The skull of Epidolops ameghinoi from the early Eocene Itaboraí fauna, southeastern
Brazil, and the affinities of the extinct marsupialiform order Polydolopimorphia

Beck, RMD

Abstract

The skull of the polydolopimorphian marsupialiform Epidolops ameghinoi is described
in detail for the first time, based on a single well-preserved cranium and associated left
and right dentaries plus additional craniodental fragments, all from the early Eocene
(53-50 million year old) Itaboraí fauna in southeastern Brazil. Notable craniodental
features of E. ameghinoi include absence of a masseteric process, very small
maxillopalatine fenestrae, a prominent pterygoid fossa enclosed laterally by a
prominent ectopterygoid crest, an absent or tiny transverse canal foramen, a simple,
planar glenoid fossa, and a postglenoid foramen that is immediately posterior to the
postglenoid process. Most strikingly, the floor of the hypotympanic sinus was
apparently unossified, a feature found in several stem marsupials but absent in all
known crown marsupials. "Type II" marsupialiform petrosals previously described from
Itaboraí plausibly belong to E. ameghinoi; in published phylogenetic analyses, these
petrosals fell outside (crown-clade) Marsupialia. "IMG VII" tarsals previously referred to
E. ameghinoi do not share obvious synapomorphies with any crown marsupial clade,
nor do they resemble those of the only other putative polydolopimorphians represented
by tarsal remains, namely the argyrolagids. Most studies have placed
Polydolopimorphia within Marsupialia, related to either Paucituberculata, or to
Microbiotheria and Diprotodontia. However, diprotodonty almost certainly evolved
independently in polydolopimorphians, paucituberculatans and diprotodontians, and
Epidolops does not share obvious synapomorphies with any marsupial order.
Epidolops is dentally specialized, but several morphological features appear to be
more plesiomorphic than any crown marsupial. It seems likely Epidolops that falls
outside Marsupialia, as do morphologically similar forms such as Bonapartherium and
polydolopids. Argyrolagids differ markedly in their known morphology from Epidolops
but share some potential apomorphies with paucituberculatans. It is proposed that
Polydolopimorphia as currently recognised is polyphyletic, and that argyrolagids (and
possibly other taxa currently included in Argyrolagoidea, such as groeberiids and
patagoniids) are members of Paucituberculata. This hypothesis is supported by
Bayesian non-clock phylogenetic analyses of a total evidence matrix comprising DNA
sequence data from five nuclear protein-coding genes, indels, retroposon insertions
and morphological characters: Epidolops falls outside Marsupialia, whereas
argyrolagids form a clade with the paucituberculatans Caenolestes and Palaeothentes,
regardless of whether the Type II petrosals and IMG VII tarsals are used to score
characters for Epidolops or not. There is no clear evidence for the presence of crown
marsupials at Itaboraí, and it is possible that the origin and early evolution of
Marsupialia was restricted to the "Austral Kingdom" (southern South America,
Antarctica, and Australia).

• Journal Article Type: Article
• Acceptance Date: Sep 29, 2016
• Online Publication Date: Oct 26, 2016
• Publication Date: Oct 26, 2016
• Deposit Date: Oct 4, 2016
• Publicly Available Date: Nov 2, 2016
• Journal: Journal of Mammalian Evolution
• Print ISSN: 1064-7554
• Electronic ISSN: 1573-7055
• Publisher: Springer Verlag
• Volume: 24
• Issue: 4
• Pages: 373-414
• DOI: https://doi.org/10.1007/s10914-016-9357-6
• Publisher URL: http://dx.doi.org/10.1007/s10914-016-9357-6
• Related Public URLs: http://link.springer.com/journal/10914