The shell size of 1236 ammonite species representing all known Early Jurassic faunas is analyzed. Size patterns are studied for the entire period and then at the biozone scale for the first four stages of the Jurassic (28 Myr), during which ammonites recovered from the crisis at the Triassic/Jurassic (T/J) boundary. Our analysis reveals that (1) a size continuum (normal distribution from “dwarfs” to “giants”) exists for all Early Jurassic ammonites; (2) although there are no sustained trends (e.g., no Cope's rule), the succession is not monotonous and patterns may differ conspicuously from one biozone to the next; and (3) increases and decreases in size range are the most frequent evolutionary styles of size change. The only pattern that can be connected with a particular episode of Early Jurassic ammonite history is the initial increase in size disparity during the first four biozones attributable to phyletic radiation after the T/J crisis. Subsequent correlations with environmental constraints (e.g., sea-level changes), although suspected, cannot be shown.

Previous studies have established a close relationship between the evolutionary origin of new clades of planktonic foraminifera and heterochrony. Studies of the Paleogene radiation of the genus Morozovella revealed, for example, a temporal pattern of variation consistent with paedomorphosis. Our study focused on the late Paleocene species of Acarinina, sister group of Morozovella. Shape variations related to evolution and ontogeny are appraised through a morphometric method based on outline analysis using the elliptic Fourier transform. Patterns of developmental and evolutionary changes are studied and compared within each species (Acarinina nitida, A. subsphaerica, and A. mckannai). As no congruence is found, we suggest that the evolutionary change observed within these species is not related to a heterochronic process. We also test for similarity of both evolutionary and ontogenetic changes among species. Although we observe no significant correlation between temporal patterns of shape change among species, the tight congruence of ontogenetic trajectories suggests that the developmental constraints affecting these trajectories have been preserved in spite of the evolutionary diversification of acarininids. Heterochrony is not clearly involved in the early Paleogene diversification of acarininids and therefore may not be as common as previously claimed. The role of developmental constraints in monitoring morphological evolution therefore needs to be reassessed.

Spatial and temporal variations in biological diversity are critical in understanding the role of biogeographical regulation (if any) on mass extinctions. An analysis based on a latest database of the stratigraphic ranges of 89 Permian brachiopod families, 422 genera, and 2059 species within the Boreal, Paleoequatorial, and Gondwanan Realms in the Asian–western Pacific region suggests two discrete mass extinctions, each possibly with different causes. Using species/family rarefaction analysis, we constructed diversity curves for late Artinskian–Kungurian, Roadian–Wordian, Capitanian, and Wuchiapingian intervals for filtering out uneven sampling intensities. The end-Changhsingian (latest Permian) extinction eliminated 87–90% of genera and 94–96% of species of Brachiopoda. The timing of the end-Changhsingian extinction of brachiopods in the carbonate settings of South China and southern Tibet indicates that brachiopods suffered a rapid extinction within a short interval just below the Permian/Triassic boundary.

In comparison, the end-Guadalupian/late Guadalupian extinction is less profound and varies temporally in different realms. Brachiopods in the western Pacific sector of the Boreal Realm nearly disappeared by the end-Guadalupian but experienced a relatively long-term press extinction spanning the entire Guadalupian in the Gondwanan Realm. The end-Guadalupian brachiopod diversity fall is not well reflected at the timescale used here in the Paleoequatorial Realm because the life-depleted early Wuchiapingian was overlapped by a rapid radiation phase in the late Wuchiapingian. The Guadalupian fall appears to be related to the dramatic reduction of habitat area for the brachiopods, which itself is associated with the withdrawal of seawater from continental Pangea and the closure of the Sino-Mongolian seaway by the end-Guadalupian.

Living fossils are taxonomic groups surviving for a long time without any remarkable morphological change. Most of them retain low taxonomic diversities. Although some of them have survived in refuges to avoid predators and competitors, not all living fossils live in refuges. The survival of these groups, therefore, should be discussed in the context of biological interaction. I carried out computer simulations of a model food web system, in which each species feeds on others according to its feeding preference. The system evolves via evolution of species. In the simulation, some clades, like “living fossils,” survived for a long time with low species diversities. Such clades consisted of species with low evolutionary rates, which result in high predation pressure and intraclade competition for food. Nevertheless, the clades sustainably utilize prey clades and are consequently provided with sufficient food. In addition, because of the low species diversities of the clades, predators of the clades soon become extinct through lack of food. This study strongly suggests that in an evolving food web system, the low evolutionary rates of living fossils allow the long survival of those groups with low taxonomic diversities.

