The taxonomy, chronostratigraphy and paleobiogeography of glyptosaurine lizards (Glyptosaurinae, Anguidae)

Abstract Glyptosaurine lizards (Glyptosaurinae, Anguidae) are an extinct group of heavily armored lizards known from North America, Europe and Asia. Glyptosaurine lizards, taxa that possess fully developed tuberculated dermal armor, appear to have been established in North America by late early Puercan time (To3). “Proxestops,” a taxon distinguished by a combination of vermiculate and tuberculated osteoderm sculpturing, is considered to be a non-glyptosaurine, a sister taxon of the Glyptosaurinae. Known from only fragmentary remains, its wide chronostratigraphic distribution suggests that “Proxestops” is a form genus that, in all probability, represents more than one taxon, that ranges from the middle Paleocene to the early Eocene of North America. Moreover, the taxa Odaxosaurus piger, Parodaxosaurus sanjuanensis and “Proxestops” are best considered “proto-glyptosaurines”. “Melanosaurins” and glyptosaurins were well-established by the early Eocene, especially in North America, and are here documented by their type species and chronostratigraphic levels. Both tribes are present in Europe (MP7), too, but the record is not as estensive as that of North America. The North American taxon Gaultia silvaticus (Wa0) is transitional between a “melanosaurin” and glyptosaurin. Because it lacks the well-defined hexagonal osteoderms that characterize the Glyptosaurini, it is removed from that group and considered to be a “melanosaurin”. The “melanosaurin” taxon “Xestops” savagei (Wa4–Wa6) cannot be referred to Xestops (Br2) based on non-corresponding elements and because superficial similarity does not justify assignment to this taxon. Arpadosaurus sepulchralis (Wa6?), whose holotype is a fragmentary right frontal, is considered a subjective junior synonym of A. gazinorum, based on minor differences in the epidermal scale pattern that probably represent individual variation. “Glyptosaurus” agmodon (Wa6?), based on a partial right maxilla, cannot be referred to Glyptosaurus (sensu stricto), and the material upon which this taxon is based bears strong resemblance to material identified as cf. “?Paraglyptosaurus” yatkolai (Wa5–Wa6). “Glyptosaurus” rhodinos (Wa5) is based on an incomplete parietal, and its reference to Glyptosaurus is considered problematic. Eoglyptosaurus donohoei (Wa7) is probably valid and is re-established here. Glyptosaurus (sensu stricto) is known solely from the middle Eocene (Br2) by G. sylvestris. Dimetoposaurus wyomingensis (Br3) is removed from Xestops vagans because its synonymy was based on superficial similarities. Helodermoides tuberculatus, the largest and last glyptosaurin (Ch3), is restricted to the Chadronian of North America. Only the “melanosaurin” Peltosaurus granulosus (Or2–Or3), which includes the species P. abbotti, seems to have crossed the Eocene-Oligocene boundary, and appears to be largely restricted to the Orellan, but extended into the Arikareean. European glyptosaurines are also represented by both glyptosaurins and “melanosaurins” early in the Eocene (MP7). Placosauriops-like “melanosaurins” are known from Dormaal (MP7), and the glyptosaurin taxon?Placosaurus ragei occurs at the same level. “Placosauriops abderhaldeni” has been identified from the Grube Messel (MP11), but this assignment remains dubious because the species has not been adequately diagnosed, and the holotype species is from the Geiseltal (MP13), which is some 4.5 million years younger. Placosauriops weigelti (MP13) is the only valid species of this genus. Paraxestops stehlini (MP14) is not referable to the North American taxon Xestops, and its relationship to Placosauriops has not been studied. The late Eocene glyptosaurins Placosaurus estesi (MP17) and P. rugosus (MP18) are the last glyptosaurines known from Europe and appear to have gone extinct at the Eocene-Oligocene boundary, casulties perhaps of the “Grande Coupure”. Asian glyptosaurines are known solely from one species, Stenoplacosaurus mongoliensis, from the middle Eocene (Sharamurunian) of China. Glyptosaurines most likely originated in North America, diversified by late Paleocene time, and rapidly spread across the North Atlantic into Europe by the early Eocene. Both “melanosaurins” and glyptosaurins took a foothold in Europe by the early Neustrian, but the glyptosaurins, aside from one occurrence (Dormaal, MP7), were conspicuously absent for most of Neustrian through early Robiacian time. In North America, glyptosaurins diversified during the early and middle Eocene, while in Europe small “melanosaurins” were a prominent part of the paleoherpetofauna, and glyptosaurins are unknown for most of the Neustrian through the Geiseltalian, in both the fossilferous Lagerstatten of Messel and Geiseltal. Stenoplacosaurus is the only known glyptosaurin glyptosaurine from Asia, and its abrupt appearance during the late Eocene suggests the possiblity of a Beringian dispersal from North America into Asia.