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., 398GEODIVERSITAS • 2015 • 37 (4) Muizon C. de et al. 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...

Virtual endocranial and inner ear endocasts of the Paleocene ‘condylarth’ Chriacus: new insight into the neurosensory system and evolution of early placental mammals

“…() and Muizon et al. () and references therein for cranial and petrosal anatomy, Bertrand et al. (2018b) for endocranial anatomy and Orliac & O'Leary () for bony labyrinth anatomy.…”

“…(), Orliac & O'Leary (), Ravel & Orliac (), Muizon et al. (), Orliac & Gilissen () and Orliac et al. (2012a).…”

The dentition of Carodnia vieirai (Mammalia: Xenungulata): enamel microstructure and mastication pattern

“…Bergqvist and Avilla (2003) consider the similarity in dental morphology between Xenungulata and Dinocerata a convergence between the two groups, as they present major differences in postcranial morphology. More recently, Xenungulata was linked to Tethytheria (O'Leary et al, 2013) and nested Carodnia within Pantodonta, as a sister taxon of Coryphodon (Muizon et al, 2015).…”

An exceptionally well-preserved skeleton of Thomashuxleya externa (Mammalia, Notoungulata), from the Eocene of Patagonia, Argentina

“…The multiple analyses we perform is also a case study in which different data sources are integrated and results are evaluated from multiple perspectives. We believe that the combination of morphological and molecular data can provide more precise and robust hypotheses for several extinct clades of mammals (e.g., Asher et al, 2005;Muizon et al, 2015;Pattinson et al, 2015), and we further note that combination of molecular and morphological datasets can in principle increase support for clades not present when one or both are analysed in isolation (Gatesy and Baker, 2005;Lee and Camens, 2009;Thompson et al, 2012).…”

“…This possibility is demonstrably not the case for many vertebrate groups with an extensive Paleogene fossil record (e.g., primates as shown by Pattinson et al, 2015), but it is naive to expect that such a scenario has never happened during the course of mammalian evolution. Muizon et al (2015) performed a phylogenetic analysis of placental interordinal relationships using morphological characters and including SANUs and a large sample of other Paleogene fossils. They pointed out the high level of homoplasy detected in their dataset, and our data also clearly demonstrate homoplasy (RI of 0.638 and CI of 0.345 in our unconstrained tree).…”