Paterson, JR, GD Edgecombe, MSY Lee. 2019. Trilobite evolutionary rates constrain the duration of the Cambrian explosion. Proceedings, National Academy of Sciences 116(10): 4394-4399 (March 5, 2019) DOI:10.5061/dry.ad.v7q827k
Summary. Trilobites are the most abundant and diverse fossils found in the Cambrian Explosion. Their body fossils first appear at the base of Cambrian Series 2, which is the second of four divisions of the Cambrian sedimentary record. The abrupt appearance of phylogenetically diverse trilobites suggests a cryptic history of evolutionary development that left no record of body fossils in sediments below Cambrian Series 2, although some trace fossils from stratigraphically lower sediments are attributed to trilobites. This study analyzed inferred rates of evolutionary change based on the distributions of 115 morphological traits in 107 species of Cambrian trilobites. Most families of Cambrian trilobites were included in the study. Results showed that there was no evidence of unusual rates of change of traits in the fossil record of Cambrian trilobites. This was interpreted to show that the Cambrian Explosion, in which trilobite diversity originated, must have happened before the record of trilobite body fossils began, as it is not documented in the fossil record. The explosion of trilobite diversity was truly rapid, and probably was accomplished during the first stage of the Cambrian (Terreneuvian) rather than extending deep into the Precambrian. A modern-style marine biosphere must have existed during deposition of the lowest Cambrian sediments, followed by large scale evolutionary stasis in the rest of the Cambrian.
Comment. Trilobites appear abruptly in Cambrian sediments, with 45 genera in 28 families and at least three orders appearing in the Cambrian Series 2 layers.1 No ancestors are evident in preceding layers. The usual explanation is that there was a long history of cryptic Precambrian trilobite evolution before the first fossils appear, with new types appearing gradually in the Cambrian.
Paterson et al give a different explanation –trilobites appear in a burst of diversity and abundance that continues throughout the rest of the Cambrian layers. The abrupt burst of diversity indicates that the explosion had occurred before trilobites appear in Cambrian Series 2, not afterward as is commonly understood. The rapid and steady increase in diversity through the rest of the Cambrian suggests that no long history of gradual Precambrian development is needed. Instead, trilobites must have originated in the Cambrian but were not preserved until deposition of Series 2 sediments. The authors state: “We conclude that the Cambrian explosion was over by the time the typical Cambrian fossil record commences and reject an unfossilized Precambrian history for trilobites, solving a problem that had long troubled biologists since Darwin.”
This does not seem to explain the problem of abrupt appearance at all – it makes the problem worse for evolutionary theory. It still fails to explain why trilobites appear abruptly in the sediments, with no evidence of evolutionary ancestors. The problem is worse now because the major burst of appearance of trilobite diversity is proposed to have occurred in a much shorter time than believed before, and still without any evolutionary ancestors. A better explanation may be that a diverse fauna of trilobites was created and the abrupt appearance of diversity is due to the abrupt onset of fossilization some time after the creation.