Gliding between continents: A review of the North American record of the giant flying squirrel Miopetaurista (Rodentia, Sciuridae) with the description of new material from the Gray Fossil Site (Tennessee)

doi:10.21203/rs.3.rs-5305463/v1

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Gliding between continents: A review of the North American record of the giant flying squirrel Miopetaurista (Rodentia, Sciuridae) with the description of new material from the Gray Fossil Site (Tennessee)

https://doi.org/10.21203/rs.3.rs-5305463/v1

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Flying squirrels (Rodentia, Sciuridae, Sciurinae, Pteromyini) have a long and complex history in North America. First recorded during the Late Eocene, they vanished during the early Late Miocene (at about 9 Ma) only to re-appear in the Pliocene and Pleistocene. The first flying squirrels to be recorded after this Late Miocene gap are surprisingly attributed to the Eurasian genus of giant flying squirrel Miopetaurista. These are just two specimens from Florida that purportedly belong to Miopetaurista webbi, an endemic species. In this work we review these occurrences and further describe a new specimen from the Early Pliocene (latest Hemphillian or early Blancan) Gray Fossil Site in Tennessee, which may represent the oldest record of the genus in North America. We validate their attribution to Miopetaurista webbi and found this species to be probably closely related to Miopetaurista thaleri, the only known Pliocene Eurasian species. The occurrence of Miopetaurista in eastern North America is puzzling, as it is set far away from the known geographical range of the genus and of that of its sister taxon, the extant Petaurista. We hypothesize that Miopetaurista, which was linked to warm forested environments, dispersed into North America via the Bering Land Bridge during the warm phases of the Early Pliocene in the frame of a major faunal dispersal involving many other taxa. Later climatic cooling isolated these squirrels in warmer refuges, such as Florida, until they finally became extinct during the Pleistocene.

Pteromyini

Sciurinae

paleobiogeography

Pliocene

Pleistocene

Tennessee

The North American record of flying squirrels

The history of flying squirrels (Sciuridae, Sciurinae, Pteromyini) in North America is longer than elsewhere, since it started during the Late Eocene and extends into the present. Much of this story is known mostly thanks to relatively common isolated cheek teeth, maxillary and mandibular fragments found throughout the continent (Goodwin 2008; Jackson and Thorington 2012; Lu et al. 2013; Casanovas-Vilar et al. 2018). Even though the assignment of much of the material to flying squirrels may be questionable and could only be confirmed if diagnostic postcranial elements are found (Thorington et al. 2005; Casanovas-Vilar et al. 2018), it is generally assumed that they represent pteromyins. The fossil record of flying squirrels in North America (and indeed that of the group worldwide), starts with the Late Eocene (36.6–35.8 Ma) Hesperopetes thoringtoni, one of the earliest squirrels, for which only isolated cheek teeth are known (Emry and Korth 2007). Hesperopetes disappeared during the Early Oligocene, but two other genera of flying squirrels are then recorded in North America, the minute Sciurion during the Early Oligocene, and the much larger Petauristodon during the latest Oligocene (Goodwin 2008; Jackson and Thorington 2012; Korth and Samuels 2015; Casanovas-Vilar et al. 2018). These had been previously referred to the Eurasian genera Blackia and Miopetaurista (Goodwin 2008), respectively, before they were recognized as endemic genera including multiple species. Flying squirrels are widespread during the Miocene in North America, but were never as diverse in Eurasia, where more than 10 different genera are known from Miocene deposits (Jackson and Thorington 2012; Lu et al. 2013; Casanovas-Vilar et al. 2018). In North America diversity is reduced solely to the genera Sciurion and Petauristodon. In an unpublished bachelor thesis, Biedron (2016) also reported the Eurasian genera Blackia and cf. Miopetaurista from the Middle Miocene of Cave Basin (Oregon). However, after the examination of pictures of this material we preliminarily conclude that the material identified as Blackia rather belongs to Sciurion, while the scarce material ascribed to cf. Miopetaurista belongs to a distinct genus of flying squirrel, but not related to the Eurasian genus. The endemic North American genera Sciurion and Petauristodon persisted until the beginning of the Late Miocene (late Clarendonian; Goodwin 2008; Samuels and Hopkins 2017), their extinction being probably related to the expansion of C₄-dominated grassland ecosystems in the continent (Casanovas-Vilar et al. 2018). After their disappearance, there is a flying squirrel gap of about 4 million years until the Early Pliocene (latest Hemphillian or early Blancan), when the Eurasian genus Miopetaurista is represented by a single mandibular fragment with m2–m3 in the Palmetto Fauna of Florida (Webb et al. 2008). In a previous work, a new American species of this genus, Miopetaurista webbi (Robertson 1976) (originally included within the genus Cryptopterus, see below), was been erected based on a single mandible with an m3 from Haile 15A, also from Florida, dating back to the earliest Pleistocene (late Blancan). Given the scarce material recovered at both sites the ascription to Miopetaurista remains contentious and is carefully evaluated in this work. Slightly later, the first representatives of the much smaller extant genus of North American flying squirrels, Glaucomys, are recorded in the Early Pleistocene (Irvingtonian) Inglis 1C site, also in Florida (Ruez 2001). Today, Glaucomys includes three species which are widespread in the coniferous and deciduous forests in the continent (Arbogast et al. 2017).

