“Proto-glyptosaurine” lizards are known from the early Paleocene (middle Puercan) of North America, represented mostly by isolated dentary, frontal and osteoderm material (Sullivan, 1981, Sullivan, 1986c and Sullivan and Lucas, 1986). A number of osteoderm specimens have been recovered from the Puercan by the author and reside in the collections of the Natural History Museum of Los Angeles County, California. Osteoderm, and other fragmentary skeletal remains, from both Puercan and Torrejonian age strata of New Mexico, are housed in the collections of the New Mexico Museum of Natural History and Science (pers. observation). Most of this material has yet to be studied in detail, but preliminary survey of the specimens suggests they are mostly referable to either Odaxosaurus sp. or “Proxestops”-grade taxa.

Sullivan (1981) reported on an incomplete frontal (KU 9734) from the Torrejonian (To2) of Kutz Canyon (KU locality 13), New Mexico, that displayed a combination of “pit-and-ridge” (vermiculate) sculpturing with some tuberculation, and an anterior part of a skull (KU 7897) with a frontal, largely encrusted and obscured by matrix, that appeared to be of the vermiculate (“pit- and-ridge”) morphology. Both specimens were tentatively referred to “Odaxosaurus cf. piger.” In a subsequent paper (Sullivan, 1986c), I reassigned KU 9734 to Proxestops jepseni and established a new taxon, Parodaxosaurus sanjuanensis, based on an anterior portion of a coalesced frontal (NMMNH P-12346, formerly UNM NP-1509) that retained vermiculate dermal sculpturing (seen in the Late Cretaceous taxon Odaxosaurus piger) from Torreon Wash, of New Mexico. Parodaxosaurus is Torrejonian (To3) age (late early Paleocene), equivalent to the “Pantolambda zone,” as redefined by Lofgren et al. (2004), and may represent anguid lizards that were the last to retain this primitive sculpturing. A recently recovered specimen (NMMNH P-58618) from Torreon Wash, that has yet to be described, consists of a nearly complete coalesced frontal, that is slightly constricted between the orbits and bears near fully tuberculate dermal armor. This frontal suggests the presence of “melanosaurin” glyptosaurines by early Paleocene (To3) time in North America.

Bartels (1983) reported the Late Cretaceous taxon “cf. Odaxosaurus piger” from the Plesiadapis cookei Zone (FG-6), which presumably corresponds to the Plesiadapis cookei Lineage Zone (Cf2) of Lofgren et al. (2004). To my knowledge, no definitive Paleocene occurrence of Odaxosaurus has been established, let alone a late Paleocene (Clarkforkian) occurrence.

Suffice it to say, much of the North American Puercan and Torrejonian anguid lizard material may pertain to “Proxestops,” a taxon named by Gauthier (1982), based on the type species Peltosaurus jepseni Gilmore (1942) from Princeton Quarry, Silver Coulee beds, Fort Union (formerly Polecat Bench) Formation (Tiffanian-Ti5a) of Wyoming (see Secord et al., 2006). These fossil anguid lizards display a transitional osteoderm morphology that falls between the vermiculate sculpturing characteristic of Odaxosaurus-Parodaxosaurus grade, to that of fully tuberculate sculpturing, characteristic of members of the Glyptosaurinae (sensu Sullivan, 1979), contra Estes (1983a) and Gauthier (1982), who considered both Odaxosaurus and Proxestops to be primitive glyptosaurines.

In a subsequent study, I (Sullivan, 1991, p. 297) established Proxestops silberlingii, a new taxonomic combination, synonymizing Gilmore's (1938) ?Harpagosaurus silberlingii (Gilmore, 1938), from the Paleocene Fort Union Formation Silberling Quarry (To2), along with Peltosaurus jepseni (Gilmore, 1942), Pancelosaurus piger (in part) (Meszoely, 1970), Proxestops jepseni (Gauthier, 1982),?Paraprionosaurus silberlingii (Estes, 1983) and the nomen nudum, Proxestops gastrodon (Bartels, 1988). Proxestops silberlingii has priority over Proxestops jepseni if the latter proves to be distinct from the former.

Smith (2009) assigned a number of specimens to “Proxestops sp.” from the early Wasatchian (Wa0) Willwood Formation, Bighorn Basin, Wyoming, but he stated that these differed from the Tiffanian type material. He noted too, that Proxestops displays a number of characters that are primitive to both anguid and glyptosaurine lizards. As a consequence, the stratigraphic range of “Proxestops,” which, in part, includes the recently named “Xestops” savagei (Smith and Gauthier, 2013, see below) appears, to be quite long, possibility extending from the early Paleocene (Pu2) through the early Eocene (Wa0), approximately 10 million years (65–56 mya), an unusually long range for a lizard taxon. Given that the taxon's identification is based largely on the transitional sculpturing, and that a number of features of the taxon are pleisomorphic, it seems likely that “Proxestops,” as currently understood, represents a form genus, a name given to fragmentary material distinguished, in this case, by incipient tubercular sculpturing that probably occurred among a number of early Paleocene anguids. It is quite likely that there is more than one taxon that exhibits this primitive morphology. Clearly, more complete material is needed in order for “Proxestops” to be properly assessed and its validity confirmed. For the purposes of this study, P. jepseni is tentatively recognized for specimens assigned to Proxestops (including P. silberlingii and P. gastrodon).

