A GREEN RIVER (EOCENE) POLYCHROTID (SQUAMATA: REPTILIA) AND A RE-EXAMINATION OF IGUANIAN SYSTEMATICS

Journal of Paleontology, Nov 2007 by Conrad, Jack L, Rieppel, Olivier, Grande, Lance

ABSTRACT-

A pleurodontan iguanian from the Green River Formation (Eocene) is described in detail and named. The new taxon is known only from a single specimen preserving all areas of the body. Although many of the bone surfaces are eroded, almost all of the skeleton is present and some cartilaginous elements are preserved. The new taxon shares important characteristics with the extant anisolepines and leiosaurines, including the morphology and placement of the caudal autotomy planes, the postxiphisternal inscriptional ribs, and notched or fenestrated clavicles that are expanded proximally. This is the earliest complete iguanian known from the Americas and the earliest known iguanian that may be confidently referred to an extant "family." A phylogenetic analysis including this taxon and other fossil and extant iguanians offers some support for the monophyly of Polychrotidae sensu lato, Tropiduridae sensu lato, and non-acrodont iguanians (Pleurodonta).

INTRODUCTION

FOSSIL LAKE is part of an extinct Early Cenozoic (Green River Formation) lake system from the western United States (Fig. 1) that has yielded a window into the Eocene in North America (Grande, 1984; Grande and Buchheim, 1994). Fishes are the best known component of the Green River lake system, but several tetrapods have also been recovered (Grande, 1984; Grande and Buchheim, 1994). Squamates are not a particularly common component of this assemblage, but at least four species have been identified from various areas of the squamate tree. The F-2 deposits have yielded the varanid Saniwa ensidens (Leidy, 1870), the boid snake Boavus idelmani (Grande, 1984; Grande and Buchheim, 1994), the shinisaurid Bahndwivici ammoskius (Conrad, 2006), and an unnamed iguanian (Grande and Buchheim, 1994).

Iguania possesses a rich but spotty fossil record beginning in the Cretaceous. Much of the early iguanian fossil record occurs in Asia (Gilmore, 1943; Alifanov, 1989, 1996; Gao and Hou, 1996; Gao and Nessov, 1998; Gao and Norell, 2000), but most extant, non-acrodont iguanians are found in the Americas. Little is known about the interrelationships of the non-acrodontan iguanians and even their monophyly with respect to the acrodont clade is questionable (Estes et al., 1988; Etheridge and de Queiroz, 1988; Frost and Etheridge, 1989; but see Schulte et al., 2003).

Here we describe and name the Green River iguanian first noted and figured by Grande and Buchheim (1994). The taxon comes from F-2 (shallow, near-shore) deposits of Fossil Lake. The new specimen preserves almost the entire skeleton, but most of the external bone surfaces have weathered away. Importantly, this animal shows a strong resemblance to members of the Polychrotidae (anoles, para-anoles, and relatives) and extends the known geographical and temporal ranges of that clade. The new taxon is important for understanding the reptile component of the Fossil Lake biota, the early Cenozoic iguanian record in the Americas, and broader iguanian relationships.

MATERIALS AND METHODS

The taxonomy used herein follows that of Frost and Etheridge (1989) modified to include some of the basic additions offered by Schulte et al. (2003) for the polychrotid subclades. Because the basic taxonomic scheme of Frost and Etheridge (1989) is being used, the taxa identified by Schulte et al. (2003) as Anisolepae and Leiosaurae will be referred to as Anisolepinae and Leiosaurinae, respectively, hereafter. Although the taxonomy included here should be considered unranked, the terms "family" and "subfamily" will be used occasionally as informal terms offering indication of the relative inclusiveness of a given clade.

The drawings included in Figure 2 were made by tracing digital photographs in Adobe Photoshop CS2, v9.0 while making direct observations on the skeleton.

Institutional abbreviations are: AMNH, American Museum of Natural History (New York); FMNH, Field Museum of Natural History (Chicago); REE, collection of Richard Etheridge; UF, Florida State Museum (University of Florida).

An uncatalogued Anolis carolinensis specimen was cleared and double stained following modified versions of published Alcian blue and Alizarin red protocols (Dingerkus and Uhler, 1977; Hanken and Wassersug, 1981; Potthoff, 1984). Cartilage was stained in an Alcian Blue solution (Alcian Blue 8GX added to a mixture of two parts glacial acetic acid and eight parts absolute ethanol). After passing the specimens through an alcohol series (EtOH: 95%, 75%, 50%, 25%; 24h each step), enzyme clearing of soft tissues was effected with Trypsin (Trypsin added to buffer solution, prepared by mixing seven parts saturated Borax solution with three parts H2O). Bone was stained with Alizarin Red S in 1% KOH. After staining, the specimens were washed for two hours in clear Buffer solution and then transferred to the glycerol series, allowing a minimum of two weeks for each step. Three to five drops H^sub 2^O^sub 2^ per 100 ml were added to glycerol solutions (25%, 50%, 75% glycerol in 0.5% KOH) to bleach pigments, particularly in the peritoneal wall.