The evolution of the synapsid tusk: insights from dicynodont therapsid tusk histology.

The mammalian tusk is a unique and extreme morphotype among modern vertebrate dentitions. Tusks-defined here as ever-growing incisors or canines composed of dentine-evolved independently multiple times within mammals yet have not evolved in other extant vertebrates. This suggests that there is a feature specific to mammals that facilitates the evolution of this specialized dentition. To investigate what may underpin the evolution of tusks, we histologically sampled the tusks of dicynodont therapsids: the earliest iteration of tusk evolution and the only non-mammalian synapsid clade to have acquired such a dentition. We studied the tissue composition, attachment tissues, development and replacement in 10 dicynodont taxa and show multiple developmental pathways for the adult dentitions of dicynodont tusks and tusk-like caniniforms. In a phylogenetic context, these developmental pathways reveal an evolutionary scenario for the acquisition of an ever-growing tusk-an event that occurred convergently, but only in derived members of our sample. We propose that the evolution of an ever-growing dentition, such as a tusk, is predicated on the evolution of significantly reduced tooth replacement and a permanent soft-tissue attachment. Both of these features are fixed in the dentitions of crown-group mammals, which helps to explain why tusks are restricted to this clade among extant vertebrates.

The giant crocodyliform Sarcosuchus from the Cretaceous of Africa.

New fossils of the giant African crocodyliform Sarcosuchus imperator clarify its skeletal anatomy, growth patterns, size, longevity, and phylogenetic position. The skull has an expansive narial bulla and elongate jaws studded with stout, smooth crowns that do not interlock. The jaw form suggests a generalized diet of large vertebrates, including fish and dinosaurs. S. imperator is estimated to have grown to a maximum body length of at least 11 to 12 meters and body weight of about 8 metric tons over a life-span of 50 to 60 years. Unlike its closest relatives, which lived as specialized piscivores in marginal marine habitats, S. imperator thrived in fluvial environments.

Simplification as a trend in synapsid cranial evolution.

The prevalence and meaning of morphological trends in the fossil record have undergone renewed scrutiny in recent years. Studies have typically focused on trends in body size evolution, which have yielded conflicting results, and have only rarely addressed the question as to whether other morphological characteristics show persistent directionality over long time scales. I investigated reduction in number of skull and lower jaw bones (through loss or fusion) over approximately 150 million years of premammalian synapsid history. The results of a new skull simplification metric (SSM), which is defined as a function of the number of distinct elements, show that pronounced simplification is evident on both temporal (i.e., stratigraphic) and phylogenetic scales. Postcranial evolution exhibits a similar pattern. Skull size, in contrast, bears little relationship with the number of distinct skull bones present. Synapsid skulls carried close to their observed maximum number of elements for most of the Late Carboniferous and Early Permian. The SSM decreased in the Late Permian but, coincident with the radiation of early therapsids, the range of observed SSM values widened during this interval. From derived nonmammalian cynodonts in the Early Triassic through the earliest mammals in the Early Jurassic, both the minimum and maximum SSM decreased. Data from three representative modern mammals (platypus, opossum, and human) suggest that this trend continues through the Cenozoic. In a phylogenetic context, the number of skull elements present in a taxon shows a significant negative relationship with the number of branching events passed from the root of the tree; more deeply embedded taxa have smaller SSM scores. This relationship holds for various synapsid subgroups as well. Although commonly ascribed to the effects of long-term selection, evolutionary trends can alternatively reflect an underlying intrinsic bias in morphological change. In the case of synapsid skull bones (and those of some other tetrapods lineages), the rare production of novel, or neomorphic, elements may have contributed to the observed trend toward skeletal simplification.

Age rank/clade rank metrics--sampling, taxonomy, and the meaning of "stratigraphic consistency".

Paleontologists frequently contrast clade rank (i.e., nodal or patristic distance from the base of a cladogram) with age rank (i.e., relative first known appearances of the analyzed taxa) to measure the degree of congruence between the estimated phylogeny and the fossil record. Although some potential biases of these methods have been examined (e.g., the effect of tree imbalance), other properties of age rank/clade rank (ARCR) comparisons have not been studied in detail. A basic premise of ARCR metrics is that outgroup taxa diverged earlier than ingroups and thus should first appear in older strata. For example, given phylogeny (A,(B,C)), then taxon A should be sampled before either taxon B or taxon C. We examine this premise in the context of (1) phylogenetic theory, (2) taxonomic practice, (3) sampling intensity (R), and (4) factors other than sampling intensity (including cladogram accuracy). Simulations combining clade evolution and sampling over time indicate a poor relationship between ARCR metrics and R when all taxa are apomorphy-based monophyletic groups. However, a good relationship exists when taxa are either stem-based monophyletic groups or if workers include taxa without a priori decisions about monophyly or paraphyly. These results are not surprising because cladograms predict the order in which lineages diverged (which applies to stem-based monophyletic taxa) and the order in which morphologic grades appeared (which applies to paraphyletic taxa relative to derived monophyletic groups). Other factors that increase ARCR metrics when the average R stays the same include high temporal variation in R, budding instead of bifurcating speciation patterns, low extinction rates, cladogram inaccuracy, and (to a much lesser extent) large clade size. These results suggest several plausible explanations for patterned differences in ARCR metrics among clades, thereby compromising their validity as measures of the quality of the fossil record.

Effects of testosterone administration and castration on the forelimb musculature of male leopard frogs, Rana pipiens.

In Rana pipiens, forelimb muscles that are used by males to clasp females during amplexus are sexually dimorphic in mass, protein content, and fiber composition. This experiment examined the effects of castration and exogenous testosterone on wet mass, dry mass, and protein content of the 22 major forelimb muscles of male leopard frogs to determine whether established patterns of sexual dimorphism of the muscles are reflected in differential androgen sensitivity. Muscles ranged from highly and moderately responsive to testosterone treatment (e.g., flexors of the elbow and of the carpus; adductors of the shoulder and of the first digit) to nonresponsive to testosterone (antagonists to these muscles). The mean dry mass of the testosterone-responsive muscles ranged broadly from 28-164% over control values. Castration had little or no effect on the response to testosterone, nor did it affect muscle mass in frogs not treated with hormone, as compared to sham-operated animals. Experimental treatment did not alter water content or protein concentration of muscles. The degree of testosterone sensitivity exhibited among the muscles of males closely correlated with their degree of sexual dimorphism. We postulate that androgens influence the functional attributes of male forelimb muscles through both organizational and activational effects.