It has been contended that Reif's odontode regulation theory is a rival and alternative to Stensiö and ørvig's lepidomorial theory as means of explaining the evolution of development of the vertebrate dermal and oral skeleton. The lepidomorial theory is a pattern-based theory that provides a homological framework that goes further than the odontode regulation theory in comparing dental papillae and their products, and it provides an explanatory mechanism for such relationships a posteriori. In contrast, the odontode regulation theory is process-based and observes only developmental similarity, providing no means of identifying homologies beyond this. The lepidomorial theory is superior to the odontode regulation theory in its ability to trace homology through the evolution of development of the dermal and oral skeleton. The criteria proposed to identify homology between scales—either within a given individual or taxon, or between different individuals or taxa—are, primarily, vascular architecture and, secondarily, external morphology. External morphology may be excluded on Reif's argument for the overarching principle of differentiation, a hypothesis supported by recent advances in the understanding of dental morphogenesis. Vascular architecture is potentially useful but appears to be determined by tooth/scale morphology rather than reflecting historical (phylogenetic) constraint. Data on the development of epithelial appendages, including teeth, scales, and feathers, indicate that individual primordia develop through progressive differentiation of originally larger, homogenous morphogenetic fields. Thus, there is no mechanism of ontogenetic developmental concrescence, just differentiation. Phylogenetic patterns of concrescence and differentiation are similarly achieved through ontogenetic developmental differentiation, or a lack thereof. In practice, however, it is not possible to distinguish between patterns of phylogenetic concrescence and differentiation because there is no means of identifying homology between individual elements within a squamation, or a dentition (in almost all instances). Thus, phylogenetic patterns of increase and decrease in the numbers of elements constituting dentitions or dermal elements are best described as such; further attempts to constrain precise underlying patterns remain without constraint and outside the realms of scientific enquiry. The application of the homology concept in the dermal and visceral skeletons is explored and it is determined that odontodes are serial homologs, conform only to the biological homology concept at this level of observation, and are devoid of phylogenetic meaning. It is concluded that Reif's theory is close to a universal theory of the evolution of development for the dermoskeleton and dentition, and additional components of theory, including the regulatory basis of temporal and spatial patterning, are tested and extended in light of data on the development of the chick feather array. Finally, the dermoskeleton is identified as an exemplary system for examining the regulatory basis of patterning and morphogenesis as it encompasses and surpasses the repertoire of established model organ systems.

Teeth are generally the best-preserved elements among mammal fossil remains and are highly diagnostic characters. Consequently, much mammalian paleontological, systematic, and evolutionary research focuses on teeth, so it is important to understand how they vary and covary with other characters. Dental traits within populations of carnivores appear to be more variable than cranial traits, a pattern that results only partly from their usually smaller size. Furthermore, dental traits, although highly correlated with one another, are not highly correlated with cranial traits, which are also highly correlated with one another. Thus, teeth and cranial bones may be subject to quite different selective pressures and genetic/developmental constraints and may suggest different microevolutionary scenarios. Vestigial teeth show significantly greater variability than expected, reflecting the absence of stabilizing selection.

Theropod dinosaurs were, and mammalian carnivores are, the top predators within their respective communities. Beyond that, they seem distinct, differing markedly in body form and ancestry. Nevertheless, some of the same processes that shape mammalian predators and their communities likely were important to dinosaurian predators as well. To explore this, we compared the predatory adaptations of theropod dinosaurs and mammalian carnivores, focusing primarily on aspects of their feeding morphology (skulls, jaws, and teeth). We also examined suites of sympatric species (i.e., ecological guilds) of predatory theropods and mammals, emphasizing species richness and the distribution of body sizes within guilds. The morphological comparisons indicate reduced trophic diversity among theropods relative to carnivorans, as most or all theropods with teeth appear to have been hypercarnivorous. There are no clear analogs of felids, canids, and hyaenids among theropods. Interestingly, theropods parallel canids more so than felids in cranial proportions, and all theropods appear to have had weaker jaws than carnivorans. Given the apparent trophic similarity of theropods and their large body sizes, it was surprising to find that species richness of theropod guilds was as great as or exceeded that observed among mammalian carnivore guilds. Separation by body size appears to be slightly greater among sympatric theropods than carnivorans, but the magnitude of size difference between species is not constant in either group. We suggest that, as in modern carnivoran guilds, smaller theropod species might have adapted to the threats posed by much larger species (e.g., tyrannosaurs) by hunting in groups, feeding rapidly, and avoiding encounters whenever possible. This would have favored improved hunting skills and associated adaptations such as agility, speed, intelligence, and increased sensory awareness.

The vomeronasal (VN) system is a pheromone-processing sensory system of tetrapods. Tetrapods use pheromones to communicate territorial boundaries, reproductive status, sex, and species identity. Presumed impressions of VN bulbs on phytosaur frontals led to a claim that phytosaurs possessed the VN system. However, in extant crocodilians, which lack the VN system, the corresponding impressions are associated not with cerebral tissue but with the ophthalmic nerves. Phytosaur head morphology was not conducive to pheromone collection. The extant phylogenetic bracket suggests that all extinct archosaurs, including phytosaurs, lacked the VN system. Without the pheromonal sense, they would not have used chemical means to convey territorial boundaries, reproductive status, sex, and species identity. Instead, they would have used visual, acoustic, and tactile cues, as in extant archosaurs and other tetrapods in which the VN sense is reduced or absent.