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Genera by their very nature are expected to be monotypic and geographically and environmentally restricted at their origin, and most genera do not endure past their stage of first appearance. At the same time, those genera that do endure have a capacity to expand greatly in geographic range, environmental breadth, and species richness. Here we ask what it is that allows some genera and not others to survive past their inception. Using occurrence data from the Paleobiology Database, we find that initial geographic range has the strongest effect on survival, followed by environmental breadth, with the effect of species richness weaker on average. The effect of geographic range is strongest if measured as the distances spanned by the occurrences of a genus rather than the number of distinct areas in which a genus lives. We document substantial secular variation in selectivity of early survival. The most striking aspect of this variation is that survival is only weakly selective among genera that first appear during the Mesozoic. By following genera beyond their stage of first appearance, we find that selectivity with respect to all factors becomes systematically stronger as cohorts age and genera become more differentiated in range, breadth, and richness. This may help account for a previously identified statistical effect of genus age on the chances of survival.
Preservation in the fossil record is never perfect in the sense that we cannot sample all individuals of a given population in time and space. Incomplete detection (i.e., preservation and modern-day sampling of fossils) often affects estimates of other paleobiological parameters of interest, such as occupancy and turnover. Here, I simultaneously model the occupancy and detection probability of taxa, teasing apart the zeros in data that reflect true absences and those that imply non-detection of taxa that were actually present in the space and time of interest. Occupancy modeling, an approach first developed in population ecology, can easily incorporate covariates of interest, such as sampling effort and habitat variables. I use a data set of brachiopod taxa from the Paleozoic to illustrate the utility of this approach for paleontological questions. I demonstrate a range of models, including those that allow colonization between time intervals and those that incorporate facies as site covariates. I also suggest how future data collection can be improved so that process- and sampling-oriented approaches such as occupancy modeling can be applied with ease to paleobiological settings to answer important paleoecological and evolutionary questions.
In quantitative paleobiology, one common format for image-derived information is the closed two-dimensional outline curve. Before the end of the last century a great variety of morphometric tools were on offer to deal with this data type, including diverse analyses of distance from a central point, “eigenshape analysis” of a carefully normalized tangent angle function, and Procrustes analysis of configurations of sliding semilandmarks. For the special case of outline forms that are nominally bilaterally symmetric, this paper offers a hybrid approach that fuses the Procrustes toolkit with a variant of a much older method, analysis of radial distance according to angles out of a center. The synthesis represents each outline as a regularly resampled data set of radially aligned shape coordinates analogous to bilateral Procrustes shape coordinates. When the center is iteratively located in one particular way, this representation has the same Procrustes distance statistics as a conventional radial representation, while the formalism of shape coordinates permits symmetrization, localizability, and thin-plate spline visualizations. We explain the method and sketch some of its advantages and limitations in the course of an example involving 99 Baculites inornatus outlines spanning 90 m of deposits from a high-precision measured section in the Campanian-age Rosario Formation cropping out at Punta San Jose, Baja California. This sequence shows substantial changes of pole curvature over time, changes that may involve a punctuation event regarding hydrostatics, hydrodynamics, or defense against predators. A concluding comment deals with implications of the developments here for Baculites microevolution and for morphometrics.
Within conifers, active abscission of complete penultimate branch systems is not common and has been described mainly from juveniles. Here I present evidence for the abscission of penultimate branch systems within early so-called walchian conifers—trees with a plagiotropic branching pattern. The specimens studied originate from a middle Early Permian gymnosperm-dominated flora within the middle Clear Fork Group of north-central Texas. Complete branch systems of three walchian conifer morphotypes are preserved; all have pronounced swellings and smooth separation faces at their bases. The source plants grew in a streamside habitat under seasonally dry climatic conditions. The evolution of active branch abscission appears to correspond to an increase in the size of conifers, and this combination potentially contributed to the restructuring of conifer-rich late Paleozoic landscapes. Moreover, trees shedding branch systems and producing abundant litter have the potential to affect the fire regime, which is a factor of evolutionary importance because wildfires must have been a source of frequent biotic disturbance throughout the hyperoxic Early Permian.