The purported occurrence of Miopetaurista in North America is puzzling because it is set very far away from all other records of the genus and also because it represents its youngest record, since the latest Eurasian species, Miopetaurista thaleri (Mein 1970) is last recorded during the Late Pliocene (Mein 1970; Jackson and Thorington 2012; Casanovas-Vilar et al. 2018). Miopetaurista was one of the most successful genera of flying squirrels in Eurasia, including 10 different species and ranging from Early Miocene to the Pliocene, and from Portugal to China, but being absent from the most septentrional parts of Eurasia (Jackson and Thorington 2012; Lu et al. 2013; Casanovas-Vilar et al. 2018). It is also the best known of all extinct flying squirrels thanks to the description of an exceptionally complete skeleton of Miopetaurista neogrivensis (Mein 1970) from Catalonia (Spain) and dating back to 11.64 Ma (Casanovas-Vilar et al. 2018). The postcranial skeleton of M. neogrivensis, and particularly its wrist bones, unambiguously proves that this is a flying squirrel belonging to the large-sized flying squirrel clade (subtribe Pteromyina). Furthermore, total-evidence phylogenetic analyses recognize it as sister taxon of the extant giant flying squirrels of the genus Petaurista, of which Miopetaurista only differs in details of its cranial and dental anatomy (Casanovas-Vilar et al. 2018; Grau Camats 2019). According to these analyses, Petaurista would have diverged during the Late Miocene and diversified mostly during the Pliocene and the Early Pleistocene. Now the genus occurs in various kinds of forested habitats in south and southeastern Asia, particularly favoring tropical and subtropical forests (Thorington et al. 2012; Koprowski et al. 2016). Nonetheless, during the Early Pleistocene its biogeographic range also comprised Europe (Jackson and Thorington 2012; Casanovas-Vilar et al. 2018). In this work, we review the North American material attributed to Miopetaurista and further describe new fossil remains from the Early Pliocene (latest Hemphillian or early Blancan) Gray Fossil Site in Tennessee (United States of America). Our results are discussed in the frame of the biogeography of the genera Miopetaurista and Petaurista as well as in relation to major mammal dispersal events between both continents during the Late Neogene.

