Exoskeletal architecture, hypostomal morphology and mode of life of Silurian and Lower Devonian dalmanitid trilobites

The overall exoskeletal architecture and hypostomal morphology of Silurian Dalmanites and Lower Devonian Odontochile, Reussiana and Zlichovaspis are compared and discussed. All four genera were benthic. However, a gradual shift from predator-scavenger to a scavenging feeding habit is inferred from the evolution of these dalmanitids in the Upper Silurian and Lowe Devonian. In the more conservative, probable predator-scavenger Dalmanites, the posterior hypostomal doublure has a group of robust denticles: these would have been effective shredding structures, enabling milling or cutting of food into smaller particles, and allowing Dalmanites to deal with mechanically relatively robust prey. Lower Devonian dalmanitids from the Prague Basin possess a comparable denticulate structure on the posterior hypostomal doublure. However, although overall exoskeletal dimensions are larger, the denticles are significantly finer and denser. The whole exoskeleton of these younger dalmanitids, with a large multi-segmented pygidium, is considered well adapted for close approximation to the sediment surface. Shallow burrowing in a soft substrate, and scavenging with opportunistic predation, is therefore considered to be their main feeding habit. Odontochile, Reussiana and Zlichovaspis characteristically occur in micritic limestone facies, where the sediment surface is likely to have been soft. The shift from a more predatory to a dominantly scavenging mode of life may have occurred independently in several unrelated dalmanitid groups, with “odontochilid” characteristics developing at different stratigraphical level.