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Alcidedorbignya monograph, basal pantodont from earliest Paleocene of Bolivia (free pdf)

Ben Creisler

I posted this over on the VRTPALEO list yesterday, but it might be of
interest to the DML as well since it represents a mammal the lived
right after the K-Pg extinction, and suggests placentals had already
diversified. (Edited to remove some other Cenozoic refs.)

---------- Forwarded message ----------
From: Ben Creisler <bcreisler@gmail.com>
Date: Fri, Jan 1, 2016 at 3:11 PM
Subject: Alcidedorbignya monograph + other new mammal papers in open access
To: VRTPALEO@usc.edu

Ben Creisler

Some recent fossil mammal papers in open access:

Christian de Muizon, Guillaume Billet, Christine Argot, Sandrine
Ladevèze & Florent Goussard (2015)
Alcidedorbignya inopinata, a basal pantodont (Placentalia, Mammalia)
from the early Palaeocene of Bolivia: anatomy, phylogeny and
Geodiversitas 37 (4): 397-634



Alcidedorbignya inopinata Muizon & Marshall, 1987 is a basal pantodont
(Placentalia, Mammalia) of small body size, from the early Palaeocene
of the Santa Lucia Formation at Tiupampa, Bolivia. Tiupampa is the
type locality for the Tiupampan, a South American Land Mammal age
(SALMA), which is assigned an age equivalent to the basal Torrejonian
1 of North America (c. 65 Ma). Alcidedorbignya is known by
exceptionally preserved specimens, which are described here. The two
major specimens are an almost complete skeleton (MHNC 8372) and a
partial skull (MHNC 8399), the former representing one of the
best-preserved fossil placentals from the early Palaeocene and
probably the oldest placental skeleton that is so completely known.
These specimens are also the first eutherian skulls and skeleton ever
discovered at Tiupampa, a locality which has yielded numerous
metatherian skulls and partial skeletons. The remarkable preservation
of the two skulls allows a detailed description of the cranial anatomy
with well-identified sutures and foramina, including those of the
auditory region. Through CT scanning of the skulls, the arterial and
venous circulation pattern in the basicranium as well as the bony
labyrinth of the inner ear were tentatively reconstructed. A thorough
description of the postcranial skeleton of MHNC 8372 is also provided.
Among pantodonts, Alcidedorbignya presents the closest morphological
similarities with Pantolambda, the oldest and earliest diverging North
American pantodont (known by skulls and skeletons), from the late
early Palaeocene (Torrejonian 2 and 3) of New Mexico. Alcidedorbignya
is one-third the size, much more gracile, and clearly exhibits more
plesiomorphic features than Pantolambda. It is also at least 3 Ma
older. Interesting similarities are also observed between the skull of
Alcidedorbignya and several “condylarths”, such as Maiorana,
Baioconodon, Arctocyon, and Arctocyonides. The basicranium of
Alcidedorbignya is also similar to that of some extant afrotheres
(e.g., Tenrec) or Lipotyphla (e.g., Solenodon), but most of these
similarities may represent placental symplesiomorphies. In fact, the
cranial anatomy of Alcidedorbignya, beyond the simple thorough
description of a basal pantodont, sheds light on the cranial anatomy
of placentals from the earliest Paleocene, previously unknown in this
detail. The postcranial skeleton of MHNC 8372 together with some
isolated specimens referred to A. inopinata, is compared to adequate
morphofunctional models (e.g., Solenodon, Dendrohyrax, Sciurus), which
indicates that it was a moderately agile, plantigrade, generalized
terrestrial mammal with good climbing ability (scansorial) and
occasionally capable of standing in a bipedal position. The scutiform
ungual phalanges were probably bearing nail-like hooves (or
primate-like nails) and because of the absence of claws, fossorial
habits are unlikely.

A parsimony analysis of a data matrix including 72 taxa and 426
characters (cranial and postcranial) has been undertaken with TNT and
Paup 4b10. The matrix includes several stem eutherians and
representatives of the four major clades of Placentalia:
Laurasiatheria, Euarchontoglires, Xenarthra, and Afrotheria.
Furthermore, because one of the major aims of this analysis is to test
possible close phylogenetic relationships of Alcidedorbignya inopinata
with the five orders of South American “ungulates”, 12 taxa belonging
to the Notoungulata, Astrapotheria, Litopterna, Pyrotheria and
Xenungulata, were included in the matrix. Two analyses were performed:
one unconstrained and one with a backbone constraint based on the
latest molecular phylogenies. One of the main results of these
analyses is the high level of homoplasy detected by the low retention
index (RI close to 0.5; CI inferior to 0.2), which indicates that half
of the similarities present in the matrix are interpreted as
homoplasies. Nevertheless, some interesting observations emerge. In
both analyses, all members of Pantodonta are closely related and
included in the Placentalia, and the Eocene “cimolestid” taxon
Didelphodus, which has been regarded as closely related to pantodonts,
is their sister-group. In the constrained analysis, the Pantodonta are
monophyletic and included in the Laurasiatheria; Didelphodus and the
pantodonts are clustered with the Tillodontia, either as their sister
group or included in a clade also comprising some “condylarths”. In
the unconstrained analysis the xenungulate Carodnia is included within
the Pantodonta as the sister group of Coryphodon, this clade being
part of a large basal polytomy of placental lineages. Both analyses
agree with a northern origin of the South American pantodont
Alcidedorbignya as it is nested within northern taxa. Furthermore, no
close relationship of Alcidedorbignya with South American endemic
ungulates is supported, except for pantodont-xenungulate affinities in
the unconstrained analysis. In the constrained analysis the five
groups of South American endemic “ungulates” are gathered in a single
clade included in the laurasiatheres but not directly related to
pantodonts. Finally, the rather apical position of Alcidedorbignya in
both analyses may be considered as an argument for a pre-K-Pg
placental diversification, as this taxon is dated at c. 65 Ma, nearly
coincident with the end of the Cretaceous.