Tuesday, November 2, 2010

Fossils + DNA Produce a New Squamate Tree

Molecular data and fossil data, when examined separately may differ in providing information on how groups of species are related to each other. But molecular data offer great potential to resolve the phylogeny of living taxa, particularly those that lack a fossil record. Weins et al. ask the question can molecular data improve our understanding of relationships of fossil taxa? They analyze the placement of snakes within the Squamata by combining published morphological data (363 characters) and new DNA sequence data (15,794 characters, 22 nuclear loci) for 45 living and 19 fossil taxa. The results suggest some fossil taxa undergo major changes in their phylogenetic position when molecular data are added; most fossil taxa are placed with strong support in the expected clades by the combined data Bayesian analyses, despite each having substantial numbers of missing cells and despite the recent suggestions that extensive missing data are problematic for Bayesian phylogenetics; that morphological data can change the placement of living taxa in combined analyses, even when there is an overwhelming majority of molecular characters. Also, they discovered a strong but misleading signal in the morphological data with respect to snakes, amphisbaenians, and dibamids; presumably this is due to limblessness and a fossorial life style. Of particular interest here is the position of snakes as the sister to the Anguimorphs. With the Anguimorpha consisting of the extinct marine reptiles, the Mosasauria, the monitor lizards, the heloderms, and the anguids. Within the snakes, the South American fossil species Dinilysia is the sister to all of the other snakes except the scoleocophidians, and the fossil species with hind limbs (Eupodophis, Haasiophis, Pachyrhachis) plus the Australian fossil species Wonambi are the sisters to the macrostomate snakes.

Wonambi usually considered a member of the extinct Madtsoiidae that first appears in the fossil record in late Cretaceous and extends to the late Pleistocene. The fossil family members have been found in South America, Africa, India, Southern Europe, and Australia. Madtsoiidae are known for a long, narrow skull; that are relatively inflexible upper jaws; a rounded snout; a braincase that narrows at the orbit and widens posteriorly,and elogated vertebrae. To date no evidence of hind limbs have been found.Wonambi reached an estimated length of 18 ft.

Dinilysia patagonica is known from the Cretaceous of Argentinia, South America  and is considered closely allied to the extant anilioid snakes. Dinilysia is often compared to booids and it had a relatively large head, and is thought to have been terrestrial. Comparisons of the cranial anatomy of Dinilysia to the recently described hind-limbed, Argentine Gondwanan snake, Najash rionegrina suggest similarities in the two species, but hind limbs have not been found in Dinilysia. The evidence for a Gondwanaland origin of snakes is relatively strong.

John J. Wiens, Caitlin A. Kuczynski, Ted Townsend, Tod W. Reeder, Daniel G. Mulcahy, and Jack W. Sites, Jr. 2010. Combining Phylogenomics and Fossils in Higher-Level Squamate Reptile Phylogeny: Molecular Data Change the Placement of Fossil Taxa. Systematic Biolology  doi:10.1093/sysbio/syq048

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