Fully tuberculate osteoderms have been recently reported from the latest Paleocene of North Carolina (Cicimurri et al., 2016) that clearly establish that glyptosaurines were widespread across North America by that time. However, it is unknown whether this occurrence is a “melanosaurin” or glyptosaurin.

In North America, glyptosaurine lizards become a dominate part of the lizard fauna at the onset of the Eocene. “Melanosaurin”-like lizards, including those of the “Proxestops”-like grade, continued to flourish, with Melanosaurus maximus and the enigmatic Arpadosaurus gazinorum making their appearance in the early Eocene (Wasatchian). The taxon Xestops vagans, known from the middle Eocene (Bridgerian), appears to be one of the last of these early primitive “melanosaurin”-like taxa. But then there is an apparent, unexplainable dearth of “melanosaurins” until the early Oligocene, when Peltosaurus becomes the last representative of the Glyptosaurinae.

One of the earliest glyptosaurine lizards to appear in the Eocene of North America is Gaultia silvaticus from the Willwood Formation (Wa0), Bighorn Basin, Wyoming (Smith, 2009). This glyptosaurine lizard, while sporting the fully tuberculate dermal armor that defines this group, has dermal armor that is subdivided into heterogenous plates of different sizes. Its skull roof dermal armor is not divided into discrete, subequal hexagonal plates (osteoderms), which is characteristic of the clade Glyptosaurini as defined by Sullivan (1979). Instead, Gaultia is more reminiscent of Arpadosaurus, in which the frontal and parietal dermal armors are subdivided into larger units. Therefore, I do not consider Gaultia to be a member of the Glyptosaurini, but instead place it as the sister taxon to that group.

Smith and Gauthier (2013) added to the diversity of early Wasatchian glyptosaurines by naming three new taxa, “Xestops” savagei (Wa4–Wa6?), “Arpadosaurus” sepulchralis (Wa6?) and “Glyptosaurus” agmodon (Wa6?), as well as identifying two others, glyptosaurine sp. “A” and “B.” Some of the material cited by Smith and Gauthier (2013) as pertaining to “Xestops” savagei had previously been assigned to the late Paleocene taxon Proxestops jepseni by Gauthier (1982), which underscores the problems in assigning these primitive glyptosaurines and proto-glyptosaurines to specific genera and species. I note that the holotype of this species is based on a partial parietal (UCMP 146425), whereas the holotype of Xestops vagans (USNM 16532) is based on a pair of coalesced frontals and other skeletal elements that do not include a parietal (Gilmore, 1928, Meszoely, 1970, Meszoely et al., 1978 and Sullivan, 1979). The partial right frontal UCMP 214628, while superficially resembling the frontals of the holotype of Xestops vagans, differs in that the anterior portion of the frontal is narrower and the posterolateral margins flare out compared to the holotype of X. vagans. These morphological differences, coupled with the fact that “X.” savagei is older (Wa4–Wa6?) than X. vagans (Br2), demonstrate that assignment to the genus Xestops is not defensible, so it is not accepted here. It must be pointed out that the two taxa have as their holotypes elements that do not overlap and therefore cannot be readily compared. Arpadosaurus sepulchralis from the Wasatch Formation of Wyoming was named by Smith and Gauthier (2013) based on a partial right frontal (UCMP 214431), which is questionably from the Wasatchian (Wa6?). The epidermal scalation pattern of A. sepulchralis is so similar to the holotype (USNM 25826) of A. gazinorum (Wa6–Wa7), that it must be considered to be within the realm of individual variation. The left dentary referred to this species (UCMP 214675) bears enlarged posterior teeth that are not unlike those seen in the type specimen described by Meszoely (1970). Because of the similarity to A. gazinorum, and fragmentary nature of A. sepulchralis, I consider it to be a subjective junior synonym of A. gazinorum. I note, too, that both species come from virtually the same stratigraphic level (see Appendix). Lastly, Smith and Gauthier (2013) named the taxon “Glyptosaurus” agmodon based on a partial right maxilla (UCMP 179384) from the Wasatch Formation (Wa6?) of Wyoming. The specimen is very similar to another specimen (UNM J-347, now NMMNH P-9982) published by Sullivan and Lucas (1988), in part, represented by a posterior part of a right maxilla bearing two bulbous crushing teeth, from the Regina Member, San José Formation (Wa5) of New Mexico. They are nearly the same size, and both have the same large, squared-off, obtuse teeth. The New Mexico specimen was one of many that was questionably referred to “Paraglyptosaurus” cf. “P.” yatkolai (see below).

Interestingly, the early Wasatchian is dominated by “melanosaurin” grade taxa, including Melanosaurus maximus (Wa2–Wa3) and the slightly younger Arpadosaurus gazinorum (Wa6–Wa7). There are a few specimens of the “melanosaurin”-grade that have been documented (Sullivan, 1979 and Sullivan, 1981) but have remained largely unstudied. This pattern of occurrence seems to be repeated in Europe (see below) where small “melanosaurin”-like taxa are more common in the early and middle Eocene (see below).

