A new extinct species of alligator lizard (Squamata: Elgaria) and an expanded perspective on the osteology and phylogeny of Gerrhonotinae

Abstract

Background Alligator lizards (Gerrhonotinae) are a well-known group of extant North American lizard. Although many fossils were previously referred to Gerrhonotinae, most of those fossils are isolated and fragmentary cranial elements that could not be placed in a precise phylogenetic context, and only a handful of known fossils are articulated skulls. The fossil record has provided limited information on the biogeography and phylogeny of Gerrhonotinae. Results We redescribe a nearly complete articulated fossil skull from the Pliocene sediments of the Anza-Borrego Desert in southern California, and refer the specimen to the alligator lizard genus Elgaria. The fossil is a representative of a newly described species, Elgaria peludoverde. We created a morphological matrix to assess the phylogeny of alligator lizards and facilitate identifications of fossil gerrhonotines. The matrix contains a considerably expanded taxonomic sample relative to previous morphological studies of gerrhonotines, and we sampled two specimens for many species to partially account for intraspecific variation. Specimen-based phylogenetic analyses of our dataset using Bayesian inference and parsimony inferred that Elgaria peludoverde is part of crown Elgaria. The new species is potentially related to the extant species Elgaria kingii and Elgaria paucicarinata, but that relationship was not strongly supported, probably because of extensive variation among Elgaria. We explored several alternative biogeographic scenarios implied by the geographic and temporal occurrence of the new species and its potential phylogenetic placements. Conclusions Elgaria peludoverde is the first described extinct species of Elgaria and provides new information on the biogeographic history and diversification of Elgaria. Our research expands the understanding of phylogenetic relationships and biogeography of alligator lizards and strengthens the foundation of future investigations. The osteological data and phylogenetic matrix that we provided will be critical for future efforts to place fossil gerrhonotines. Despite limited intraspecific sampled sizes, we encountered substantial variation among gerrhonotines, demonstrating the value of exploring patterns of variation for morphological phylogenetics and for the phylogenetic placement of fossils. Future osteological investigations on the species we examined and on species we did not examine will continue to augment our knowledge of patterns of variation in alligator lizards and aid in phylogenetics and fossil placement.

The affinities of the Late Triassic Cryptovaranoides and the age of crown squamates

“…The anteroposteriorly trending ridges on the vomers of † C. microlanius are also very different from the crest-like ridges of anguimorphs [ 6 ], such as Pseudopus apodus [ 57 ] and Elgaria spp. [ 58 , 59 ], where these features appear as the apices of developed wings of bone. Further, new information on the palatal anatomy of early diverging crown squamates shows that ventral ridges on the vomer are found outside Anguimorpha, including in members of the pan-scincoid clade †Paramacellodidae [ 9 ].…”

Body size estimation from isolated fossil bones reveals deep time evolutionary trends in North American lizards

“…These include anatomical measurements from a total of 283 fossil lizard specimens, with 44 recognized genera and at least 33 recognized species, from the Paleogene record of the U.S. Western Interior (Table 1 and S1 Dataset). Only fossil lizard specimens with complete cranial bones or associated limb bones were sampled (e.g., Fig 3), for the following reasons: 1) the lizard fossil record is mostly comprised of cranial bones and, fortunately, most extant phylogenetic morphological characters for lizards are found in cranial bones [62,63], making it possible to determine at least coarse taxonomic identifications (family or higher) for fossil lizard specimens from cranial material [63]; 2) historical fossil identification may lack fidelity at the genus or species level [64][65][66]; 3) cranial and limb bones can be used to estimate SVL in lizards [9,34]. A few measured specimens included complete skulls (Fig 3A and 3I) or even skeletons (Figs 4A and 5).…”

“…When estimating body length or mass for a fossil specimen that included more than one complete cranial or limb element, the corresponding regression that offered the lowest standard error of the estimate (SEE) was used (S1-S7 Tables). All analyses were conducted at the level of crown-group assignments [64][65][66]. Fossil data were grouped by North American Land Mammal Ages (NALMAs), which compose a relative chronology used to divide the Paleogene based on taxonomic turnover in the mammal fossil record [78,79].…”

A Palaeogene stem crotaphytid ( Aciprion formosum ) and the phylogenetic affinities of early fossil pleurodontan iguanians

“…In iguanians, the surangular process is the larger of the two posterodorsal processes of the dentary, and the coronoid process (the dorsalmost process) is relatively less developed in its posterior extent and overall size.In some anguids (e.g. anguines, some Abronia ), the coronoid process is well-developed and extends relatively far posteriorly, and in several gerrhonotines, such as most Elgaria , the surangular process is absent [112]. Regardless, no posterior process of the dentary exceeds the coronoid apex in anguids. A.12.…”

“…In some anguids (e.g. anguines, some Abronia ), the coronoid process is well-developed and extends relatively far posteriorly, and in several gerrhonotines, such as most Elgaria , the surangular process is absent [ 112 ]. Regardless, no posterior process of the dentary exceeds the coronoid apex in anguids.…”