Larval ecology, life history strategies, and patterns of extinction and survivorship among Ordovician trilobites

Abstract

Differences in the larval ecology of Ordovician trilobites directly influenced the outcome of the Ashgill extinction (latest Ordovician) and indirectly governed the pattern of evolution in post-Ordovician trilobites. Larval ecology also affected survivorship patterns within the Ordovician, particularly between the Llandeilo and the Caradoc stages. All taxa with pelagic adults became extinct by the end of the Ordovician. Similarly, trilobites with entirely planktonic larvae had all but disappeared by the end of the Ordovician. Although suffering a significant loss in diversity, taxa with benthic larvae provided the ancestral stock for the majority of post-Ordovician lineages. Trilobites with a two-stage protaspid period (single planktonic followed by benthic larvae) suffered least during the Ashgill extinction, giving rise to approximately 23% of the new genera appearing in the early Silurian.Patterns of extinction/survivorship among trilobite taxa with different developmental strategies indicate that the so-called Ashgill extinction was most likely the result of a composite phenomenon, including environmental perturbations, ecosystem breakdown and biogeographic restriction, and was not the consequence of a single catastrophic event (e.g., bolide collision). Indeed, our results are consistent with extinction models which invoke periodic global cooling and sea level regression associated with glaciation. Taxa with planktonic larvae first suffered a marked decline in diversity between the Llandeilo and the Caradoc (end of Middle Ordovician), coincident with the onset of the Ordovician-Silurian glaciation. During this time interval, the paleogeographic ranges of most surviving genera with solely planktonic larvae were severely constricted to lower latitude paleobiogeographic provinces. During the Ashgill (Rawtheyan to Hirnantian), climatic and sea level fluctuations were most extreme because of continued and more extensive glaciation. At this time, when extinctions were particularly severe, there was also a significant effect upon the survivorship of taxa with benthic larvae.Our results indicate that the proportion of the life cycle spent in the water column, dependent upon a planktonic food source, was a critical factor in the survivorship of trilobite taxa during Middle to Late Ordovician time. Consequently, differences in life history patterns predisposed individual taxa to survival or extinction.

Biogeographic and bathymetric determinants of brachiopod extinction and survival during the Late Ordovician mass extinction

“…An alternative interpretation is that the extinction of deeper-water genera reflects expansion of oxygen minimum zones and associated sulfidic conditions [14], but recent studies [46] do not support this hypothesis. Changes in oxygenation would not necessarily affect all taxa similarly, and whether such changes can explain other selective patterns such as the disproportionate extinction of trilobites with planktonic life stages [17] is not clear.…”

Investigating age‐dependency of species extinction rates using dynamic survivorship analysis

“…The most serious of these is: Do different sub-groups of a taxon (whether true clades or ecological groups) have markedly different extinction rates? Much evidence exists for various fossil groups that this is sometimes the case (see Foote, 1988;Norris, 1988;Stanley and others, 1988;Chatterton andSpeyer, 1989, Baumiller, 1993). For example, Baumiller (1993) recently showed that the mean duration of camerate crinoid genera was significantly shorter than the mean duration of nonDownloaded by [University of Calgary] at 15:57 29 March 2015 pinnulate genera and argued that this may be linked to the more 'specialised' nature of the camerates.…”

Phylogenetic and Biogeographic Analysis of Sphaerexochine Trilobites

“…The event is particularly important for trilobites, as its selectivity profoundly affected the evolution of the group. Previous research suggests that trilobites with a planktonic larval stage are more strongly affected by the extinction event than trilobites with benthic larvae [13]. Furthermore, trilobite groups with a presumed pelagic adult stage completely go extinct at the Ordovician-Silurian boundary.…”