In North America, early Eocene glyptosaurins are first represented by the enigmatic taxon “Glyptosaurus” (“Paraglyptosaurus”) yatkolai (holotype AMNH 5181) and specimens referred to “cf. P. yatkolai” and “?P. cf. P. yatkolai” (Sullivan, 1979 and Sullivan and Lucas, 1988). The holotype (AMNH 5181) and other AMNH referred specimens are from the Regina Member of the San José Formation, which mostly likely corresponds to Wa5, based on the correlations of Robinson et al. (2004). The referred UNM material (above), described by Sullivan and Lucas (1988), which now resides in the collections of the NMMNH, also comes from the Regina Member of the San José Formation. The NMMNH specimens are presently being re-evaluated by me.

Smith (2011a) named “Glyptosaurus” rhodinos based on a partial parietal (USNM 527814) and designated a partial right maxilla (USNM 527978) as the paratype. Both specimens are from the Willwood Formation (Wa5), Wyoming. They are of the same age as specimens referred to “Glyptosaurus” (Paraglyptosaurus) yatkolai noted above and may pertain to that taxon.

The species “Glyptosaurus” donohoei was established for an incomplete and somewhat crushed skull, partial left mandible, other incomplete postcranial material and osteoderms (USNM 18316), from the Boysen Reservoir region, of Fremont County, Wyoming (White, 1952). The specimen came from the Lost Cabin Member of the Wind River Formation (contra Sullivan, 1979; 1989) of the northern Wind River Basin, which spans Wa7–Br1a (Robinson et al., 2004). Sullivan (1979) established the genus Eoglyptosaurus for the holotype of “G.” donohoei and referred another more complete skull (AMNH 7431) to it. Later, Sullivan (1986a) described a new, more complete skull (UCMP 12600) of Glyptosaurus sylvestris and revised the taxon accordingly. As a consequence of this revision, Hirsch et al. (1987), based on unpublished and unsubstantiated data, contended that Eoglyptosaurus donohoei could not be separated from Glyptosaurus sylvestris on the basis in differences in the pterygoid teeth and raised sub-conical osteoderms. This, in turn, prompted me (Sullivan, 1989) to synonymize E. donohoei with Glyptosaurus sylvestris and establish a new taxon, Proglyptosaurus huerfanensis for AMNH 7431, from the Huerfano Formation, of Colorado. However, it now seems that the late Wasatchian (Wa7) Eoglyptosaurus donohoei is distinct not only from the younger Bridgerian (Br2) Glyptosaurus sylvestris, but also from the Wasatchian (Br1a) Proglyptosaurus huerfanensis based on the features noted by me (Sullivan, 1979, Sullivan, 1986a and Sullivan, 1989). Smith (2011a) indicated the holotype of Eoglytosaurus (“Glyptosaurus”) has yet to be properly described and that further study of the homodont dentition of Proglyptosaurus huerfanensis needs to be made. The taxon Paraglyptosaurus (Glyptosaurus) princeps was placed into synonymy with G. sylvestris (Sullivan, 1989).

“Glyptosaurus” (Paraglyptosaurus) hillsi and Proglyptosaurus huerfanensis are two very different glyptosaurins and are two of the few that actually co-occur, not only in the same geographic area, but nearly at the same stratigraphic level (Br1a). “Glyptosaurus” (Paraglyptosaurus) hillsi (holotype USNM 6004) is characterized, in part, by having a broad skull, with flat hexagonal osteoderms, heterodont dentition, and the posterior teeth expanded into a massive crushing dentition. Thus, it departs from Proglyptosaurus huerfanensis which has a relative narrow skull, more numerous inflated hexagonal osteoderms and homodont dentition. The Huerfano Formation material, that previously formed part of the hypodigm of “Paraglyptosaurus” princeps and was subsequently synonymized with Glyptosaurus sylvestris (Sullivan, 1989), probably pertains to either “Glyptosaurus” (Paraglyptosaurus) hillsi or Proglyptosaurus huerfanensis on the basis of their geographic and stratigraphic origin.

The enigmatic taxon Xestops vagans (holotype USNM 16532, formerly YPM 541, see Meszoely, 1970), which is considered a primitive “melanosaurin” glyptosaurine, is known from only one specimen from the Bridger “B” Formation, Grizzly Buttes, Wyoming. It is characterized by the definitive tuberculate dermal armor that is characteristic of all glyptosaurines, but it retains a primitive anguid epidermal scale pattern, features that are seen in a number of obscure and little known glyptosaurine lizards. This primitive morphology caused Meszoely et al. (1978) to refer a number of European “Xestops”-like taxa to this genus. However, it has since been demonstrated that limited and superficial resemblances cannot be used to define similar-looking glyptosaurine lizards, and that the European “melanosaurins,” while similar in some respects to the North American taxon Xestops, have proven to be distinct (see Keller et al., 1991 and Keller, 2009).

Hecht (1959) named Dimetoposaurus wyomingensis, based on a single complete left frontal (AMNH 3819) from the Tabernacle Butte area, upper Bridger Formation (Br3) of Wyoming, which he considered to be a primitive diploglossine and not a glyptosaurine. Although it is a glyptosaurine lizard, it differs from Xestops vagans in a number of respects, most notably having narrow frontals and a larger interparietal scute, contrary to the observations of Meszoely (1970). It was synonymized with Xestops vagans by Meszoely (1970) based largely on having a similar morphology and the same stratigraphic position. Estes (1983a), following Meszoely (1970), also considered it to be a synonym of Xestops vagans. However, Dimetoposaurus wyomingensis comes from slightly younger deposits (Br3, not Br2). Given the morphological differences, as well as the different stratigraphic level from which it came, I reject the synonymy of Meszoely (1970) and, accordingly, resurrect Dimetoposaurus wyomingensis as a valid melanosaurin glyptosaurine.

