Patterns in the bony skull development of marsupials: high variation in onset of ossifcation and conserved regions of bone contact
Stephan N. F. Spiekman1,2,3 & Ingmar Werneburg3,4,5
1Paläontologisches Institut und Museum der Universität Zürich, Karl-Schmid-Strasse 4, 8006 Zürich, Switzerland.
2Institute of Biology Leiden (IBL) at Leiden University, Sylviusweg 72, 2333 BE Leiden, the Netherlands.
3Museum für Naturkunde, Leibniz-Institut für Evolutions- and Biodiversitätsforschung an der Humboldt-Universität zu Berlin, Invalidenstraße 43, 10115 Berlin, Germany.
4Senckenberg Center for Human Evolution and Palaeoenvironment (HEP) at Eberhard Karls Universität, Sigwartstraße 10, 72074 Tübingen, Germany.
5Eberhard Karls Universität, Hölderlinstraße 12, room: 308g, D-72076 Tübingen, Germany.
Development in marsupials is specialized towards an extremely short gestation and highly altricial newborns. As a result, marsupial neonates display morphological adaptations at birth related to functional constraints. However, little is known about the variability of marsupial skull development and its relation to morphological diversity. We studied bony skull development in fve marsupial species. The relative timing of the onset of ossifcation was compared to literature data and the ossifcation sequence of the marsupial ancestor was reconstructed using squared-change parsimony. The high range of variation in the onset of ossifcation meant that no patterns could be observed that diﬀerentiate species. This finding challenges traditional studies concentrating on the onset of ossifcation as a marker for phylogeny or as a functional proxy. Our study presents observations on the developmental timing of cranial bone-to-bone contacts and their evolutionary implications. Although certain bone contacts display high levels of variation, connections of early and late development are quite conserved and informative. Bones that surround the oral cavity are generally the frst to connect and the bones of the occipital region are among the last. We conclude that bone contact is preferable over onset of ossifcation for studying cranial bone development.