Thursday, October 11, 2018

[PaleoMammalogy • 2018] Miopetaurista neogrivensis • Oldest Skeleton of A Fossil Flying Squirrel Casts New Light on the Phylogeny of the Group

Miopetaurista neogrivensis Mein 1970

in Casanovas-Vilar, Garcia-Porta, Fortuny, et al., 2018. 

Flying squirrels are the only group of gliding mammals with a remarkable diversity and wide geographical range. However, their evolutionary story is not well known. Thus far, identification of extinct flying squirrels has been exclusively based on dental features, which, contrary to certain postcranial characters, are not unique to them. Therefore, fossils attributed to this clade may indeed belong to other squirrel groups. Here we report the oldest fossil skeleton of a flying squirrel (11.6 Ma) that displays the gliding-related diagnostic features shared by extant forms and allows for a recalibration of the divergence time between tree and flying squirrels. Our phylogenetic analyses combining morphological and molecular data generally support older dates than previous molecular estimates (~23 Ma), being congruent with the inclusion of some of the earliest fossils (~36 Ma) into this clade. They also show that flying squirrels experienced little morphological change for almost 12 million years.

Fig 1: The fossil flying squirrel Miopetaurista neogrivensis.
 (a) Reconstruction of the skeleton based in the partial skeleton IPS56468 from Abocador de Can Mata. Missing elements are based on extant giant flying squirrel Petaurista petaurista and are colored in blue.
(b) Life appearance of Miopetaurista neogrivensis showing the animal ready to land on a tree branch. Coat pattern and color are based in extant Petaurista species, the sister taxon of Miopetaurista.  Scale bar is 4 cm.

Fig 3: Mandible and cheek teeth of Miopetaurista neogrivensis.
 (a to c) Partial left hemimandible (IPS56468j) in lateral, medial and dorsal views. (d to e) Partial right hemimandible (IPS56468i) in lateral and medial views. A caudal vertebra and a bone fragment are attached to the lateral side of the mandibular ramus. Both hemimandibles were associated to the partial skeleton IPS56468 from ACM/C5-D1. (f to g) Partial hemimandible (IPS87560) from ACM/C8-B sector in lateral and medial views. (h) Left upper cheek teeth series (P3–M3) of IPS56468h (Figure 6—Figure supplement 1 ). (i) Left lower cheek teeth series (p4–m3) of IPS56468j. Cheek teeth measurements are given in Supplementary file 4 whereas mandibular measurements are given in Supplementary file 6. For a detailed description and comparisons of cheek teeth and mandible morphology see Appendix 3.1 and 3.2. an, angular process; ar, articular process; co, coronoid process. Scale bar is 1 cm in figs. a to g; 2 mm in (h to i).

Fig 7: Flying squirrel phylogeny and node dating estimates based on a Bayesian total evidence analysis including Miopetaurista neogrivensis.

Fig 8: Fossil record of ‘flying squirrels’ and paleoclimatic data. Temporal ranges of purported flying squirrel genera in Europe, Asia and North America. The 95% highest posterior density (HPD) intervals for flying squirrel divergence as derived from total evidence and node dating analyses are indicated in orange shading (see Figure 7 and Figure 7—figure supplement 1 ). Darker shading indicates the time interval where both independently calculated estimates overlap, thus defining the most likely time interval for flying squirrel divergence. Global paleoclimatic data are taken from Zachos et al., 2001.

Miopetaurista neogrivensis is the oldest unquestionable flying squirrel and dates back to the middle/late Miocene boundary (11.6 Ma). Its diagnostic wrist anatomy indicates that the two subtribes of flying squirrels had already diverged at that time. Moreover, this new fossil allows for a recalibration of flying squirrel time of origin and diversification, generally providing somewhat older estimates than previous molecular analyses. These differ according to the phylogenetic method used, total evidence analysis estimates an interval of 36.6 – 24.9 Ma while node dating results in a younger estimate of 30.6 – 17.4 Ma. Therefore, we cannot rule out that at least some of the oldest (ca. 36 Ma) fossils tentatively identified as flying squirrels may indeed belong to this group. However, the estimates of both independent phylogenetic approaches overlap for the late Oligocene (31 – 25 Ma), which should be considered the most likely interval for flying squirrel divergence. The two flying squirrel subtribes are found to have diverged during the early Miocene (22 – 18 Ma) while most extant genera would do so during the Miocene, although they are not recorded until the Pleistocene. Miopetaurista neogrivensis is estimated to have diverged from Petaurista spp., its sister taxon, between 18.8 – 12.4 Ma, the oldest boundary overlapping with the earliest record of the genus Miopetaurista (18 – 17 Ma). Perhaps not surprisingly, the skeletons of both genera show little differences. Sciurids are often regarded as a morphologically conservative group and flying squirrels are no exception having experienced few morphological changes for almost 12 million years.

Isaac Casanovas-Vilar, Joan Garcia-Porta, Josep Fortuny, Óscar Sanisidro, Jérôme Prieto, Marina Querejeta, Sergio Llácer, Josep M Robles, Federico Bernardini, and David M Alba. 2018. Oldest Skeleton of A Fossil Flying Squirrel Casts New Light on the Phylogeny of the Group.  eLife. 7; e39270 DOI:  10.7554/eLife.39270.001

Oldest fossil of a flying squirrel sheds new light on its evolutionary tree via @elife @EurekAlert

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