The continuous fossil record of therocephalian therapsids (Eutheriodontia) across the Permo-Triassic boundary and their differential survivorship of the end-Permian extinction offer an exceptional deep-time perspective on vertebrate life-history evolution during episodes of large-scale ecological perturbation. To examine potential impacts of the extinction on body size evolution (e.g., “Lilliput” effects) and growth patterns, we investigated cranial sizes and limb bone histology in the therocephalian Moschorhinus kitchingi both before and after the end-Permian extinction, facilitated by analysis of thin-sections of 23 limb bones from an ontogenetic sample of ten individuals across the Permo-Triassic boundary. In general, early subadult Moschorhinus displayed propodial cortices with extensive woven- and parallel-fibered bone (PFB) with dense radial and reticular vascularization and a moderately thickened bone wall with few growth marks. The outer cortex of propodials and epipodials showed a transition to PFB and lamellar bone with longitudinally oriented canals in individuals interpreted as late subadults or adults (>80% largest size). Most elements displayed several (3 ) growth marks, though growth marks were more faithfully recorded in the epipodials of Permian individuals. Pearson product-moment correlation tests were performed to examine the relationship between size and robusticity on growth proxies (% cortical vascularity, mean primary osteon diameter), but variation in histomorphology could not be explained by size alone. Variation in body size may be affected by differences in juvenile growth rate and duration, which are highly variable in environmentally stressed extant reptile species. Geologic stage was a more consistent predictor of cortical vascularity. We suggest that Permian and Triassic Moschorhinus exhibited differential rates of early skeletal growth, corroborating the hypothesis that increased environmental variability in the earliest Triassic was associated with rapid growth to a minimum body size requirement and, consequently, shortened developmental times.
Using tangential thin sections, we examined variation in porosity and water vapor conductance across two eggs of Troodon formosus, a small (∼50 kg) theropod dinosaur from the North American Upper Cretaceous, testing two hypotheses of egg incubation: (1) full burial within sediments or vegetation and (2) partial burial with exposed upper egg portions. We divided and sampled the eggs in five zones, 1 through 5 from blunt top to more pointed bottom. A geometric model composed of a hemisphere, cone, and paraboloid was used to estimate total and zonal volumes and surface areas. The 138 × 67 mm idealized Troodon egg has a volume, surface area, and mass of 296.4 cm3, 239.23 cm2, and 314.2 g, respectively. Zonal surface areas and volumes highlight the strongly asymmetric and elongate form of the Troodon egg. Geometric modeling provides better estimates of volume and surface area where egg shape diverges markedly from that of a typical bird egg. Porosity varies significantly across both Troodon eggs, with zones 2 and 3 having the largest pores and a majority (70–78%) of total conductance, whereas zone 5 has very low conductance. Total water vapor conductance in the two eggs are 31.85 and 40.62 mg H2O day− Torr−, values 76% and 97% of those predicted for an avian egg of similar size. Low total conductance compares favorably to values in extant birds and non-avian reptiles that incubate in open nests, arguing against full burial incubation. Together with nesting site evidence, low conductance values favor partial burial and incubation by a Troodon adult. Asymmetric egg shape concentrates volume, surface area, and conductance near or at the point of subaerial exposure. Among non-avian dinosaurs, the eggs of Troodon and troodontids are most similar to those of modern birds in having an asymmetric shape, low porosity, no ornamentation, and three structural eggshell layers.
Nectocaridids are soft-bodied early to middle Cambrian organisms known from Burgess Shale-type deposits in Canada, China, and Australia. Originally described as unrelated species, they have recently been interpreted as a clade; their flexible tentacles, camera-type eyes, lateral fins, internal gills, axial cavity, and funnel point to a relationship with the cephalopods. However, aspects of this reinterpretation, including the relevance of the group to cephalopod evolution, have been called into question.
Here, I examine new and existing nectocaridid material, including a large new form that may represent a sexual dimorph of Nectocaris pteryx. Differences between existing taxa largely represent taphonomic variation between sites and specimens—which provides further constraint on the organisms' anatomy. I revise the morphology of the tentacles and fins, and describe mouthparts and phosphatized gills for the first time. A mathematical analysis supports the presence of the earliest known camera-type eyes, and fluid mechanical considerations suggest that the funnel is optimized for efficient jet propulsion in a low Reynolds number flow regime.
Nectocaridids closely resemble coleoid cephalopods, but a position deeper within Cephalopoda raises fewer stratigraphic challenges. Whether its coleoid-like construction reflects common ancestry or profound convergence, the Nectocaris body plan adds substantially to Cambrian disparity, demonstrating the rapid colonization of nektobenthic niches after the Cambrian explosion.