While we wait out the winter, I may as well talk trilobites. Of the now 14 established orders, the Agnostida are an odd outlier in the trilobite clade. These diminutive trilobites emerged during the Cambrian "explosion" and vanished at the tail end of the Ordovician. There has even been dispute as to whether to include them as trilobites at all! Perhaps the most well known of the Order would be the very common Itagnostus interstrictus from the Wheeler shale in Utah. Agnostids were blind trilobites that may have been semi-pelagic or even benthic crawlers -- it is still a matter of debate. It can be challenging to determine what side is "up"! Not all agnostids have effaced features, as there are some from the Cambrian deposits in Russia that are quite textured.
I can't say that I have too many specimens of this unique Order (just a few) as I don't collect much material from the Cambrian. They are not particularly showy given their size and simple morphology, and they are closely related to the eodiscids, which have since been erected to their own standalone Order. Shergold (1991) argues that they are distinctly different. In many ways, they resemble other trilobitomorphs, but they have just enough similar morphological traits to include them under the trilobite banner. Those interested in learning more about the agnostida are encouraged to delve into the Treatise O: Arthropoda. See as well: Cotton, T.J. & R.A. Fortey. 2005. Comparative morphology and relationships of the Agnostida. In: Koenemann, S. & Jenner, R. (eds.). Crustacean Issues 16, Crustacea and Arthropod Relationships (CRC Press: Boca Raton). Comments are closed.
|
Kane Faucher
Archives
February 2024
|