Hadromeros: A Trilobite Survivor

Reconstruction of Hadromeros subulatus, from Kielan-Jaworowska et al. (1991).

Trilobites of the genus Hadromeros were widespread in the Late Ordovician and Early Silurian of Eurasia and North America. They are classified in the Cheiruridae, the same family that includes another trilobite genus that has been featured on this site, Sphaerexochus. However, Hadromeros differs from Sphaerexochus in that its glabella (its 'nose') is not as large. Whereas in my earlier post I suggested that Sphaerexochus may have been a predator, Hadromeros was probably a less aggressive feeder. It was possibly a detritivore, picking bits of nutritious material out of the sand and mud. This interpretation is supported by the known leg morphology of a closely related genus, Ceraurus, in which the legs are fairly generalised and show little adaptation for food processing (Bergström 1973).

Hadromeros and Ceraurus are placed in the Cheirurinae, a distinct subfamily from the Sphaerexochinae that includes Sphaerexochus. One characteristic feature of many members of the Cheirurinae is that the bases of the pleura, the plates that run down each side of the thorax, are swollen in dorsal view. The purpose of these swellings remains unknown. You might expect that, trilobites being as abundant in the fossil record as they are, we would know a great deal about them, and in many respects that is quite true. However, in other respects our knowledge is also frustratingly incomplete. Trilobites have an extensive fossil record because their dorsal exoskeleton was mineralised, and it is this that is usually preserved. The ventral section of the body, on the other hand, was not mineralised, and is only preserved under exceptional circumstances. This includes such significant features as the legs and mouthparts (as indicated above, we have some knowledge of the leg morphology of Cerarurus, but no direct evidence for Hadromeros). It is possible that the cavities underneath the cheirurine pleural bases housed some modification of the gills, if the gills in trilobites were comparable to those of living crustaceans. But how or to what purpose the gills were modified can only be speculated upon.

Morphologically, Hadromeros was a fairly unremarkable trilobite, but it stands out from the other genera of the Cheirurinae in one important respect (that has, indeed, already been alluded to in this post). The end of the Ordovician saw a mass extinction in marine life, by some measures the second largest to have ever occurred. Few groups of animals made it through unscathed, and the cheirurids were no exception. Of the eight subfamilies of Cheiruridae recognised by Přibyl et al. (1985), only three made it through to the Silurian: the Cheirurinae, Sphaerexochinae and Deiphoninae. Within the Cheirurinae, Hadromeros is the only genus currently known from both sides of the Ordovician-Silurian boundary, and may have been ancestral to all other post-Ordovician cheirurines. While other genera were whisked away, Hadromeros became the Trilobite that Lived.

REFERENCES

Bergström, J. 1983. Palaeoecologic aspects of an Ordovician Tretaspis fauna. Acta Geologica Polonica 23 (2): 179-206.

Kielan-Jaworowska, Z., J. Bergström & P. Ahlberg. 1991. Cheirurina (Trilobita) from the Upper Ordovician of Västergötland and other regions of Sweden. Geologiska Föreningen i Stockholm Förhandlingar 113 (2-3): 219-244.

Přibyl, A., J. Vaněk & I. Pek. 1985. Phylogeny and taxonomy of family Cheiruridae (Trilobita). Acta Universitatis Palackianae Olomucensis Facultas Rerum Naturalium Geographica-Geologica XXIV 83: 107-193.