Decline in phylogenetic diversity of Arthrodira(stem-group Gnathostomata)correlates with major Devonian bioevents
Arthrodira,the most diverse subgroup of placoderms,or jawed stem gnathostomes,is noted for their occupation of high trophic levels,especially in the later stage of their evolutionary history.Despite the relatively complete fossil record of arthrodires,the results of previous studies on the arthrodire diversity,based on counting the numbers of taxa in each time interval(raw or taxic diversity),correlate poorly with major Devonian bioevents.Here,we assemble a new,exhaustive dataset of arthrodires,comprising 450 species of 219 genera.Most taxa are integrated into a supertree integrating the results from various phylogenetic investigations.Our analysis of the phylogenetic diversity,accounting for the presence of ghost lineages,reveals a very different pattern compared to the raw diversity.The phylogenetic diversity of arthrodires exhibited a typical early burst pattern,peaking in the Early Devonian(Lochkovian-Pragian Boundary),and followed by declines that aligned well with several major bioevents.Near each of the first four events,the arthrodire diversity experienced only minor drops and generally persisted at high levels.The later three events,particularly the Frasnian-Famennian Boundary and Devonian-Carboniferous Boundary Events,led to steep declines in arthrodire diversity,from which they never recovered before their complete extinction in the end-Devonian.All these declines were not evident in the raw or taxic diversity pattern,except that related to the Frasnian-Famennian Boundary Event.
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