Crocodilian Evolution and Systematics

 

A joint effort between molecular biology and paleontology to elucidate the evolutionary history of Crocodilians
 

 Primary Researchers           

 

    L.D. Densmore

 

    Christopher Brochu

 

 

 Crocodylians are used in research programs that range in diversity from developmental biology to reptile genomics. Some species are critically endangered, and others provide important economic resources for impoverished nations. They are the closest living relatives of birds and critical to analyses of bird phylogeny. Crocodylia bridges neontology
and paleontology with a rich fossil record that includes more than 100 recognized species over some 80 million years. Known fossil species outnumber the living by six to one. This allows calibration of divergence times and comparisons of stratigraphy and such things as the molecular clock.

Some aspects of crocodylian phylogeny are stable. All data support monophyly of Alligatoridae, of the caimans and, with the exception of a single species, of Crocodylus. They broadly agree that Crocodylus, Osteolaemus, and Tomistoma are closer to each other than to the alligatorids. Stratigraphy and molecules typically support similar divergence times.

At the same time many phylogenetic problems remain, most notably the placement and divergence time of the Indian gharial (Gavialis). Recent analyses only serve to illustrate the challenges that exist within Crocodylia; many of the problems are the opposite of what researchers face with other amniote clades, and most of Crocodylia’s known diversity is extinct. Divergences within the order range from the Late Cretaceous to the Quaternary, making it necessary to use multiple markers that are informative at different phylogenetic levels. In addition, relationships within some genera (e.g. within the true crododiles) and among related genera (e.g., the caimans) have yet to be resolved. Similarly, we are no longer sure about the placement of the dwarf crocodiles (Osteolaemus) or how even many species the genus includes.

The focus of this project is a resolution of crocodylian phylogenetics using a combined molecular and morphological approach. Our immediate goals are to:

  • Acquire complete mitochondrial genome sequences for representativesof all living crocodylian species
  • Acquire DNA sequences for at least 6 selected nuclear gene products
  • Expand morphological data sets to include soft-tissue anatomy
  • Obtain high-resolution computed tomographic images from the skulls of important living and extinct crocodylians
  • Revisit fossils within critical stratigraphic windows during the
    Tertiary

The long-term goals of this project are to:

  • Resolve outstanding phylogenetic problems within the order
  • Empirically explore methods for obtaining divergence time estimates
    from molecules
  • Examine novel gene orders found in the crocodylian mitochondrial
    genome
  • Construct a comprehensive evolutionary narrative for Crocodylia

To achieve these goals, we will provide an interdisciplinary environment for our students and collaborators including advanced techniques of molecular phylogenetics, digital morphology, and paleontology. The products of this research, including a digital atlas of the crocodylian skull, will be invaluable for high school and undergraduate teaching; furthermore, parts of this work will be important in ongoing crocodylian conservation efforts.

last modified

8/30/04

david.rodriguez@ttu.edu