Morphologic evolution in the cephalon of the Ordovician-Devonian Phacopida trilobites and its responds to environmental changes

The Phacopida are a representative order of trilobites from the Ordovician to the Devonian periods, occupying an important position in the marine ecosystem during the early Phanerozoic spannedmany significant geological events in the Paleozoic. The taxonomic diversity of Phacopida has been well investigated, but the co-evolution of its morphological disparity with geological events still needs to be clarified. This study focuses on the morphological evolution of Phacopida trilobites and its response to geological events ranging from the Ordovician to Devonian. Utilizing a geometric morphometric approach, we analyzed the shape of cephalon of 351 distinct genera and quantified the morphological changes of Phacopida from the Ordovician to Devonian based on morphospaces. We found that the peak of morphological disparity in the cephalon of Phacopida occurred in the Darriwilian and the Eifelian, and there were three major recessions, respectively: the Katian, the latest Silurian, and the Famennian, all of them appeared in periods of major environmental disturbances such as anoxic events and regressions, suggesting that the morphological evolution in the cephalon of Phacopida was closely related to oxygen content in sea water and the changes of sea level, and these environmental factors greatly affected the variation of their morphological disparity. Meanwhile, the number of new genus of Phacopida decreased during the Eifelian, but morphological disparity in the cephalon(SOV and SOR) increased in the same time, which means the peak of morphological disparity appeared after the peak of taxonomic diversity. This could be because the rising Eifelian competitors which have similar ecological niche of Phacopida, restrained the taxonomic diversity of Phacopida, but these competitors has also increased competition between species in the same ecological niche and promoted the morphological innovation in the cephalon of new species of Phacopida.