Basal Mesoeucrocodylian Morphology and Phylogenetic Systematics
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Issue Date
1-May-11
Authors
Sertich, Joseph John-Warren
Publisher
The Graduate School, Stony Brook University: Stony Brook, NY.
Keywords
Abstract
The Mesozoic was a time period of major biologic and geologic change in Earth's history, culminating in the breakup of the southern supercontinent Gondwana during the Cretaceous. Vertebrate fossils recovered from this interval have generated dramatic insights into the biogeographic and plate tectonic history of Gondwana, instigating competing hypotheses regarding the early pattern and sequence of the breakup of the southern landmasses and the biogeographic processes that shaped them. Among the most abundant, best preserved, and most complete remains yet recovered from the supercontinent are crocodyliforms, a clade that includes the extinct ancestors of today's crocodiles. Basal mesoeucrocodyilans represent a diverse, mostly Gondwanan radiation of terrestrial and semi-aquatic crocodyliforms occupying ecospace not typically associated with extant crocodiles. Recent paleontological fieldwork in the Cretaceous of Madagascar and Tanzania has resulted in the discovery of new crocodyliform remains that provide insight into the morphology and evolutionary relationships of basal mesoeucrocodylians vital to understanding their biogeographic history. Detailed investigation of the postcranial appendicular skeleton of Simosuchus clarki, a bizarre, pug-nosed, herbivorous crocodyliform from the Late Cretaceous (Maastrichtian) of northwestern Madagascar reveals previously unrecognized diversity in the postcranial skeleton of basal mesoeucrocodylians. Although typically crocodyliform in overall morphology, the appendicular skeleton of Simosuchus preserves a number of derived features indicating a primarily semi-erect posture, in contrast to the erect posture inferred for other basal mesoeucrocodylians. New and vastly more complete remains of the enigmatic taxon Miadanasuchus oblita, also from the Late Cretaceous of Madagascar, have provided taxonomic and phylogenetic insight into the poorly understood Trematochampsidae, a diverse group of Gondwanan crocodyliforms. A new trematochampsid crocodyliform from the middle Cretaceous deposits of southwestern Tanzania extends the range of this group into sub-equatorial Africa and provides the first faunal link between emerging southern African Cretaceous faunas and well known circum-Saharan faunas. Critical reevaluation of previous phylogenetic analyses combined with additional taxon and character sampling has resulted in the construction of a new phylogenetic dataset targeting basal mesoeucrocodylian relationships. Results of this phylogenetic analysis significantly clarify basal mesoeucrocodylian interrelationships and provide a foundation for biogeographic investigations into the processes that have shaped their distribution through the Cretaceous.