Recovery from the end-Triassic mass extinction in European shelf seas: body size changes in marine invertebrates and the role of temperature and anoxia

Fossil communities in the wake of mass extinction are often characterised by small sized individuals, this is often ascribed to the Lilliput Effect (a transient body size reduction within a surviving species). During periods of biotic recovery body size should increase, this, and its relationship with other recovery parameters are seldom documented. In order to explore the relationships between size, diversity and environments bivalve taxonomic richness and body size are recorded from the latest Triassic and Lower Jurassic of the British Isles. This interval encompasses the end-Triassic mass extinction event and its subsequent recovery. During the post-extinction interval oxygen depleted marine waters are reported to have become widespread and allegedly delayed onset of biotic recovery.
During the Lower Jurassic three phases of size change are noted: an initial size increase from the extinction event through the Hettangian Stage (Lower Jurassic); a phase of size decrease characterises the next Stage, the Sinemurian; another phase of size increase then followed during the Pliensbachian Stage. These trends are reported for whole communities as well as among- and within-species. The latter is well exemplified by the dramatic size increase seen in Plagiostoma giganteum J. Sowerby which increases in size by 179% over the Hettangian. It is here proposed that within-species size increase in newly originating taxa following mass extinction be termed the Brobdingnag Effect, in honour of the giants in Gulliver’s Travels.
Such patterns in body size act independent of other recovery metrics and appear unrelated to marine oxygenation which, for the Hettangian and lowermost Sinemurian, was oxic-dysoxic interspersed by comparably brief, localised bouts of anoxia. Temperature often exerts a control on the body size with creatures in cooler climates often being larger sized (i.e. Bergmann’s Rule). Temperature appears unrelated to body size patterns in the Lower Jurassic, however there are gaps at critical intervals.