 3 ). The posterior process is long. The presence of an anterolateral process could not be determined. The maxillae preserve the facial, palatine, premaxillary and orbital processes ( Figs 2D View Figure 2 , 3C, D View Figure 3 ). There are six or seven nutrient foramina on the labial surface of both maxillae. There are 18 teeth on the left maxilla, filling all available positions, and on the right maxilla there are 16 teeth and 20 tooth positions. The maxillary lappet projects anterodorsally from the medial surface of the premaxillary process. There is a single foramen (the anterior inferior alveolar foramen) near the anterior edge of the facial process where it meets the premaxillary process. The facial process is inflected medially but does not contact the frontal. The anteroposterior dimension of the facial process is narrow. The palatine process has a slightly rounded subtriangular medial projection. The orbital process is long relative to the facial and premaxillary processes. The posteriormost portion of the left orbital process is broken. Rugose texturing is present on the lateral surface of the maxilla, particularly on the facial process, and some osteoderms are fused to the facial process. The lacrimal is long, extending for nearly half the length of the orbital process of the maxilla, and has rugose sculpturing on its lateral surface. The lacrimal extends both medially and dorsally to enclose, in part, the lacrimal foramen.

The right jugal is present but is missing the posterior portion of the orbital process and dorsal portion of the temporal ramus ( Fig. 3H View Figure 3 ). The temporal ramus has a broad anteroposterior dimension, but the orbital process is more gracile. The orbital process has a ventral lamina that overhangs the dorsal margin of the orbital process of the maxilla. The jugal spur (quadratojugal process) is large and has a pointed posterior projection. The palpebral is triangular and externally visible.

Both prefrontals are present but were difficult to separate from the matrix. The frontal process is long and projects posterodorsally. The ventral process is short and lacks an anteroventral projection (xy slices 740–750). The prefrontal has a large articulation facet for the facial process of the maxilla. The frontal is complete but broken into an anterior and a posterior piece that are slightly separated from each other ( Fig. 2E, F View Figure 2 ). The frontal is azygous and wide relative to its length. An osteodermal crust covers the dorsal exposure of the frontal, and some osteoderms are fused to the frontal. The anterior process of the frontal is triradiate. The interorbital region is constricted. The facet for the prefrontal on the anterolateral surface of the frontal has small, anterolaterally facing processes at its posterior margin.

Both vomers are present, and the right vomer was segmented ( Fig. 3G View Figure 3 ). There is a dorsally extending posterolateral flange. The anterior (vomeronasal) portion of the bone extends to a lower ventral level than the rest of the element. The foramen for the medial palatine nerve penetrates the right vomer posteriorly and exits anteriorly (xy slices 528–573). The palatines were difficult to segment from the matrix, especially posteriorly. Only the vomerine process was segmented ( Fig. 3I View Figure 3 ), but from the CT slices it was evident that there are no palatine teeth. The vomerine process lacks a posteroventral ridge demarcating the facet for the vomer. There is no anterior flange dorsal to the choana.

The dentaries preserve all but the posterior portion of the bone ( Fig. 2B, D View Figure 2 ). The Meckelian groove is open and is directed ventromedially and ventrally in the anterior and posterior portions of the dentary, respectively. Neither dentary is preserved well enough to ascertain the presence of a surangular process or a posterior groove ventral to the parapet. The free margin of the intramandibular septum is incomplete but present on both sides (for the right dentary, see xy slice 1044; Supporting Information, Fig. S2 View Figure 2 ). There are 22 or 23 tooth positions and 17 teeth on the right dentary and at least ten teeth on the left dentary. Dentition is pleurodont. Crowns on the mesial teeth are unicuspid and sharp, and the crowns on distal teeth are near bicuspid. Teeth are recurved throughout the tooth row of both the dentary and maxilla except for the distalmost teeth. Some of the crowns are missing from the mesial teeth of the right dentary and left maxilla. The mesial teeth and some of the mid-tooth row teeth of the right maxilla are particularly long, recurved and sharp.