Glyptosaurins are more prominent in the middle Eocene (Bridgerian) and the latest Eocene (Chadronian) of North America. The holotype of Glyptosaurus sylvestris (UNSM 16523) is a single left frontal from the Bridger “B” Formation of Grizzly Buttes, Wyoming, which is Br2 level (Robinson et al., 2004). Numerous specimens from this interval have been assigned to the taxon (Sullivan, 1979) and subsequently, following the synonymy of Paraglyptosaurus princeps with G. sylvestris (Sullivan, 1989). This material was augmented by a complete skull from the (upper) Black's Fork Member, Bridger “B” Formation, Uinta County, Wyoming of the same level and age (Br2).

In contrast to middle Eocene taxa, late Eocene, Uintan-age glyptosaurine lizards from North America are poorly known. Schatzinger (1975) was the first to report dentary and osteoderm material from the Uintan of San Diego County, California (Friars and Mission Valley formations), but because of the fragmentary nature of the material, no generic and specific identifications could be made. More recently, Moscato (2013) described the most complete specimen recovered to date from the Santiago Formation (Ui3, see Robinson et al., 2004) of San Diego County, California. This specimen (SDNHM 75932) consists of the anterior portions of both frontals, a right prefrontal, posterior portion of the right maxilla, both jugals, greater posterior part of the left dentary, left humerus, and numerous body osteoderms, some of which remain articulated. It appears that this glyptosaurin is a sub-adult based on the incomplete fusion of cranial osteoderms on the frontals and absence of osteoderms on the jugals. Comparison of SDNHM 75932 to Glyptosaurus sylvestris, in particular to UCMP 12500 (see Sullivan, 1986a), led Moscato to conclude that it was referable to that species based on having: 1) flattened frontals; 2) broad cranial osteoderms; and 3) concentric rows of tubercles on the osteoderms. The frontals are extremely narrow, and the frontal/nasal articulation facet is clearly visible on both the left and right frontal. Two cranial, or cephalic, osteoderms are preserved along the right lateral side of the right frontal, representing the remnants of the right orbital row adjacent to the prefrontal. All the remaining cranial osteoderms were unfused and presumably detached from the frontals prior to burial, as is commonly reported for Helodermoides (Sullivan, 1979). The two frontals are in near contact, thus they indicate the anterior width of these two elements. The left lateral side of the right frontal suggests room for only a half of a single row of cephalic osteoderms, so there would be only three rostral-caudal rows across the width of the frontals at this location. This is even fewer than in Stenoplacosaurus (i.e. four--two on each frontal). Considering the incomplete and sub-adult nature of the specimen, and given the stratigraphic and geographic occurrence, it seems probable that the San Diego specimen represents an unnamed glyptosaurin and thus it is not referable to the Bridgerian taxon Glyptosaurus sylvestris. The only other published accounts of glyptosaurine lizards from Uintan-age strata were by Westgate (1989), based on isolated glyptosaurine osteoderms from the Laredo Formation (also Ui3, see Robinson et al., 2004) of West Texas, and a glyptosaurin cephalic osteoderm from the Devil's Graveyard Formation (Ui3), also of West Texas (Stocker and Kirk, 2016).

The late Eocene (Chadronian) glyptosaurin Helodermoides tuberculatus is the best known of all the glyptosaurins and is represented by a number of specimens that span Ch-2 to Ch-4 (middle-late Chadronian). A few specimens, and some of the largest, purportedly came from the Orellan (Gilmore, 1938). However, both late Chadronian and early Orellan strata occur in the area where they were found, and the marker beds that divide these two ages in this part of eastern Wyoming were not recognized at the time they were collected (Prothero, pers. comm. to RMS, 2019). It seems more likely that the specimens originated from the latest Chadronian age strata of eastern Wyoming rather than early Orellan strata. Nevertheless, the Orellan represents a small amount of time, and it seems clear that Helodermoides became extinct at, or shortly after, the Grande Coupure event in Europe and the Eocene/Oligocene transition in North America.

