Jurassic ichthyosaurs dominated upper trophic levels of marine ecosystems. Many species coexisted alongside each another, and it is uncertain whether they competed for the same array of food or divided dietary resources, each specializing in different kinds of prey. Here, we test whether feeding differences existed between species, applying finite element analysis to ichthyosaurs for the first time. We examine two juvenile ichthyosaur specimens, referred to Hauffiopteryx typicus and Stenopterygius triscissus, from the Strawberry Bank Lagerstätte, a shallow marine environment from the Early Jurassic of southern England (Toarcian, ~183 Ma). Snout and cranial robusticity differ between the species, with S. triscissus having a more robust snout and cranium and specializing in slow biting of hard prey, and H. typicus with its slender snout specializing in fast, but weaker bites on fast-moving, but soft prey. The two species did not differ in muscle forces, but stress distributions varied in the nasal area, reflecting differences when biting at different points along the tooth row: the more robustly snouted Stenopterygius resisted increases or shifts in stress distribution when the bite point was shifted from the posterior to the mid-point of the tooth row, but the slender-snouted Hauffiopteryx showed shifts and increases in stress distributions between these two bite points. The differences in cranial morphology, dentition and inferred stresses between the two species suggest adaptations for dietary niche partitioning.
The study of trait evolution in modular animals is more complicated than that in solitary animals, because a single genotype of a modular colony can express an enormous range of phenotypic variation. Furthermore, traits can occur either at the module level or at the colony level. However, it is unclear how the traits at the colony level evolve. We test whether colony-level aggregate traits, defined as the summary statistics of a phenotypic distribution, can evolve. To quantify this evolutionary potential, we use parent-offspring pairs in two sister species of the bryozoan Stylopoma, grown and bred in a common garden breeding experiment. We find that the medians of phenotypic distributions are evolvable between generations of colonies. We also find that the structure of this evolutionary potential differs between these two species. Ancestral species align more closely with the direction of species divergence than the descendent species. This result indicates that aggregate trait evolvability can itself evolve.
The evolution of trait variation among populations of animals is difficult to study due to the many overlapping genetic and environmental influences that control phenotypic expression. In a group of animals, bryozoans, it is possible to isolate genetic contributions to phenotypic variation, due to the modular nature of bryozoan colonies. Each bryozoan colony represents a snapshot of the phenotypes that correspond to a single genotype, which can be summarized as a phenotypic distribution. We test whether these phenotypic distributions are heritable across generations of colonies in two sister species of the bryozoan Stylopoma, grown and bred in a common garden breeding experiment. We find that components of phenotypic distributions, specifically median trait values of colony members, are heritable between generations of colonies. Furthermore, this heredity has macroevolutionary importance because it correlates with the morphological distance between these two species. Because parts of phenotypic distributions are heritable, and this heritability corresponds to evolutionary divergence between species, we infer that these distributions have the potential to evolve. The evolutionary potential of these phenotypic distributions may underpin the emergence of colony-level traits, like division of labor in colonies.
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