Deep-time geometricians and hints on motor control evolution of marine invertebrates
Abstract Motor control is an important component of animal movement that relies on the nervous system to conduct coordinated locomotion. Marine trace fossils, recording a myriad of movement patterns of principally invertebrate bilaterians, provide unique geological evidences for the evolution of animal locomotor behaviour. Here we report highly geometrical circular trace fossils from the Upper Ordovician deep-water slope deposits of North China. They consist of a small-sized ring-like main burrow with around 8 or 12 regularly-spaced tangential branches, contributing to semi-octagonal or semi-dodecagonal patterns with an average turning angle between successive branches of about 45° and 30°, respectively. The individual specimens also display regular and locally highly symmetric combination patterns like a Venn diagram. The ordered geometric expression of tiny circular trace fossils here suggests a relatively high level of motor control from minute invertebrate animals. Bilaterian motor control evolution in the early Palaeozoic as evidenced by the marine trace fossil record could be a key ingredient of biological evolutions during the Cambrian and Ordovician radiations. The Ordovician Radiation as an ecological expansion in the marine realm was accompanied by distinct offshore migration of the shallow-shelf ichnofauna and compositional change in the deep-sea ichnofauna. The greatly ameliorated deep-sea food content and relatively homogenous deep-sea food distribution may have promoted more geometrical and patterned trace fossils with a high level of motor control in the deep-sea regime during the Late Ordovician. It is speculated that after the latest Ediacaran–early Cambrian that set up the bilaterian behavioural toolkit, the Ordovician and Cretaceous represent two periods with distinct behavioural and motor control evolution of the deep-sea ichnofauna, which were in pace with revolutions in pelagic organic matter circulation during the corresponding periods.