The “melanosaurin” Peltosaurus granulosus (which here includes the species P. abbotti and P. floridanus, see Sullivan and Holman, 1996) is known from numerous specimens from the Orellan of North America, although there are unverified reports that a specimen is known from the Chadronian (“Titantotherium beds”) by Gilmore (1928) and Estes (1983a). Smith (2011b) questionably referred a parietal fragment and other material (dentary and maxillary fragments) to cf. Peltosaurus sp. Because the parietal lacks any hint of tuberculation, I regard this assignment to Peltosaurus to be erroneous. Peltosaurus was briefly addressed by Meszoely (1970), but no comprehensive study of Peltosaurus has been completed. Suffice to say, a number of fossil lizards have been identified as “Peltosaurus,” but these have subsequently been assigned to other taxa (see Estes, 1983a, Meszoely, 1970 and Sullivan and Holman, 1996). A single early Arikareean (early late “middle” Oligocene) record of Peltosaurus has been reported (see Sullivan and Holman, 1996), but this specimen needs to be re-evaluated. Recently, Scarpetta (2019) documented Peltosaurus granulosus from the upper Sharps and Monroe Creek formations of Sharps Corner, South Dakota, extending its range into the early late Oligocene (Ar1 and Ar2 of the Arikareean, respectively; Tedford et al., 2004). Putative Miocene occurrences of Peltosaurus have been reported as well, but these latter specimens have been misidentified (Sullivan and Holman, 1996). Thus, with the exception of P. granulosus, it seems that nearly all glyptosaurine lizards became extinct shortly after the Grande Coupure event in Europe and the Eocene-Oligocene transition in North America.

European Paleocene herpetofaunas were recently summarized by Rage (2012) to include only caudates, anurans, lizards, amphisbaenians, scolecophidians and boids. With respect to the lizards, only scincids and necrosaurids are known. This lies in stark contrast to the Paleocene of North America, where Paleocene herpetofaunas are far more diverse (Estes, 1975, Estes, 1976, Gilmore, 1942 and Sullivan, 1991). Moreover, anguids, represented primarily by the proto-glyptosaurines Parodaxosaurus-Proxestops-like taxa, dominate part of the herpetofauna in North America.

In Europe, Glyptosaurine lizards, and as a consequence the family Anguidae, together with both tribes, the Glyptosaurini and “Melanosaurini,” appeared abruptly in lowermost Eocene reference level MP7.

Hecht and Hoffstetter (1962) were the first to report on fossil amphibians and squamates from Dormaal (MP7), Belgium, but without figuring any of the material. Years later, additional glyptosaurine specimens from the Dormaal sands (Landen Formation) were the subject of a study by Sullivan et al. (2012), where they described a medial part of a right frontal pertaining to a “melanosaurin” (IRScNB R 266) with unusual epidermal scale impressions on the dorsal surface of the frontal. Since that study, one of the “melanosaurin” frontals described by Hecht and Hoffstetter (1962) resurfaced and was recently examined by one of us (RMS). This frontal (IRScNB R 392, Fig. 2), figured here for the first time, described by Hecht and Hoffstetter (1962), is nearly complete, is a little over 20 mm in length, the dorsal surface is covered with minute tubercles, and it has a sagittal suture that is clearly visible on the dorsal surface (Fig. 2a). The ventral surface (Fig. 2b) is completely fused, leaving no trace of the sagittal suture. This frontal departs from IRScNB R 266, figured by Sullivan et al. (2012, fig. 4), in that it lacks the anterior positioned epidermal scale impressions, but it bears the frontoparietal scale impression typically present in “melanosaurins”. The interparietal portion of the coalesced frontals is missing, as is the right posterolateral side, so the corresponding interparietal and right frontoparietal scale impressions are not preserved. This frontal appears to be similar to that seen in both species of Placosauriops, but I refrain from assigning it to that taxon because of the limited amount of material upon which to base its identity.

Sullivan et al. (2012) also described a nearly complete left dentary (IRScNB R 263), which they questionably referred to Placosaurus, and established a new species for its reception, ?P. ragei. A nearly complete parietal table (IRScNB R264) was referred to the same species (cf.?P. ragei), as it originated from the same locality as the dentary. The dentary of?Placosaurus ragei, which is remarkably well-preserved, has a weakly developed heterodont dentition bearing space for only 19 teeth. The dentary had previously been referred to the stratigraphically younger cf. Paraplacosauriops quercyi (see Augé and Sullivan, 2006).

In 1940, Oskar Kuhn described a number of anguid lizards from the middle Eocene brown coals of Geiseltal, Germany (MP13), which is approximately 43.5 Ma. Among them were two “melanosaurins” that he named Placosauriops abderhaldeni and P. weigelti. Their morphological similarity to the North American Xestops vagans caused Meszoely et al. (1978) to synonymize the Geiseltal species with the genus Xestops, together with Paraxestops stehlini (Hoffsetter, 1962). While these Eocene lizards superficially resemble the general pattern of cranial epidermal scutes seen in the North American Xestops, the shape and the size of the European taxa are readily distinguishable from Xestops. This fact led Keller et al. (1991) to suggest that they be returned to their respective genera and that the synonymy with their North American cousin Xestops be rejected. In their review of the European “melanosaurins,” Augé and Sullivan (2006) reaffirmed the distinct nature of the European Placosauriops from the North American taxon Xestops based large on the morphology of the frontals, including the patterns of epidermal scale impressions, and noted that the characters used by Meszoely et al. (1978) to combine the two taxa were either inaccurate or misinterpreted. More importantly, Augé (2005) and Augé and Sullivan (2006) synonymized Placosauriops abderhaldeni with P. weigelti, because the differences between the taxa were viewed as trivial and because they came from the same deposit (Lagerstätte).