The Gray Fossil Site

The Gray Fossil Site (GFS) of northeast Tennessee presents an amazingly detailed record of Early Pliocene life in the southern Appalachian region. The GFS biota is incredibly diverse and well-preserved, with over 100 species of plants and 100 species of animals recovered from site (e.g., Parmalee et al. 2002; Wallace and Wang 2004; Boardman and Schubert 2011; Zobaa et al. 2011; Mead et al. 2012; Ochoa et al. 2012; Worobiec et al. 2013; Bourque and Schubert 2015; Ochoa et al. 2016; Jasinski and Moscato 2017; Jasinski 2018; Short et al. 2019; Siegert and Hermsen 2020; Quirk and Hermsen 2021; Bōgner and Samuels 2022; Oberg and Samuels 2022; Hermsen 2023). Sediments at GFS are interpreted as representing an ancient sinkhole with a small, deep lake that filled over a span of thousands of years (Shunk et al. 2006; Shunk et al. 2009). Age of the site is inferred to be early Pliocene (late Hemphillian or early Blancan NALMA) based on mammal biochronology, between ca. 4.9–4.5 Ma (Samuels et al. 2018; Samuels and Schap 2021). Both the flora and fauna of the site are interpreted as representing a relatively closed forested ecosystem (Ochoa et al. 2016; Samuels et al. 2018; Quirk and Hermsen 2021), with plant communities consisting predominantly of oak, hickory, and pine forest (Ochoa et al. 2016), similar to current floras of lower elevation habitats in the southern Appalachian Mountains (Wallace and Wang 2004; Gong et al. 2010). Components of the flora at the site indicate Asian influences (Liu and Jacques 2010; Gong et al. 2010; Ochoa et al. 2012; Quirk and Hermsen 2021; Hermsen 2021), as do some elements of the mammal fauna (Wallace and Wang 2004; Oberg and Samuels 2022). Presence of abundant fish, frogs, neotenic salamanders, aquatic turtles, alligators, beavers, and desmans (Parmalee et al. 2002; Boardman and Schubert 2011; Mead et al. 2012; Bourque and Schubert 2015; Jasinski 2018; Oberg and Samuels 2022) all indicate the lake was persistent year-round. Exceptionally abundant tapirs (Tapirus polkensis, Hulbert et al. 2009), relatively common occurrences of the ailurid Pristinailurus (Wallace and Wang 2004) and tree squirrels (Crowe 2017), coupled with pronounced rarity of cursorial mammals at the site reflect the forested nature of the habitat. The presence of some plants indicative of warmer climates and reptiles intolerant of freezing conditions (Alligator, Heloderma) (Parmalee et al. 2002; Mead et al. 2012) combined with ecometric estimates (Schap et al. 2021) indicate that the early Pliocene climate of Gray was warmer and wetter than at present.

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.

Affinities of American Miopetaurista

Robertson (1976) compared M. webbi to Miopetaurista tobieni, the youngest European species of the genus. De Bruijn (1995) noted that M. tobieni (type locality Wölfersheim, Germany, MN15) basically differs from M. thaleri (type locality Marnes de Celleneuve, France, MN14) by its slightly larger size and considered them to be synonyms. Both species were erected by Mein (1970) but page priority determines that the name M. thaleri has precedence. Miopetaurista thaleri is known from the latest Miocene (MN13) to the Late Pliocene (MN16) and ranges from France to Greece. Miopetaurista webbi is indeed a large-sized Miopetaurista species, but it is somewhat smaller than M. thaleri, being more comparable to the older Middle and Late Miocene species M. neogrivensis (Fig. 3, Table 1). Furthermore, the m3 are generally narrower than in other large-sized Miopetaurista species, such as M. neogrivensis and M. thaleri (Fig. 3, Table 1). The shared morphological features between M. thaleri and M. webbi outlined by Robertson (1976) include the distinct anterosinusid, short and backwards-directed metalophid, distinct entoconid and complete entolophid. As discussed before, the presence of a hypoconulid in M. webbi, which according to Robertson (1976) would distinguish this species from all other Miopetaurista, is a misinterpretation of a worn part of the thick posterolophid in the type specimen (UF 12353). On the other hand, almost all the morphological features that align M. webbi and M. thaleri are general diagnostic characters of the genus Miopetaurista (see Mein 1970; Daxner-Höck and Mein 1975), only the presence of a complete entolophid distinguishes this species as well as M. crusafonti (see Mein 1970; Casanovas-Vilar et al. 2015), M. neogrivensis (see Mein 1970; Casanovas-Vilar et al. 2018), and M. thaleri (see Mein 1970). All these species further present well-developed mesostylids. The complete entolophid is absent in all the older (Early and Middle Miocene) Miopetaurista species, such as Miopetaurista gaillardi, which shows an incomplete and discontinuous ridge (see Mein 1970). It is also absent in the poorly-known and similarly-sized (Fig. 3, Table 1) Miopetaurista asiatica Qiu (2002), from the Late Miocene (ca. 7 Ma) Lufeng hominoid locality in China, but this may be attributed to the high dental wear of the available lower molars. The entolophid of M. webbi is apparently higher than in both M. crusafonti and M. neogrivensis being most similar to M. thaleri. In addition, M. thaleri shows higher lophids than both M. crusafonti and M. neogrivensis, and the metalophid is conspicuously more developed and pointing backwards, thus resembling M. webbi. Finally, M. thaleri also shows a hint of the sinuous ridges in the trigonid basin (‘metaconid-metastylid [=mesostylid] chaos’ of McKenna, 1962) as seen in all specimens of M. webbi (UF 123353, UF 95701) other than the specimen from the GFS (ETMNH 34820). This feature is absent in both M. crusafonti and M. neogrivensis, but is diagnostic of the genus Petaurista (see McKenna 1962), being already present in the Pleistocene species of the genus (see Dehm 1962; Tiunov and Gimranov 2020). Yet, the lower molars of Petaurista clearly differ from those of Miopetaurista because of their higher crown, higher ridges in the trigonid basin, and absence of mesoconid. Nevertheless, we note that some characteristic morphological features of Petaurista, namely the continuous and high entolophid and a ‘draft’ of the complex ridges of the trigonid basin, already occur in the youngest Miopetaurista species, such as M. thaleri and M. webbi. Therefore, we concur with Robertson (1976) that both species are closely related, although the available material for M. webbi is too limited to conduct any phylogenetic analysis, at the very least until upper cheek teeth of this species are recovered. It is also worth noting that previous phylogenetic analyses found Miopetaurista to be the sister taxon of the extant Petaurista (Casanovas-Vilar et al. 2018) and showed that the best-known species, the Middle and early Late Miocene M. neogrivensis, only differed in cranial and especially dental morphology. It is therefore not surprising that later Miopetaurista species show further dental similarities with Petaurista.

