Alcidedorbignya inopinata, a basal pantodont (Placentalia, Mammalia) from the early Palaeocene of Bolivia: anatomy, phylogeny and palaeobiology

ABSTRACT 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.