Keller (2009) was the first to describe, in great detail, the excellently preserved specimens from Messel (MP11), considered by Franzen (2005) to be 47 Ma, and he assigned all the material to “Placosauriops abderhaldeni”. As pointed out by Keller (2009), Kuhn (1940) did not provide a diagnosis for either Placosauriops abderhaldeni or P. weigelti, and the diagnoses provided for these species by Meszoely et al. (1978) were based solely on comparisons to the North American taxon Xestops vagans. The type species “P. abderhaldeni” is distinguished by frontals with concave orbital borders and being slightly smaller than P. weigelti, which, in turn, is characterized by dorsal keeled osteoderms (Estes, 1983a, Keller, 2009 and Meszoely et al., 1978). Keller (2009) not only suggested that “P. abderhaldeni” was not properly diagnosed, but that some of the features noted by Meszoely et al. (1978) should be removed from the diagnosis based on the Messel specimens. Although Keller (2009) suggested some features for discriminating P. abderhaldeni, primarily concerning the osteoderms of the orbital region of the frontal, number of presacral dorsal osteoderms transverse series, as well as the overall length and snout/vent lengths, he was unable to formally diagnose the species. Moreover, he did not offer any clarity as to why the Messel specimens should be referred to the species “Placosauriops abderhaldeni,” let alone to the genus Placosauriops, despite the fact that there are approximately 4.5 Ma between the faunas of the Grube Messel and the younger Geiseltal Lagerstätte.

Hoffstetter (1962) named Paraxestops stehlini based on a nearly complete fused frontal and complete mandible from the upper Eocene (MP14) of Mormont-Saint-Loup, Switzerland. The species was synonymized with Xestops by Meszoely et al. (1978), largely because of its superficial resemblance to the North American taxon. Augé and Sullivan (2006) again pointed out differences between the frontals of the two taxa and thus resurrected the genus Paraxestops for “X.” stehlini.

Augé and Sullivan (2006) established the genus Paraplacosauriops for material previously referred to “Placosaurus” quercyi from the Phosphorites du Quercy (MP16). Paraplacosaurus quercyi is the chronostratigraphically youngest and largest European “melanosaurin”. It has a pronounced heterodont dentition and rectangular plate-like osteoderms fused to the lateral ascending portion of the maxilla. Other dentary material, some very fragmentary, was assigned to cf. Paraplacosaurus quercyi, and all are from the early Eocene (MP7, 8 + 9). However, the most complete specimen, a left dentary with heterodont dentition (IRScNB D 02) from Dormaal (MP7), is more gracile than the neotype (MNHH QU-16569), in which the dentary is not as deep or robust. The fragmentary dentary bearing heterodont teeth (MNHN CB 16445) from the early Eocene of Condé-en-Brie, France (MP 8 + 9) was considered too equivocal to be referred to P. quercyi. It seems that none of these European specimens are referable to Paraplacosauriops quercyi, rather they are incomplete remains most likely from some unknown taxa.

It should be abundantly clear, based on the conclusions presented by Augé and Sullivan (2006), and on the foregoing review of known “melanosaurin” taxa, that the European “melanosaurins” are in need of a thorough restudy and revision. This is similar to the situation with the North American genera “Proxestops” and “Glyptosaurus”, where the assignment of fragmentary material to these genera from outside the type localities is problematic at best. It is doubtful that Placosauriops and/or Paraplacosauriops can be confidently assigned to the Dormaal material, nor can the Swiss taxon Paraxestops stehlini be referred to these taxa based on the known material. Even the nearly complete, beautifully preserved Messel specimens assigned to “Placosaurus abderhaldeni” by Keller (2009) must be viewed with skepticism.

It is noteworthy, if not perplexing, that aside from the occurrence of?Placosaurus ragei from Dormaal, glyptosaurins are largely absent for most of the early and middle Eocene of Europe. One exception is a specimen consisting of articulated osteoderms from the posterior border of the cranial region that Kuhn (1940) described and illustrated as “Placotherium (Loricotherium) weigelti.” This specimen (Kuhn, 1940 Tafel VIII, figs. 1a, b) is of the dorsal neck region, based on the hexagonal osteoderms from the posterior portion of the parietal area and rectangular dorsal osteoderms from the anterior part of the shoulder area. Estes (1983a) reviewed the nature of the material, and the taxonomic history behind the interpretation of this unnumbered specimen. Other unnumbered isolated osteoderms, some hexagonal, were also figured in the same plate (see Kuhn, 1940, Tafel VIII, figs. 2–5), and were identified as belonging to “Placosaurus waltheri.” Estes (1983a) regarded this taxon as a nomen dubium. Whether this material can be referred to Placosaurus or to some other taxon, the importance of these specimens lies in the fact that they indicate the presence of glyptosaurins in the Lagerstätte of Geiseltal.

The holotype of the glyptosaurin Placosaurus rugosus (MNHN 1906-25) from the late Eocene of La Débruge (MP18) was redescribed and rediagnosed by Sullivan and Augé (2006) together with a new species, P. estesi (holotype MNHN QU-1773), from the new standard level of Fons 4, MP17 (Pierrère, Phosphorites du Quercyi). Aside from being slightly chronostratigraphically younger than Placosaurus rugosus, P. estesi differs from P. rugosus in being more robust, dorsally having many more hexagonal cephalic osteoderms along the posterior midline region of the coalesced frontals, and, ventrally, the cristae cranii frontalis are less divergent posteriorly.

