ScienceDaily (Dec. 17, 2009) — A University of Florida researcher has co-authored a study tracing the evolution of the modern opossum back to the extinction of the dinosaurs and finding evidence to support North America as the center of origin for all living marsupials.
The study, to be published in PLoS One on Dec. 16, shows that peradectids, a family of marsupials known from fossils mostly found in North America and Eurasia, are a sister group of all living opossums. The findings are based in part on high-resolution CT scans of a 55-million-year-old skull found in freshwater limestone from the Bighorn Basin of Wyoming.
Restorations of (A) Herpetotherium and (B) Mimoperadectes. (Credit: Jorge González (La Plata, Argentina), courtesy Horovitz et al, PLoS One, doi:10.1371/journal.pone.0008278)
"The extinction of the dinosaurs was a pivotal moment in the evolution of mammals," said Jonathan Bloch, study co-author and associate curator of vertebrate paleontology at UF's Florida Museum of Natural History. "We're tracing the beginnings of a major group of mammals that began in North America."
Opossum-like peradectids first appeared on the continent about 65 million years ago, at the time of the Cretaceous-Paleogene extinction event, which killed the dinosaurs.
"North America is a critical area for understanding marsupial and opossum origins because of its extensive and varied fossil record," said lead author Inés Horovitz, an assistant adjunct professor at the University of California, Los Angeles. "Unfortunately, most of its species are known only from teeth."
The study also analyzes two 30-million-year-old skeletons of Herpetotheriidae, the sister group of all living marsupials.
Based on fossil evidence from the skull and two skeletons, the study's authors concluded the evolutionary split between the ancestor of opossums and the ancestor of all other living marsupials occurred at least 65 million years ago, Horovitz said.
Marsupials migrated between North and South America until the two continents separated after the end of the Cretaceous period. Marsupials in South America diversified and also migrated into Antarctica and Australia, which were still connected at that time, Bloch said.
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Cranial Anatomy of the Earliest Marsupials and the Origin of Opossums
Inés Horovitz1*, Thomas Martin2, Jonathan Bloch3, Sandrine Ladevèze4¤, Cornelia Kurz5, Marcelo R. Sánchez-Villagra4*
1 Department of Ecology and Evolutionary Biology, University of California Los Angeles, Los Angeles, California, United States of America, 2 Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Universität Bonn, Bonn, Germany, 3 Florida Museum of Natural History, University of Florida, Gainesville, Florida, United States of America, 4 Palaeontologisches Institut und Museum, Zürich, Switzerland, 5 Naturkundemuseum im Ottoneum Kassel, Kassel, Germany
Abstract Top
Background
The early evolution of living marsupials is poorly understood in part because the early offshoots of this group are known almost exclusively from jaws and teeth. Filling this gap is essential for a better understanding of the phylogenetic relationships among living marsupials, the biogeographic pathways that led to their current distribution as well as the successive evolutionary steps that led to their current diversity, habits and various specializations that distinguish them from placental mammals.
Methodology/Principal Findings
Here we report the first skull of a 55 million year old peradectid marsupial from the early Eocene of North America and exceptionally preserved skeletons of an Oligocene herpetotheriid, both representing critical groups to understand early marsupial evolution. A comprehensive phylogenetic cladistic analysis of Marsupialia including the new findings and close relatives of marsupials show that peradectids are the sister group of living opossums and herpetotheriids are the sister group of all living marsupials.
Conclusions/Significance
The results imply that North America played an important role in early Cenozoic marsupial evolutionary history and may have even been the center of origin of living marsupials and opossums. New data from the herpetotheriid postcranium support the view that the ancestral morphotype of Marsupialia was more terrestrial than opossums are. The resolution of the phylogenetic position of peradectids reveals an older calibration point for molecular estimates of divergence times among living marsupials than those currently used.
Citation: Horovitz I, Martin T, Bloch J, Ladevèze S, Kurz C, et al. (2009) Cranial Anatomy of the Earliest Marsupials and the Origin of Opossums. PLoS ONE 4(12): e8278. doi:10.1371/journal.pone.0008278
Editor: Robert DeSalle, American Museum of Natural History, United States of America
Received: June 22, 2009; Accepted: November 17, 2009; Published: December 16, 2009
Copyright: © 2009 Horovitz et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: University of Zurich and Swiss National Fond (3100A0-116013) gave Financial support to Marcelo R. Sanchez-Villagra. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
* E-mail: horovitz@ucla.edu (IH); m.sanchez@pim.uzh.ch (MRSV)
¤ Current address: Royal Belgian Institute of Natural Sciences, Department of Paleontology, Bruxelles, Belgium
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