Reinvestigation of the basicranium of Haldanodon exspectatus (Mammaliaformes, Docodonta)

ABSTRACT

The docodont Haldanodon exspectatus, a mammaliaform from the Late Jurassic of Portugal, is a crucial taxon for studying higher-level relationships of mammaliaforms and their morphological evolution. Based on high-resolution computed tomography scanning of three specimens of Haldanodon, we developed a new reconstruction of the basicranium including the petrosal and the inner ear. Our study confirms that Haldanodon and other basal mammaliaforms, such as Morganucodon and Sinoconodon, are similar in the main characteristics of their external anatomy of the petrosal. However, Haldanodon shows several derived features that support a phylogenetic position of docodonts more derived than Morganucodon and Sinoconodon: (1) elongated and curved cochlear canal (nearly 180°), (2) single lateral flange foramen of petrosal, (3) absence of anterior paroccipital process, and (4) squamosal constriction. The bony labyrinth reveals a secondary crus commune, which is regarded to be a plesiomorphic feature of the mammaliaform groundplan. The cochlear canal shows an apical inflation connected to a distinct sulcus and a separate notch in the internal acoustic meatus, supporting the existence of a lagenar nerve and macula as in monotremes. Haldanodon is unique among Mesozoic mammaliaforms in having a hypertrophied paroccipital region that is excavated into large tympanic pneumatic recesses, which are connected with the extensive porous and probably also pneumatic internal structures of the surrounding bones. The curved cochlear canal and the pneumatized middle ear region support the hypothesis that Haldanodon had more effective low-frequency hearing as an adaptation to a fossorial mode of life.

ACKNOWLEDGMENTS

We thank L. Kallien and W. Leis (Steinbeis-Transferzentrum Gießerei Technologie Aalen, Fachhochschule Aalen, Germany) and T. R. Ryan (Department of Anthropology and Center for Quantitative Imaging, Pennsylvania State University, U.S.A.) for producing the μCT scans, and G. Oleschinski (Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Universität Bonn, Germany) for preparing the photographs. We also thank S. Ladevèze, J. A. Lillegraven, J. R. Wible, R. J. Asher, and an anonymous reviewer for their helpful comments that greatly improved the manuscript. This research has been supported by National Science Foundation (U.S.A.) grants (DEB 0316558 and EF0629959), a Humboldt-Forschungspreis from Alexander von Humboldt-Stiftung (Bonn, Germany) to Z.-X.L., and a Max Kade Foundation Fellowship (New York) to T. M.

Handling editor: Guillermo Rougier