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Robert R Reisz

Publications and source records attributed to Robert R Reisz.

7 recordsLinked to original sources
PubMed2006

Origin of dental occlusion in tetrapods: signal for terrestrial vertebrate evolution?

Evolutionary changes of the dentition in tetrapods can be associated with major events in the history of terrestrial vertebrates. Dental occlusion, the process by which teeth from the upper jaw come in contact with those in the lower jaw, appears first in the fossil record in amniotes and their close relatives near the Permo-Carboniferous boundary approximately 300 million years ago. This evolutionary innovation permitted a dramatic increase in the level of oral processing of food in these early tetrapods, and has been generally associated with herbivory. Whereas herbivory in extinct vertebrates is based on circumstantial evidence, dental occlusion provides direct evidence about feeding strategies because jaw movements can be reconstructed from the wear patterns of the teeth. Examination of the evolution of dental occlusion in Paleozoic tetrapods within a phylogenetic framework reveals that this innovation developed independently in several lineages of amniotes, and is represented by a wide range of dental and mandibular morphologies. Dental occlusion also developed within diadectomorphs, the sister taxon of amniotes. The independent, multiple acquisition of this feeding strategy represents an important signal in the evolution of complex terrestrial vertebrate communities, and the first steps in the profound changes in the pattern of trophic interactions in terrestrial ecosystems.

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PubMed2006

The phylogeny of early eureptiles: comparing parsimony and Bayesian approaches in the investigation of a basal fossil clade.

For the first time the phylogenetic relationships of early eureptiles, consisting of captorhinids, diapsids, and protorothyridids, are investigated in a modern phylogenetic context using both parsimony and Bayesian approaches. Ninety parsimony-informative characters and 25 taxa were included in the analyses. The Bayesian analysis was run with and without a gamma-shape parameter allowing for variable rates across characters. In addition, we ran two more Bayesian analyses that included 42 autapomorphies and thus parsimony-uninformative characters in order to test the effect of variable branch lengths. The different analyses largely converged to the same topology, suggesting that the "protorothyridid" Coelostegus is the sister taxon of all other eureptiles and that the remaining "protorothyridids" are paraphyletic. Also, there is a close relationship between diapsids and Anthracodromeus, Cephalerpeton, and Protorothyris, a grouping of Thuringothyris with captorhinids, and a variable position of the "protorothyridids" Brouffia, Hylonomus, and Paleothyris. The lack of resolution in some parts of the tree might be due to "hard polytomies" and short divergence times between the respective taxa. The tree topology is consistent with the hypothesis that the temporal fenestrations of diapsid reptiles appear to be the consequence of a more lightly built skeleton, indicating a significant ecological shift in the early stages of diapsid evolution. Bayesian analysis is a very useful additional approach in studies of fossil taxa in which more traditional statistical support like the bootstrap is often weak. However, the exclusive use of the Mk model appears suitable only if autapomorphic characters are included, whereas the Mk+gamma model performed well with or without autapomorphies.

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PubMed2005

Embryos of an early Jurassic prosauropod dinosaur and their evolutionary significance.

Articulated embryos from the Lower Jurassic Elliot Formation of South Africa are referable to the prosauropod Massospondylus carinatus and, together with other material, provide substantial insights into the ontogenetic development in this early dinosaur. The large forelimbs and head and the horizontally held neck indicate that the hatchlings were obligate quadrupeds. In contrast, adult Massospondylus were at least facultatively bipedal. This suggests that the quadrupedal gait of giant sauropods may have evolved by retardation of postnatal negative allometry of the forelimbs. Embryonic body proportions and an absence of well-developed teeth suggest that hatchlings of this dinosaur may have required parental care.

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PubMed2005

Four well-constrained calibration points from the vertebrate fossil record for molecular clock estimates.

Recent controversy about the use of the vertebrate fossil record for external calibration of molecular clocks centers on two issues, the number of dates used for calibration and the reliability of the fossil calibration date. Viewing matters from a palaeontological perspective, we propose three qualitative, phylogenetic criteria that can be used within a comparative framework for the selection of well-constrained calibration dates from the vertebrate fossil record. On the basis of these criteria, we identify three highly suitable new fossil calibration dates for molecular clock estimates: the lungfish-tetrapod split (between 419 and 408 Ma), the bird-crocodile split (between 251 and 243 Ma), and the alligator-caiman split (between 71 and 66 Ma). Together with our previously suggested bird-lizard split (between 252 and 257 Ma), these four fossil dates span a range of very different ages. They are, in our opinion, more suitable for molecular clock calibration than the traditionally used mammal-bird split, which is less well constrained. We plea for closer interactions between paleontologists and molecular biologists in studying the timescale of vertebrate evolution.

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PubMed2003

The Permian mammal-like herbivore Diictodon, the oldest known example of sexually dimorphic armament.

Dicynodonts, a highly successful group of Palaeozoic tetrapods, were herbivores with keratinous beaks, and were frequently equipped with large, neomorphic tusks. Diictodon is a particularly abundant dicynodont genus, allowing statistical investigation of its palaeobiology. Anatomical, morphometric and distributional analyses provide evidence of sexual dimorphism, based on the presence or absence of formidable tusks. Tusk occurrence is also correlated with the presence of a cranial boss on the skull roof and, possibly, with greater cranial size. This earliest well-documented example of dimorphic armament suggests that sexual dimorphism, and the complex social behaviour that accompanies it, have long been characteristic of the synapsid lineage.

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