In summary, the European glyptosaurines are represented by both glyptosaurins and “melanosaurins” early in the Eocene (MP7), but their generic and specific identities are far from certain due to the incomplete material upon which many taxa have been based. Comparisons of the somewhat complete Geiseltal (MP13) “melanosaurins” to excellently preserved “melanosaurins” of the older Messel Lagerstätte (MP11) are problematic based on our currently understanding of characters used to distinguish the species. In general, it seems that a number of small “melanosaurins” are known throughout the early and early late Eocene (Robiacian) of Europe (MP7–MP14). Glyptosaurins are known throughout the Eocene of Europe, but their record is spotty. They are conspicuously absent from the Messel Lagerstätte and represented by only isolated osteoderms and part of a dorsal neck region from the younger Geiseltal brown coals.

The record of glyptosaurines in Asia is extremely poor. None have been recovered from the Paleocene deposits, in which only anguimorphs (Anguiodea indet.) have been documented (Ni et al., 2016 and Van Itterbeeck et al., 2007) and Varaniformes (Dong et al., 2016). The Eocene record for Asia is only slightly better. The glyptosaurin Stenoplacosaurus mongoliensis is the only reported Asian glyptosaurine known (Sullivan and Dong, 2018). Up until recently only two specimens have been documented, the holotype AMNH FR 6669, first described by Gilmore (1943) and IVPP V898, described by Chow (1957) from the Shara Murun of Inner Mongolia China and the Heti Formation of Henan, respectively. New referred frontal material from near the type locality, which has yet to be described, further documents this taxon. However, these new specimens provide little additional knowledge regarding the morphology of these armored lizards.

The origin of glyptosaurine lizards probably occurred in North America based on the occurrence of the Late Cretaceous taxon Odaxosaurus piger, known mostly from deposits of Wyoming. Estes (1983a) considered Odaxosaurus to be “broadly ancestral” to the Glyptosaurinae. Paleocene proto-glyptosaurines, represented by the form genus “Proxestops,” are known from the early Paleocene to the earliest Eocene of the North American Rocky Mountain region. These proto-glyptosaurines are considered transitional to the bona fide glyptosaurines and occur from the early Paleocene (Puercan) through early Eocene (Wasatchian). They probably represent a number of genera and species, but because of a poor fossil record and fragmentary remains, their taxonomic identities remain elusive. Interestingly, proto-glyptosaurines co-existed alongside the earliest glyptosaurines, suggesting an early and rather rapid diversification of species.

The earliest “melanosaurin” is now known from the early late Paleocene (To3) of New Mexico based on an undescribed nearly complete frontal (NMMNH P-58618) that displays prominent tuberculated dermal armor and retains the primitive pattern of epidermal scales and underlying osteoderms. The appearance of the “melanosaurins” this early is not surprising considering that similar taxa are known from the earliest Eocene (MP7) of Belgium. Dispersal of the early “melanosaurins” and glyptosaurins from North America to western Europe no doubt occurred prior to the Paleocene-Eocene thermal maximum and may have continued into the earliest Eocene.

Small “melanosaurins” represented by Gaultia silvaticus (Wa0), a species that seems transitional to glyptosaurins and considered to be the sister taxon to the glyptosaurins, is one of the earliest Eocene glyptosaurines. “Melanosaurins” continued to flourish throughout the Wasatchian of North America, represented by larger taxa such as Melanosaurus maximus (Wa2–Wa3) and Arpadosaurus gazinorum (Wa6–Wa7). The Wasatchian glyptosaurin taxa “Glyptosaurus” rhodinos (Wa5), “Glyptosaurus” (“Paraglyptosaurus’) yatkolai, including material referred to cf. “P. yatkolai” (Wa5–Wa6), “Glyptosaurus” agmodon (Wa6?), and Eoglyptosaurus donohoei (Wa7) suggest a major radiation and diversification of glyptosaurin lizards before the Bridgerian. A single glyptosaurine caudal vertebra, reported by Estes and Hutchison (1980) from the late Wasatchian Margaret Formation (Eureka Sound Group) of Ellesmere Island, Canadian Arctic Archipelago (see Stidham and Eberle, 2016), suggests these lizards were thriving there in the peak of global warmth before they dispersed into western Europe.

In western Europe, “melanosaurins” and glyptosaurins were well established by the early Eocene (MP7), represented by a small unnamed “melanosaurin” and a medium-sized glyptosaurin, ?Placosaurus ragei. Due to the poor fossil record, little is known about glyptosaurine evolution during the early Eocene of western Europe. It's not until the middle Geiseltalian (equivalent to the earliest Uintan of North America, see Fig. 1) that “Placosauriops abderhaleni” from the Messel fauna (MP11) makes an appearance. The amount of interchange between North America and Europe during the early Eocene based on glyptosaurines cannot be adequately demonstrated. Bridgerian (late early Eocene) glyptosaurines continued to diversify in North America. The small “melanosaurins” Xestops vagans (Br2) and Dimetoposaurus wyomingensis (Br3) are known from limited material, and it seems that larger “melanosaurins” were already extinct by the outset of the Bridgerian. Glyptosaurins include the sympatric taxa Proglyptosaurus huerfanensis (Br1a) and “Glyptosaurus” hillsi (also Br1a) and suggest further diversity among the Glyptosaurini. Arguably, the best known North American glyptosaurin, Glyptosaurus sylvestris, occurs later in the Bridgerian (Br2). There is no evidence that these later glyptosaurins had any connection to Europe during Bridgerian time. Late early Eocene (Bridgerian equivalent) glyptosaurines are not known from this interval in Europe or Asia.

