Ontogeny of Polycotylid Long Bone Microanatomy and Histology.

Plesiosauria is an extinct clade of diapsid marine reptiles that evolved in the Late Triassic and radiated globally for the remainder of the Mesozoic. The recent description of a pregnant specimen of Polycotylus latipinnis demonstrates that some plesiosaurs were viviparous. To establish a baseline of histological data on plesiosaur ontogeny, we sampled the mother and fetus of the gravid plesiosaur specimen. To widen the base of data concerning ontogeny and life history of plesiosaurs, we gathered additional morphologic and histologic data from a securely identified growth series of polycotylids from the Pierre Shale of South Dakota. Paleohistological thin sections were prepared from the three humeri. Both adults show a dense, heavily remodeled cortex consisting entirely of longitudinally oriented secondary osteons, except for a thin rind of superficial primary bone. The mother exhibits an external fundamental system, indicating it was fully mature; the other adult does not. In both adults the cortex grades into a spongy medulla, comprising large vascular canals and erosion rooms surrounded by secondary lamellar trabecular bone, and lacking a marrow cavity. The fetal humerus possesses a medullary region similar to that of the Dolichorhynchops bonneri adult, although its lamellar bone is primary and deposited around calcified cartilage. The medulla is demarcated from the cortex by a prominent Kastschenko's line. The cortex of the fetus is a relatively thin layer of periosteal woven bone, longitudinally to radially vascularized, and interfingered with columns of osteoblasts surrounded by rapidly-deposited extracellular matrix. The neonate humerus resembles the fetus, with its trabeculae identical in both size and histology, although it lacks calcified cartilage. The cortex is also similar but much thicker, consisting entirely of rapidly deposited, radially vascularized, woven to fibrolamellar bone. The cortex carries a line near its surface. This feature is not a line of arrested growth, but a sudden change in vascular angle and increase in bone density. We argue this feature is a birth line indicating a change in growth regime, possibly in response to increased hydrodynamic forces after birth. The birth line indicates that the neonate was about 40% of maternal length when born. Our histological data demonstrate that polycotylids had very high fetal growth rates, and that birth size was large. Comparison with the geologically oldest plesiosaur confirms that rapid growth evolved in the Triassic, although histological details differ, and the degree to which the polycotylid ontogenetic pattern is generalizable to other plesiosaurs is currently unknown. Further histological research utilizing full growth series is needed, particularly for Jurassic taxa.

Evolution in the sabre-tooth cat, Smilodon fatalis, in response to Pleistocene climate change.

The late Pleistocene was a time of environmental change, culminating in an extinction event. Few fossil localities record a temporal series of carnivore fossil populations from this interesting interval as well as Rancho La Brea (RLB). We analysed mandibles of Smilodon fatalis from RLB using 2-D geometric morphometrics to examine whether, and how, mandibular shape changes through time. Smilodon fatalis shows mandibular evolution with oscillations between a small, ancestral-type morph in pits 77 (≈37 Kybp) and 2051 (≈26 Kybp), a larger, more derived morph in pits 91 (≈28 Kybp) and 61-67 (≈13.6 Kybp), and an intermediate morph from pit 13 (≈17.7 Kybp). These oscillations end in pit 61-67, with greatest body size, and are estimated to have its widest gape and lowest bite force. Additionally, variation is lowest in pit 61-67, which was deposited concurrent with the Bølling­Allerød warming event, which may have important implications for the timing or conditions during the extinction event. Contra to a temporal Bergmann's rule, such rapid warming events appear to be correlated with larger, derived, morphologies whereas static, cooler, climates correlate with gracile, ancestral morphologies.

Viviparity and K-selected life history in a Mesozoic marine plesiosaur (Reptilia, Sauropterygia).

Viviparity is known in several clades of Mesozoic aquatic reptiles, but evidence for it is lacking in the Plesiosauria. Here, we report a Late Cretaceous plesiosaur fossil consisting of a fetus preserved within an adult of the same taxon. We interpret this occurrence as a gravid female and unborn young and hence as definitive evidence for plesiosaur viviparity. Quantitative analysis indicates that plesiosaurs gave birth to large, probably single progeny. The combination of viviparity, large offspring size, and small brood number differs markedly from the pattern seen in other marine reptiles but does resemble the K-selected strategy of all extant marine mammals and a few extant lizards. Plesiosaurs may have shared other life history traits with these clades, such as sociality and maternal care.

Inferring and testing hypotheses of cladistic character dependence by using character compatibility.

The notion that two characters evolve independently is of interest for two reasons. First, theories of biological integration often predict that change in one character requires complementary change in another. Second, character independence is a basic assumption of most phylogenetic inference methods, and dependent characters might confound attempts at phylogenetic inference. Previously proposed tests of correlated character evolution require a model phylogeny and therefore assume that nonphylogenetic correlation has a negligible effect on initial tree construction. This paper develops "tree-free" methods for testing the independence of cladistic characters. These methods can test the character independence model as a hypothesis before phylogeny reconstruction, or can be used simply to test for correlated evolution. We first develop an approach for visualizing suites of correlated characters by using character compatibility. Two characters are compatible if they can be used to construct a tree without homoplasy. The approach is based on the examination of mutual compatibilities between characters. The number of times two characters i and j share compatibility with a third character is calculated, and a pairwise shared compatibility matrix is constructed. From this matrix, an association matrix analogous to a dissimilarity matrix is derived. Eigenvector analyses of this association matrix reveal suites of characters with similar compatibility patterns. A priori character subsets can be tested for significant correlation on these axes. Monte Carlo tests are performed to determine the expected distribution of mutual compatibilities, given various criteria from the original data set. These simulated distributions are then used to test whether the observed amounts of nonphylogenetic correlation in character suites can be attributed to chance alone. We have applied these methods to published morphological data for caecilian amphibians. The analyses corroborate instances of dependent evolution hypothesized by previous workers and also identify novel partitions. Phylogenetic analysis is performed after reducing correlated suites to single characters. The resulting cladogram has greater topological resolution and implies appreciably less change among the remaining characters than does a tree derived from the raw data matrix.