American Miopetaurista biogeography and Early Pliocene faunal dispersals

The presence of Miopetaurista in Pliocene and earliest Pleistocene sites in south-eastern North America opens interesting paleobiogeographical questions, most notably because these occurrences are far from the range of this genus as well as from that of Petaurista, both fossil and extant (Fig. 4). Miopetaurista is known from the Miocene and Pliocene of Europe and Anatolia, with a few Late Miocene occurrences in eastern Asia. Its sister taxon, Petaurista, includes almost 20 extant species distributed across forests of southern and eastern Asia; from Pakistan and Nepal to eastern China, and from Java and Borneo (Indonesia) to Honshu (Japan) (Fig. 4; Thorington and Hoffmann 2005; Thorington et al. 2012; Koprowski et al. 2016). However, during the Pleistocene it is also known from higher latitudes (Fig. 4), including Germany (Dehm 1962; referred to as Petauria), Poland (Black and Kowalski 1979; also referred to as Petauria) and north-eastern Russia (Tiunov and Gimranov 2020). Miopetaurista likely had a similarly broad distribution, although it is currently unknown from the most septentrional regions of Eurasia. Indeed, in Eurasia during the Pliocene the genus is solely represented by M. thaleri that persists in Europe, although it is very rare. Its last occurrence is a single broken M1/2 from Rebielice Królewskie 1, Poland (MN16, Late Pliocene). Miopetaurista is thought to have favored relatively warm (tropical to warm-temperate) forest environments, and probably contracted its geographical range as global cooling occurred during the late Neogene (Lu et al. 2013; Casanovas-Vilar et al. 2018). Conversely, brief warm phases may have allowed temporary range extensions. Mixed mesophytic forests are interpreted as having occurred across much of the Northern Hemisphere through the Miocene (Graham 1999; Baskin and Baskin 2016), providing possible dispersal routes for Miopetaurista from Eurasia to North America in the Late Miocene or Early Pliocene. Floral evidence and ecometric climate estimates for the GFS suggest it was a warm-temperate forest environment, similar to mesophytic forests that occur in southeastern US today (Ochoa et al. 2016; Schap et al. 2021). Such kind of forest is also comparable to the habitats occupied by most extant Petaurista species in Asia (Thorington et al. 2012; Koprowski et al. 2016), which favor subhumid subtropical to temperate climates. Pollen data indicate that the GFS was surrounded by a mesophytic forest dominated by a Quercus-Carya-Pinus assemblage comparable to that currently existing a few degrees south of this location (Pfadenhauer and Klötzli 2020), but lacking the common substory and understory taxa and including an array of herbaceous taxa indicative of more open environments instead (Ochoa et al. 2012; Ochoa et al. 2016). This kind of forests are species-rich and structurally diverse, being composed of two or more tree canopies, with the highest trees attaining up to 40 m (Pfadenhauer and Klötzli 2020). Such humid and multi-layered forested habitats are adequate for large-sized flying squirrels, which favor taller, emerging trees, as launching platforms for very long glides of almost 100 m (Stafford et al. 2002; Jackson 2012).