Isolated osteoderms from the Baca Formation of New Mexico were also considered to be glyptosaurin, cf. Glyptosaurus (Lucas et al., 1983 and Lucas, 1997), but their identity is less certain. The age of the lower part of the Baca Formation (also referred to as the Hart Mine Formation) is considered to be Bridgerian (Lucas, 1997; pers. comm., 2019) and not Duchesnean age, as indicated by Robinson et al. (2004). The North American middle Eocene (Uintan) glyptosaurines are represented by few accounts. Isolated glyptosaurine osteoderms are known from Texas, (southern) California and North Carolina; all three of these records are known to be glyptosaurins (see above). One specimen, SDNHM 75932, from the late Uintan of San Diego, probably represents a new genus and species of glyptosaurin. European glyptosaurines of Uintan equivalent age, approximately middle to late Geiseltalian (MP11–MP13), are better known, but not adequately diagnosed or studied. These fossil lizards include a small “melanosaurin” taxon referred to “Placosauriops abderhaldeni,” known from many exquisitely preserved specimens from the Grube Messel (MP11) (Keller, 2009) and from the younger Placosauriops weigelti (MP13) from the brown coals of Geiseltal. Only one undiagnostic glyptosaurin specimen is known from the Geisetalian interval, demonstrating how incomplete the fossil record is for these armored lizards. There are no Uintan, middle or late Geiseltalian age equivalent glyptosaurines known from Asia. Presumably the Geiseltalian “melanosaurins” evolved from the early “melanosaurins” that took a foothold early on at the beginning of the Eocene (MP7) in Europe. In Europe, the small enigmatic “melanosaurin” Paraxestops stehlini (MP14) from Mont-Saint-Loup, Switzerland, is roughly equivalent in age to the early Duchesnean of North America. Rage and Augé (2010) reported on a small herpetofauna from the middle Eocene of Lissieu, France (MP14), that included the posterior portion of a left maxilla, which they identified as an indeterminate glyptosaurine. Based on their illustration (Rage and Augé, 2010, fig. 1.3, 1.4), the maxilla is clearly from a small “melanosaurin” glyptosaurine. It occurs at about the same level as the holotype of Paraxestops stehlini and may pertain to that species. The Asian glyptosaurin Stenoplacosaurus mongoliensis is known from the early Duchesnian time equivalent, approximately 41 Ma. Indeterminate glyptosaurine osteoderms were recently reported from the Brembridge Limestone (Bed 18) from the Hampshire Basin of England (MP20) by Klembara and Green (2010). They considered them to be Bartonian age, but they are now known to be younger (Priabonian) based on the correlation presented in Fig. 1. They are probably related to the late Eocene genus Placosaurus based on the known paleogeographic connection between what is now France and the United Kingdom.

The sudden appearance, and only glyptosaurine, Stenoplacosaurus from Asia (Inner Mongolia, China), poses an interesting question. Is this taxon more closely related to the North American taxa (i.e. Helodermoides, or the unnamed taxon represented by SDNMH 75932) or to the European Placosaurus? One can speculate that it is more closely related to the European taxa, a precursor to Placosaurus, or that it was derived from a western North America population of glyptosaurins, such as the unnamed San Diego specimen (SDNHM 75932), as the Beringian region provided a dispersal conduit for taxa from North America to Asia for much of the Paleogene (Beard and Dawson, 1999). In North America, the Chadronian record of glyptosaurin lizards is robust. Numerous specimens of Helodermoides tuberculatus are known (Sullivan, 1979), and the taxon is found through most of this land mammal age (Ch2–Ch4). It is arguably the most well-known and best documented glyptosaurine. Curiously, no North American “melanosaurins” have been reported for this time interval. In Europe, the “melanosaurin” Paraplacosauriops quercyi (MP16), and the glyptosaurins Placosaurus estesi (MP 17) and P. rugosus (MP18), are known from the North American Chadronian age equivalent. Finally, the “melanosaurin” Peltosaurus granulosus appears in the early Oligocene (Orellan) of North America, where it is known from hundreds of specimens. It apparently ranged through the middle Oligocene, and possibly later (Scarpetta, 2019), thus, it represents the youngest occurrence and the last of the glyptosaurine lizards.

In summary, it seems that glyptosaurines originated and dispersed from North America into northwestern Europe during the Paleocene/Eocene thermal maximum via the North Atlantic. Glyptosaurine populations in North America continued to diversify as endemic taxa throughout the early and middle Eocene. A notable decline in glyptosaurine species occurs during post-Bridgerian times in North America. Because of the brief intercontinental land bridge connection, no additional interchanges between North America and Europe occurred, and Europe was totally isolated by late Ypresian time (early Bridgerian in North America). The intermittent Beringia land corridor may have provided a dispersal route for late Eocene glyptosaurins into Asia, but a detailed study comparing Stenoplacosaurus to its North American cousins will need to be made in order to confirm this and will depend on acquiring more complete specimens.