The distal portion of the right squamosal is preserved in the separate mudstone block ( Figs 2G–I View Figure 2 , 3F View Figure 3 ). The ventral process curves anterolaterally to articulate with the right quadrate. The element is uniform in width. The right quadrate also is preserved in the block. The quadrate has a narrow mediolateral dimension, with roughly parallel lateral and medial margins. The cephalic condyle is well developed. The mandibular condyle is shaped like a hyperbolic paraboloid and is slightly narrower than the rest of the quadrate. The anteromedial surface of the quadrate is concave, especially dorsally. The pterygoid lamina is shallow, and there is a slightly protruding flange of bone dorsal to it. The column and the tympanic crest are both well developed. The dorsal and ventral portions of the conch are roughly the same width.

The osteoderms are robust and have heavy vermiculate sculpturing ( Figs 2A–F View Figure 2 , 4A View Figure 4 ). Osteoderms become more rectangular posteriorly. Many osteoderms have midline keels, including osteoderms anterior to the frontal and the osteodermal crust of the frontal itself. An articulated series of osteoderms (potential temporal osteoderms) surround the quadrate dorsally, laterally and ventrolaterally ( Fig. 2G–I View Figure 2 ). The osteoderms must have slid ventrally during fossilization, because osteoderms could not occur directly ventral to the quadrate in a live lizard owing to articulation between the quadrate and the mandible. The posterior osteoderms above the quadrate have keels or ‘sails’ that are more than twice as tall as the osteoderm itself ( Fig. 2H, I View Figure 2 ). The prefrontal, frontonasal and posterior internasal osteoderms are present, and the prefrontals and posterior internasals are in broad contact ( Fig. 2E, F View Figure 2 ). The left prefrontal osteoderm appears to have slipped ventrally from its natural position. Supranasal and anterior internasal osteoderms are not preserved. There is a frontoparietal osteoderm on the left side of the frontal; the osteoderm might have fallen off on the right side. No anteroventral or sublabial osteoderms were preserved.

One of the vials of associated material contains two unidentified bones, which might be weathered osteoderms. The other vial contains two small pieces of mudstone matrix, each with several visible and apparently articulated osteoderms. One of the matrix pieces also contains a tubular bone that is probably an epipteryogid. The osteoderms have the same heavy, vermiculate sculpturing as do those on the skull and in the large mudstone block, and several osteoderms preserve distinct keels.

Comparisons: Comparisons are largely based on the specimens examined by Scarpetta et al. (2021). If no specimen number is listed, the comparison accommodates all specimens of a given taxon that we examined for that dataset.

Abronia cuyama has the wide frontal and heavily sculptured vermiculate-textured osteoderms that are characteristic of Abronia , excluding, among examined specimens, Abronia monticola Cope, 1878 , Abronia moreletti Bocourt, 1872 and Abronia gadoƲii Boulenger, 1913 (i.e. species previously placed in Mesaspis ). The anterodorsal osteoderms of Barisia imbricata Wiegmann, 1828 and Barisia ciliaris Smith, 1942 are also heavily sculptured. The facial process of the maxilla is narrow in Abronia campbelli Brodie & Savage, 1993 , Abronia lythrochila Smith & Alvarez del Toro, 1963, A. moreletti , A. monticola , A. gadoƲii , G. parƲus SRSU 5538 and some observed specimens of Elgaria panamintina Stebbins, 1958 and Elgaria Ʋelazquezi Grismer & Hollingsworth, 2001 (MVZ 191076 and SDNHM 68678, respectively). Several species of Abronia (e.g. Abronia graminea Cope, 1864 , A. ornelasi and A. gadoƲii ), Barisia ( Barisia leƲicollis Stejneger, 1890 , B. ciliaris and B. imbricata ) and D. lugoi have nasals that are separated near their longitudinal midpoint. The quadrate conch is largely uniform in width in A. campbelli , A. lythrochila , A. gadoƲii and several specimens of Barisia (e.g. B. ciliaris FMNH 30707). The vomeronasal region of the vomer is ventrally displaced in A. campbelli , A. lythrochila , A. graminea , A. ornelasi and A. gadoƲii . The vomerine process of the palatine lacks a ventral ridge in most Abronia , but the ridge is present in Abronia mixteca Bogert & Porter, 1967 and A. gadoƲii . Species previously referred to Mesaspis and Abronia taeniata Wiegmann, 1828 (TCWC 30660) have an osteoderm overlying the frontoparietal scute. Examined specimens of Barisia and Abronia , excluding A. ornelasi and species previously placed in Mesaspis , display contact of the posterior internasal and prefrontal osteoderms. Abronia cuyama has a frontonasal osteoderm, which is absent in Barisia ( Fig. 4D View Figure 4 ), and has a flat anterior surface of the nasal process of the premaxilla, unlike Barisia .