Pliocene European occurrences of Miopetaurista are close in age to North American ones, but are separated by thousands of kilometers, as no Miopetaurista remains have been reported from the Asian Pliocene. We hypothesize that a species similar to M. thaleri crossed the Bering Strait during the earliest Pliocene, probably favored by a northward expansion of the geographical range of this genus during the warm episodes that characterized the Early Pliocene from about 5 to 3 Ma (Haywood et al. 2011; Fedorov et al. 2013; Burke et al. 2018; Westerhold et al. 2020). Subsequent cooling would have contracted or driven the range of American Miopetaurista to the south, so that its last occurrence in the continent is in the earliest Pleistocene (late Blancan) of Florida. This giant flying squirrel surely disappeared as result of Pleistocene glacial dynamics, when the subtropical climate of Florida became colder and more seasonal (DeSantis et al. 2009; Yann and DeSantis 2014). At the other side of the Bering Strait, the genus Petaurista would have diverged during the Late Miocene and diversified mostly during the Pliocene and the Early Pleistocene (Casanovas-Vilar et al. 2018), but apparently never dispersed into North America.

The early Pliocene occurrences of Miopetaurista in Tennessee and Florida correspond to a time of major intercontinental dispersals of mammal from Asia to North America (Woodburne 2004a). That time is when the first North American records of cervids (Eocoileus and Bretzia), ailurines (Pristinailurus), some mustelids (Neogale, Sminthosinis, Trigonictis, Arctomeles, Gulo), and the machairodontine Megantereon occur (Webb 2000; Woodburne 2004a; Wallace and Wang 2004; Gustafson 2015; Samuels et al. 2018). In addition to the GFS documenting occurrences of Eocoileus (Samuels et al. in prep.), Pristinailurus and Arctomeles (Wallace and Wang 2004), and Gulo (Samuels et al. 2018), multiple genera of eulipotyphlans and rodents from the site reflect immigration from Eurasia. Recently described talpids from the GFS (Oberg and Samuels 2022) include Parascalops and Neurotrichus, which are known from the Pliocene of Europe, and a desman (Magnatalpa) representing one of two occurrences outside of Eurasia. Shrews from the site include records of Paenelimnoecus and Crusafontina (Doby 2015), also immigrants from Eurasia. Besides Miopetaurista, the sciurids from the GFS include the chipmunk Eutamias, representing the only known occurrence of the genus in North America (Crowe 2017). Immigrant taxa likely travelled west to east across Canada through the Late Miocene or Early Pliocene, then spreading south into the temperate forests of the eastern US. As already stated, the forests of much of the Northern Hemisphere showed stronger similarity in the late Neogene (Baskin and Baskin 2016) making them a suitable route for dispersal, but relatively arid habitats in the western and central US and the presence of the Rocky Mountains likely formed a natural land barrier for many species. A northern dispersal route in the late Neogene is supported by the presence of the meline Arctomeles at both the GFS (Wallace and Wang 2004) and the high arctic of Canada (Tedford and Harington 2003), while it is absent in the western part of North America.

We confirm that the Eurasian genus of giant flying squirrels Miopetaurista occurs in eastern North America where it is represented by the endemic and poorly known species M. webbi. Currently only a few lower molars of this species are known from Pliocene to earliest Pleistocene sites of Florida and Tennessee. This is a large-sized Miopetaurista species characterized by a complete ectolophid and, in most specimens, complex ridges in the trigonid basin. Such morphology recalls that of M. thaleri, the youngest Eurasian species in the genus, and extant Petaurista. However, until additional material, particularly upper cheek teeth, is found, detailed phylogenetic analyses are impossible. Miopetaurista webbi is the youngest species in the genus and we hypothesize that these giant flying squirrels, linked to warm-temperate forested environments, dispersed into North America through the Bering land bridge during the warm episodes of the Early Pliocene. This is framed within a major late Neogene faunal dispersal that included other rodents (Eutamias), talpids (Neurotrichus, Parascalops), soricids (Paenelimnoecus, Crusafontina), capreoline cervids (Eocoileus, Bretzia), the meline Arctomeles, and an ailurine (Pristinailurus). Pleistocene glacial dynamics resulted in colder and more seasonal climates in North America that likely initially implied the confinement of Miopetaurista to warmer refuges, such as Florida where it is last recorded, and as climate turned harsher, its final extinction.

