A new trilobite fossil reveals the cephalic specialization of trilobites in the central Cambrian

Trilobites reached their greatest genetic diversity in the Cambrian. However, unlike this measure of diversity, the morphological disparity of trilobites based on cranial profile peaked in the mid to late Ordovician.

Mid-early Cambrian trilobites with specialized cephalon are rare, especially among ptychoparioids. Even with a few exceptions, ptychoparioids exhibit a monotonous pattern of head specialization, characterized by additional cephalic border spines.

Recently, led by Prof. Zhao Fangchen, postgraduate Sun Zhixin and Dr. Zeng Han of the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences (NIGPAS) described a ptychopariid trilobite with an unusual cephalic morphology called Phantaspis auritus gen. et sp. nov. from Cambrian Central Mantou Formation in Shandong Province, Northern China.

This unique trilobite provides new insight into the morphological range and structural basis of cephalic specialization in Cambrian trilobites. The study was published in Acta Palaeontologica Polonica.

Phantaspis is characterized by a cephalon with an extended anterior area of ​​double lobed shape resembling a pair of rabbit ears in later ontogenetic stages, representing a form of specialization in a Cambrian trilobite that has not been repeated in any younger trilobite . This illustrates the diversity of Cambrian trilobites in morphotypes and provides an example of variation of the ptychoparioid cranial profile during the Middle Cambrian caused by specialization.

The Phantaspis extended gray mullet resembles some sediment feeders with a specialized gray mullet, for example the Harpina and Trinucleidae species. However, in Phantaspis the anterior border was not as thickened as those of the above groups. In addition to adapting to a particular lifestyle habit, additional possibilities should be considered.

The cephalic form observed in Phantaspis may have reduced the risk of predation by increasing their effective size, thus making it more difficult for predators to eat, similar to other trilobites.

Furthermore, the development and stabilization of cranial morphology associated with sexual maturity suggests a possibility of sexual selection, similar to “scarab-like” horns known from other trilobites, which are presumed to reflect this type of selective strategy.