Studied materials, CT-scanning and rendering
The described specimen is curated at the collections of the Gray Fossil Site & Museum (acronym ETMNH) in Gray, Tennessee, with collection number ETMNH 34820. Comparison material of Miopetaurista webbi is curated at the Florida Museum of Natural History in Gainesville, Florida. Specimens were photographed using either a Nikon D810 DSLR camera with an AF-S Micro Nikkor 60 mm lens or a DinoLite Edge AM4815ZT digital microscope camera.
The specimen described herein was scanned by microfocus X-ray computed tomography (micro-CT) using a Skyscan model 1273 (https://www.bruker.com) at East Tennessee State University. The data (as a stack of 8-bit tiff files) were later processed with the software Avizo 7.1 (Thermo Fisher Scientific, Waltham, Massachusetts, USA) using the semi-automatic image segmentation tools to create surface renderings. The CT image series and mesh models of the specimen are available at MorphoSource (see Availability of Data and Material).
Anatomical terminology and measurement methods
Dental terminology (Fig. 1) follows Casanovas-Vilar et al. (2015) and Sinitsa (2018), while the measurement method is after Casanovas-Vilar et al. (2015). The sciurid classification follows Steppan et al. (2004) and Thorington and Hoffmann (2005). All measurements are given in millimeters and were taken from the digital model of ETMNH 34820 using the measuring tools in the software Avizo 7.1 (Thermo Fisher Scientific, Waltham, MA, USA). Measurements for the comparative sample were taken from published sources.
Occurrence data and mapping
Fossil occurrences of the genus Miopetaurista were compiled after Casanovas-Vilar et al. (2015) and also downloaded from the NOW database of fossil mammals (The NOW Community 2024; see also Žliobaitė et al. 2023) after cross-checking the data with published literature. Data on the geographic range of extant Petaurista species were downloaded from the IUCN Red List of Threatened Species (IUCN 2024). Fossil occurrences of Petaurista were compiled from the literature and the NOW database. Data were plotted using the GeoMapApp (http://www.geomapapp.org).
Abbreviations
Measurement abbreviations
L=length; W=maximum width; w1= width at the anterior border of the molar; w2=width at the posterior border of the molar
Institutional abbreviations
ETMNH, Gray Fossil Site & Museum, East Tennessee State University, Gray, Tennessee
UF, Florida Museum of Natural History, University of Florida, Gainesville, Florida
Other abbreviations
GFS, Gray Fossil Site (Gray, Tennessee, USA)
MN, Mammal Neogene Zones for Europe (after Mein 1975, 1999, age boundaries follow Hilgen et al. 2012)
NALMA, North American Land Mammal Age (after Woodburne 2004)
Systematic paleontology
Order Rodentia Bowdich 1821
Family Sciuridae Fischer [von Waldheim] 1817
Subfamily Sciurinae Fischer [von Waldheim] 1817
Tribe Pteromyini Brandt 1855
Subtribe Pteromyina Thorington and Hoffmann 2005
Genus Miopetaurista Kretzoi 1962
Miopetaurista webbi (Robertson 1976)
Figure 1
Material. An isolated left m3 (ETMNH 34820)
Measurements. (L x w1 x w2) 4.65 x 3.66 x 2.53
Remarks on the taxonomic history of the genus Miopetaurista
The genus Miopetaurista has a complex history that can be particularly confusing when consulting old literature. Kretzoi (1962: 307) introduced this genus name, for which he did not provide a diagnosis, for the large-sized flying squirrels of the European Miocene and proposed Sciurus gibberosus Hoffmann (1893) as its type species. Later on, Mein in his review of European Neogene flying squirrels (Mein 1970) considered this species to be synonymous with Sciuropterus albanensis Forsyth Major (1893), so the genus name Miopetaurista and the emended diagnosis proposed by Mein (1970) was matched with this species. In the same work, Mein (1970) introduced the genus Cryptopterus for the largest flying squirrels in the European Neogene, including the species Cryptopterus gaillardi, Cryptopterus neogrivensis and Cryptopterus thaleri, amongst others. These would differ from Miopetaurista by their larger size, protoloph and metaloph not converging towards the protocone in the upper cheek teeth and presence of an anterosinusid in the lower cheek teeth, amongst other features (Mein 1970). However, Daxner-Höck (1975) subsequently reviewed the type of Sciurus gibberosus and found that the crushed specimen had been incorrectly glued and heavily varnished so that its morphology and proportions could not be adequately determined, even before its first description and depiction by Hoffmann (1893). Unfortunately, Pierre Mein's (1970) taxonomic conclusions were based on these wrong descriptions and figures. After Gudrun Daxner-Höck’s detailed observations on the holotype Daxner-Höck and Mein (1975) concluded that Sciurus gibberosus better fit the diagnosis for Mein’s genus Cryptopterus, which would automatically be replaced by Miopetaurista, because it had been erected previously. The smaller flying squirrel species that Mein (1970) had placed within the genus Miopetaurista would then be transferred to the new genus Albanensia Daxner-Höck and Mein (1975) with Sciuropterus albanensis Forsyth Major (1893) as type species.
This taxonomic history must be taken into account when reviewing the North American record of flying squirrels. For example, the differential diagnosis of the North American flying squirrel genus Petauristodon (Engesser 1979) refers to Cryptopterus, which includes the species currently allocated within Miopetaurista. Perhaps more importantly, the first citation of the genus Miopetaurista in North America is that by Robertson (1976), who erected the species Cryptopterus webbi for a specimen found at Haile 15A (Florida). Although published in 1976, Robertson’s work was accepted for publication in 1974 (Robertson 1976: 111), so the synonymy between Cryptopterus and Miopetaurista was not yet known. Later reports of the genus in North America, such as Webb et al. (2008), correctly use the combination Miopetaurista webbi, but surprisingly the combination Cryptopterus webbi still is used by Goodwin (2008) in his review of the North American fossil record of the Sciuridae in Janis et al. (2008).