Funding

This work is part of R + D + I project PID2020-117289GBI00 funded by the Agencia Estatal de Investigación of the Spanish Ministry of Science and Innovation (MCIN/AEI/10.13039/501100011033/). Research has also been supported by the Generalitat de Catalunya/CERCA Programme. I. C-V. is member of the consolidated research group 2017 SGR 116 GR of the Agència de Gestió d’Ajuts Universitaris i de Recerca of the Generalitat de Catalunya. Specimen collection at the Gray Fossil Site in Tennessee was partially funded through a National Science Foundation Grant (NSF Grant #0958985) to SC Wallace and BW Schubert. Funding for the project was also provided by the Center of Excellence in Paleontology at East Tennessee State University and ETSU RDC Major Grant (#23-011M) to J. S.

Author Contribution

J. S., I. C-V. and M. G-C. designed the study. J. S. scanned the specimens and took the photographs of all specimens. M. G-C. drafted the descriptions, took the measurements and prepared all figures with input from the other authors. M. G-C., I. C-V. and J. S. wrote the manuscript. C. C. identified and sorted microfossil remains from GFS, documented sciurids from the fauna, and reviewed the manuscript.

Acknowledgement

We are indebted to Samantha Hopkins (University of Oregon) who kindly provided pictures and data of the specimens of the Middle Miocene Cave Basin (Oregon). We also thank the collection managers who kindly allowed access to specimens in their care: M Inabinett and B Compton (ETMNH), Aaron Woodruff and R Narducci (UF). GFS microfossil specimens were collected through the important efforts of L Emmert, S Haugrud, and many volunteers at the Gray Fossil Site. This work is part of the PhD dissertation of the first author, in the framework of the PhD Programme in Geology of the Universitat Autònoma de Barcelona.

Data Availability

All data associated to this manuscript are provided in the main text. The CT image series and mesh models of the Gray Fossil Site Miopetaurista webbi specimen (ETMNH 34820) are available at MorphoSource (https://www.morphosource.org/media/000481704; https://www.morphosource.org/concern/media/000672220?locale=en). ETMNH34820 is housed and adequately curated at the Gray Fossil Site & Museum, being accessible to other researchers.

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Table 1. Measurements of the studied specimen as compared to that of the m3s of other Miopetaurista species plotted in Fig. 3.

Species	Catalog number	Length	Width	Reference for the measurements
M. webbi	ETMNH 34820	4.65	3.66	this work
M. lappi	M.L. MC 726	4.43	3.70	Mein 1970
M. gibberosus	L.J.G. 2048	5.00	3.00	Hoffmann 1893
M. neogrivensis	F.S.L. 65418	4.85	4.50	Mein 1970
M. neogrivensis	IPS56468i	5.21	4.41	Casanovas-Vilar et al. 2015
M. neogrivensis	IPS56468j	5.14	4.55	Casanovas-Vilar et al. 2015
M. neogrivensis	IPS43505	4.43	3.70	Casanovas-Vilar et al. 2015
M. neogrivensis	IPS43505	5.22	4.38	Casanovas-Vilar et al. 2015
M. gaillardi	F.S.L. Lgr. 85	3.35	3.20	Mein 1970
M. crusafonti	IPS 72355	4.29	3.62	Casanovas-Vilar et al. 2018
M. crusafonti	IPS 72356	4.17	3.96	Casanovas-Vilar et al. 2018
M. crusafonti	IPS 63257	4.28	3.59	Casanovas-Vilar et al. 2018
Miopetaurista sp.	NHMW2004z0055/0001	4.65	4.20	Daxner-Höck 2004
M. asiatica	V 13148. 1	5.10	4.10	Qiu 2002
M. thaleri	M.n.P. 1922-1	5.46	5.10	Dahlmann 2001
M. thaleri	Included in D.S.T. Wö 4	5.39	4.97	Dahlmann 2001
M. thaleri	Included in D.S.T.Wö 239/13	5.39	4.89	Dahlmann 2001
M. thaleri	Included in D.S.T. Wö 239/13	5.63	4.68	Dahlmann 2001
M. webbi	UF 95701	4.91	3.70	Robertson 1976
M. webbi	UF 12353	4.54	3.79	Webb et al. 2008

No competing interests reported.

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Gliding between continents: A review of the North American record of the giant flying squirrel Miopetaurista (Rodentia, Sciuridae) with the description of new material from the Gray Fossil Site (Tennessee)

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Abstract

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Introduction

The North American record of flying squirrels

The Gray Fossil Site

Materials and methods

Discussion

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Funding

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Acknowledgement

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References

Tables

Additional Declarations

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