Diversification events and the effects of mass extinctions on Crocodyliformes evolutionary history

Abstract

The rich fossil record of Crocodyliformes shows a much greater diversity in the past than today in terms of morphological disparity and occupation of niches. We conducted topology-based analyses seeking diversification shifts along the evolutionary history of the group. Our results support previous studies, indicating an initial radiation of the group following the Triassic/Jurassic mass extinction, here assumed to be related to the diversification of terrestrial protosuchians, marine thalattosuchians and semi-aquatic lineages within Neosuchia. During the Cretaceous, notosuchians embodied a second diversification event in terrestrial habitats and eusuchian lineages started diversifying before the end of the Mesozoic. Our results also support previous arguments for a minor impact of the Cretaceous/Palaeogene mass extinction on the evolutionary history of the group. This argument is not only based on the information from the fossil record, which shows basal groups surviving the mass extinction and the decline of other Mesozoic lineages before the event, but also by the diversification event encompassing only the alligatoroids in the earliest period after the extinction. Our results also indicate that, instead of a continuous process through time, Crocodyliformes diversification was patchy, with events restricted to specific subgroups in particular environments and time intervals.

Keywords: Crocodyliformes; diversification; mass extinction; phylogeny; topology-based methods.

Figure 1.

Summary of the Crocodyliformes phylogeny depicting diversification events. Each terminal taxon has its fossil record, represented by black lines, depicted against geological time. Clades are collapsed and numbers in parentheses represent the total number of terminal taxa within each branch (some clades are represented by the name of higher level groups and others by the name of a genus within the clade). The node related to clades for which diversification shifts were obtained in the analyses are marked—the colour assigned to the symbol matches the colour of the period related to the shift and red was assigned to those interpreted as artefacts. Numbers within < > correspond to the intervals on which these shifts were identified. The ages of cladogenetic events, within the grey lines, do not correspond to time intervals in which they are depicted on the figure. Names of clades related to shifts identified in the analysis—A: Crocodyliformes; B: unnamed clade (all taxa more closely related to Zosuchus than to Gobiosuchus); C: unnamed clade (Hsisosuchus+Mesoeucrocodylia); D: Mesoeucrocodylia; E: Notosuchia; F: Sebecosuchia; G: Neosuchia; H: Metriorhynchidae; I: unnamed clade (all taxa more closely related to Goniopholis than to Theriosuchus); J: Goniopholididae; K: unnamed clade (all taxa more closely related to Bernissartia than to Laganosuchus); L: Eusuchia; M: Brevirostrines; N: Alligatoroidea; O: Crocodyloidea.