Description
The molar has a subrectangular occlusal outline. The anterior margin is straight, while the posterior one is slightly curved. The enamel is not crenulated. There are four major cuspids: metaconid, protoconid, hypoconid and entoconid. The metaconid is the largest and most prominent cuspid and it presents a metaconid crista originating from its posterolingual corner that ends in a prominent cuspid as high as the entoconid. This cuspid is followed posteriorly by the double mesostylid, which consists of two slightly lower cuspids. The groove separating the two mesostylid cuspids is noticeably shallower than those that separate the mesostylid from the metaconid and the entoconid. The metaconid connects with the protoconid by means of a vestigial anterolophid. There is a small and deep anterosinusid in the anterobuccal corner of the molar, which is partially closed by a short anterior cingulid. The metalophid is short and bifurcates into two branches: a short and thick one that curves anteriorly to merge the metaconid base, and a thinner and more sinuous one which is directed posterolingually towards the mesostylid but disappears before reaching it. The ectolophid is continuous and M-shaped, presenting a well-developed mesoconid. The sinusid, deep and transverse, is not closed by any cingular formations. The entolophid is high, thick, straight and continuous. This ridge merges the entoconid with the ectolophid just posterior to the mesoconid and anterior to the hypoconid. The mesolophid is absent. The posterolophid is continuous and very thick, further being as high as the entoconid. This ridge is constricted in its posterolabial border. The tooth presents four cylindrical roots, albeit only its base is preserved. There is one large root below the hypoconid and three smaller ones below the protoconid, metaconid and entoconid, respectively, which are of subequal size.
Discussion
ETMNH 34820 can be assigned to the genus Miopetaurista because of the following diagnostic features (Mein 1970): weakly to moderately crenulated enamel, presence of anterosinusid, well-developed anterolophid, isolated mesostylid and m3 with non-reduced distal part. It differs from the North American genus Petauristodon Engesser (1979) most notably by its larger size, less crenulated enamel, and presence of anterosinusid. It also differs from Petaurista, which is first reported from Pleistocene sites of Eurasia (Jackson and Thorington 2012), by its different shape, less complex dental pattern and presence of a well-defined mesoconid. Miopetaurista is widespread and represented by various species in Eurasia (Jackson and Thorington 2012; Casanovas-Vilar et al. 2018) during the Miocene and the Pliocene, but in North America only the species Miopetaurista webbi (Robertson 1976) occurs. In turn, this is incredibly rare and only two additional specimens are known (Fig. 2): the holotype (UF 12353), a mandible with an m3 from Haile 15A (earliest Pleistocene, Florida); and a mandibular fragment with m2–m3 (UF 95701) from the Palmetto Fauna (Early Pliocene, Florida; Webb et al. 2008). Both specimens are clearly distinguished from Petauristodon and show the diagnostic features for the lower molars of Miopetaurista (see above and Mein 1970), such as the larger size, simpler dental morphology and presence of anterosinusid. The size of ETMNH 34820 fits that of the species M. webbi (Robertson 1976; Webb et al. 2008), albeit it is slightly shorter (Fig. 3, Table 1). It also shares with this species the complete transverse entolophid (referred to as hypolophid in Robertson 1976), incomplete metalophid (referred to as protolophid in Robertson 1976) and weakly crenulated enamel. Robertson (1976) remarks that M. webbi differs from all other Miopetaurista species in having a small hypoconulid. However, this feature is a misinterpretation of a worn part of the thick posterolophid in the type specimen. ETMNH 34820 also differs from previously described specimens of M. webbi in certain features, such as the absence of the low sinuous ridges in the trigonid basin, just at the base of the metaconid, which are called ‘metaconid-metastylid [=mesostylid] chaos’ by McKenna (1962). These are evident in all other specimens of M. webbi, although they are considerably worn in the holotype (UF 12353, Fig. 2), and resemble the morphology of the genus Petaurista, in which these ridges are conspicuously higher (McKenna 1962). However, there is no trace of them in ETMNH 34820, and this cannot be attributed to dental wear, which is almost nonexistent. Also, in all the Florida specimens of M. webbi the mesostylid is simple, whereas in ETMNH 34820 it is double. Finally, in M. webbi, particularly in the specimen from the Palmetto Fauna (UF 95701, Fig. 2), the enamel in the trigonid basin is clearly more crenulated than in the specimen from the GFS. Whether these morphological differences are intraspecific is questionable. Large collections of Miopetaurista are rare, so intraspecific variability has not been adequately assessed. Yet, relatively large samples have been described for Miopetaurista crusafonti (Casanovas-Vilar et al. 2015), Miopetaurista neogrivensis (Casanovas-Vilar et al. 2018) and Miopetaurista thaleri (de Bruijn 1995), and these show some variability in the lower molars, particularly in the m1/m2, and for M. thaleri in the m3. Intraspecific variation mostly refers to the development of the mesostylid, which may be simple or double in M. crusafonti (Casanovas-Vilar et al. 2015), to the length of the entolophid, and to the development of the mesolophid. Once this variation is taken into account, and further considering that M. webbi is the only species known from North America, we ascribe the specimen form the GFS to this taxon, although we acknowledge that this conclusion is subject to an evaluation of the intraspecific variability in this poorly known species. The GFS may represent the oldest record of Miopetaurista in North America, though the Palmetto Fauna of Florida and Gray share a number of taxa and are currently considered roughly contemporaneous.