The extinct gerrhonotines P. ricardensis and G. mungerorum were previously suggested to be part of or closely related to Abronia based primarily on the presence of heavily sculptured osteoderms and relatively long, fang-like teeth ( Good, 1988a), but neither was ever formally placed in Abronia . Paragerrhonotus ricardensis was described from a partial skull ( Estes, 1963). Phylogenetic studies that included P. ricardensis placed the taxon in a polytomy with Gerrhonotus , Barisia and Abronia or as a stem gerrhonotine ( Conrad & Norell, 2008; Norell et al., 2008). Based on examination of the holotype and with reference to the diagnostic characters listed by Estes (1963), P. ricardensis and A. cuyama share heavily sculptured osteoderms (although the texture is less vermiculate in Paragerrhonotus , similar to Barisia ), a large jugal spur, and sharp, recurved mesial teeth on the maxilla. A large jugal spur and sharp and recurved maxillary teeth are present in many gerrhonotines ( Gauthier, 1982; Ledesma et al., 2021; Scarpetta et al., 2021), and neither was used here as a phylogenetic character. Paragerrhonotus ricardensis differs from A. cuyama in possessing an elongate frontal, in lacking osteoderm keels, in lacking a ventral lamina of the orbital process of the jugal, in possessing a distinctive arrangement of small osteoderms on the posterior portion of the frontal (not adapted as a phylogenetic character) and in possessing a premaxillary nasal process with a concave dorsal surface. Abronia cuyama and P. ricardensis do not share any unique synapomorphies or apomorphic character state combinations.

We also note that P. ricardensis has a single row of four pterygoid teeth, as originally stated by Estes (1963) and contra Good (1988a). The loss of pterygoid teeth is another feature mentioned by Good (1988a) to suggest a close relationship between P. ricardensis and Abronia . The loss of pterygoid teeth was interpreted by several authors as a derived feature of Abronia and Barisia ( Good, 1987; Scarpetta et al., 2021). A few examined specimens of Abronia and Barisia have one to three irregularly arranged pterygoid teeth (e.g. A. campbelli UTA 95952; B. ciliaris FMNH 30707).

Gerrhonotus mungerorum was described based on a single, isolated frontal ( Wilson, 1968). Additional fossils, including partial dentaries, maxillae and a parietal, were later attributed to the taxon from the type locality and other localities (e.g. Holman, 1973, 1975), but those identifications were either explicitly tentative or based on features not unique to G. mungerorum with respect to other gerrhonotines. Gerrhonotus mungerorum requires further study, especially the tentatively referred fossils, and a systematic assessment of the species would benefit from the procurement of additional fossils from the type locality. We have not examined the holotype or attributed specimens of G. mungerorum in person; therefore, based on the description of the holotype and descriptions of referred fossils, G. mungerorum and A. cuyama share a heavily sculptured frontal, sharp and recurved teeth, and the presence of near bicuspid crowns on the distal teeth. Sharp and recurved teeth and near bicuspid distal crowns are common to many gerrhonotines ( Scarpetta et al., 2021), and the former was not used as a phylogenetic character. Gerrhonotus mungerorum differs from A. cuyama in having an elongate frontal and in lacking keels on the frontal osteoderm crust.

Cranial osteoderm keels are present in Elgaria multicarinata Blainville, 1835 and Elgaria nana Fitch, 1934 , although neither of those species has osteoderm keels anterior to the frontal or on the frontal ( Ledesma et al., 2021; Scarpetta et al., 2021). Among other squamates, nuchal osteoderm sails were observed on Cordylus namakuiyus Stanley, Ceríaco, Bandeira, Valerio, Bates & Branch, 2016 and Cordylus angolensis Bocage, 1895 (CAS 254912 and AMNH 47333, respectively). Among other anguimorphs, postcranial osteoderm keels were observed in Shinisaurus crocodilurus Ahl, 1930 (UF 45615 and FMNH 215541), and keels are present on some cranial osteoderms and the frontal osteodermal crust in some Xenosaurus (e.g. Xenosaurus grandis Gray, 1856 ; FMNH 123702).