The asaphids are an incredibly diverse and successful Order of trilobites, and one of my absolute favourites. Not all asaphids have such stout, simple body plans. If we consider the large panoramic-eyed pelagic types or Hypodicranotus with the longest hypostome in relation to the body, we see that they are anything but "simple." In Canada, perhaps the most common trilobite to find in Ordovician rocks is the classic Isotelus. Flourishing from the Cambrian to the end of the Ordovician, they certainly dominated the seas on account of the impressive sizes they could attain. In fact, the biggest complete trilobite ever found is Isotelus rex, measuring 72 cm in length, and found by Dave Rudkin and his team in Churchill, Manitoba in 1999. The asaphids have had a lot of twists and turns in their development. If we consider the Russian asaphids, we can plot out the gradual increase in eye stalk length beginning with Asaphus cornutus, and then a bit more with A. punctatus, A intermedius, and finally the classic longest eye stalks belonging to A. kowalewskii. Eye stalk length is likely indicative of an adaptive trait in being able to burrow into sea floor sediment with just the eyes poking out. Other evolutionary traits include the elongation of the cephalon and pygidium, as can be seen in Megistaspidella and Ectenaspis. It is uncertain what purpose this may have served, but it is possible it assisted in more effective burrowing in sediment, if not also possibly a hydrodynamic benefit for swimming. Of course, most asaphids (not all) were nekto-benthic. They could certainly attain gigantic sizes; in Canada, giant Isotelus could reach a length of over 30 cm. Although size can be a great adaptive trait, it also can lead to some problems. We can reasonably assume the most fierce predator of the Ordovician seas would be nautiloids, and so being a bit bulkier may have proven an advantage. That being said, maintaining that size means a heavier dependence on available nutrition. Since trilobites moulted up to 30 times in their lives, energy requirements were likely quite high. Another unique feature to many asaphids is the hypostome. These quite often take on the appearance of a "fork." The purpose of the hypostome is still debated. Was it for rasping and feeding? We can credit the excellent research of Thomas Hegna for exploring this in more detail. For reference, see Hegna, T.A. 2010: The function of forks: Isotelus‐type hypostomes and trilobite feeding. Lethaia, Vol. 43, pp. 411–419. But perhaps the most interesting hypostome among the asaphids belongs to Hypodicranotus where the very extended fork runs almost the entire length of the body.
There is no doubt that I am a big fan of asaphids! They make up about a fifth of my collection in terms of distinct species. They were fairly resilient, too. If we consider the deep anoxic environment of the upper Ordovician that laid down the black shales of the Whitby Formation, it was an asaphid and one ptychopariid genus that was able to withstand that environment. In fact, the very first trilobite to be described was an asaphid, and the first trilobite in Canada to be described was... an asaphid! Although they were beneficiaries of the Ordovician biodiversity event (ODE), theirs was a candle that burnt twice as bright and was snuffed by the widespread extinction events that closed out the Ordovician. The torch would be passed to the other remaining Orders of trilobites. Comments are closed.
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Kane Faucher
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February 2024
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