Escalation reflected in ornamentation and diversity history of brachiopod clades during the Mesozoic marine revolution

Abstract

After the end-Permian mass extinction, the phylum Brachiopoda attained a secondary peak of diversity in the Mesozoic. Although two orders (the Athyridida and Spiriferinida) became extinct in the Jurassic, the extants Rhynchonellida and Terebratulida diversified and apparently demonstrated escalatory trends in response to increasing predation pressure during the Mesozoic marine revolution. It has been already shown that brachiopods and other clades of shelly benthos increased their ornamentation as antipredatory defences in the course of the mid-Palaeozoic marine revolution. This study examines the temporal changes in ornamentation of Mesozoic rhynchonellid and terebratulid brachiopods to determine if analogous escalation-related adaptation existed. Temporal changes in ornamentation and taxonomic diversity of Mesozoic Rhynchonellida and Terebratulida is documented in detail at the genus level. The number of rhynchonellid genera (generic diversity) increased during the Triassic from 7 to 46; then, after minor decreases, rose to 57 in the mid-Jurassic, and rapidly fell in the Late Jurassic to the end of the Mesozoic. The strength of ornamentation showed a similar temporal change: steady increase through the Triassic and Jurassic, with the appearance of strongly costate and spinose forms, and afterwards, a sudden decrease during the latest Jurassic and Early Cretaceous. The early Mesozoic trends are interpreted to reflect escalation, i.e. the continuous effort to adapt to the threat of gradually increasing durophagous and boring predation. The Late Jurassic decline and Cretaceous minima in diversity and ornamentation suggest that the rhynchonellid clade became a victim in the Mesozoic marine revolution. By contrast, terebratulids seem to keep pace with the amplifying predation: the degree of their ornamentation increased and their diversity only moderately decreased after the Jurassic maximum. From that time until the present, the two clades followed diverging evolutionary pathways. Rhynchonellids gave up the arms race, the coarsely ornamented forms gradually disappeared from the fossil record, and the clade abandoned the level-bottom communities to survive in environments with less predation pressure. Terebratulids increased their ornamentation and other antipredatory tools, and remained a fairly successful clade even in the shallow marine habitats.