Good-tidings and well-wishes!
I realize that I’ve been rather neglectful of my blog of late – an unfortunate fact which has been due to my recent trip to my ‘native land’ of New York to visit my family, see the AMNH (don’t worry, I’ll post pictures!), check out the paleontology department at SUNY Stony Brook , and gain a bit of weight (which I’m currently attempting to shed) from enjoying too much Italian and sea food, among other things. In any event, I’ve decided to try something different for this week’s ‘Wednesday Wonders’ installment: covering an invertebrate. Though I’m most certainly a devoted vertebrate paleontology enthusiast, I can’t deny that certain fossil inverts intrigue me quite a bit. Since I’m not nearly as knowledgeable about spineless organisms as I am about chordates (and in particular, tetrapods), I can’t post about these animals very often. However, I feel that in this case, I can make an exception, for I’ve been recently enjoying my newly-acquired copy of David L. Meyer and Richard A. Davis’ “A Sea Without Fish: Life In The Ordovician Sea Of The Cincinnati Region.” A sizable portion of this excellent book’s chapter on arthropods is dedicated to the eurypterids of the period and area, and considering the fact that these critters are all over New York state (whose official fossil isEurypterus) as well, I can’t help but feeling a certain attachment to them, which is aided in part by their fascinating appearance.
As Meyer and Davis write,
“Eurypterids are chelicerate arthropods, distinguished by having the first pair of appendages (chelicerae) equipped with small pincers. Modern cheliceratesare the horseshoe crabs and arachnids…The body of a eurypterid is unique, with a distinct head (prosoma) bearing compound eyes followed by an elongated, segmented section called the opisthosoma, divided into a wider preabdomen and a narrower, tail-like postabdomen…Six pairs of appendages were attatched to the underside of the head and served functions of feeding and locomotion. Because the exoskeleton was not calcified, preservation of the chitinous remains of eurypterids was unlikely, and therefore they are usually very rare fossils.”
For a general review of eurypterid anatomy, do go here.
Within the Eurypterida class, Megalograptus is the name-bearing genus of the Megalograptoidea superfamily and the Megalograptidae family. Palaeos.com describes the latter as
“huge, spectacular, spiny, predatory Ordovician forms, clearly active hunters, seem to have taken over from the waning Endocerid nautiloids as the superpredators of the late Ordovician seas. Nevertheless, they were a short-lived group, geographically restricted, and seem to have been wiped out by the terminal Ordovician mass extinction, although not before giving rise to the Mixopteridae. The compound eyes face forward, and there is a tongue-like extension at the front of the prosoma bearing large marginal spines. The cheliceraesmall and short, 1st and 3rd walking legs relatively short, with closely set spines, second pair of legs enormously developed, with long paired spines for snatching prey. The preabdomen narrow with axial furrows [(plow-like structures running along the preabdomen region of the body)]; the postabdomen moderately narrow; telsonnarrow lanceolate [(leaf-shape)], with what seem to be side claspers. The epistoma short and broad; metastoma subovate to cordate; genital appendage type of type “B” form club-shaped, of type “A” form short and narrow.”
The animal was first described on the basis of fragmentary material in 1874 by S.A. Miller, who believed them to be parts of an enormous graptolite, so he named this new genus “Megalograptus” or “great graptolite”. But, as Meyer and Davis reveal,
“Later workers corrected the error on the basis of additional, albeit fragmentary discoveries. A truly phenomenal discovery made in 1938 of exceptionally well-preserved and nearly complete specimens from a single bed in the uppermost Cincinnatian Elkhorn Formation of the Richmond Group in Adams County, Ohio, led to a better understanding of the animal. This material, which included male and female specimens [(females were presumably somewhat larger)], became the basis for a new species, M. ohioensis, described by Kenneth E. Caster and Erik Kjellesvig-Waering in 1964.”
The authors go on to cite this creature as “one of the largest creatures in the Cincinnatian sea floor community, reaching a length of over 50 cm.” Regarding the aforementioned spines characteristic of the group, they concede that
“Exactly how the eurypterid used these spines is uncertain. Caster and Kjellesvig-Waering considered Megalograptus to have been a predator, and thus the appendages likely had some function in grasping prey. The basket-like structure of the long spines of the third appendage suggests that the animal might have raked them through the sediment in order to extract prey in a sieving fashion. Only a few other eurypterids have similar long spiny appendages. Tubular castings filled with fragments of eurypterid integument associated with the eurypterid material could represent feces of the animal, indicating cannibalistic behavior like that found in living eurypterid relatives among the scorpions and spiders. The fourth pair of appendages lack spines, and the fifth pair has expanded, flattened segments giving them a paddle-like appearance…These were most likely employed in swimming.
One of the most peculiar features of Megalograptus is the development at the end of the postabdomen of a pair of expanded, hook-like cercal blades flanking a spine-like telson [(the large, comma-shaped structures on either side of the spike at the tip of the creature’s tail)]…Caster and Kjellesvig-Waering thought that the cerical blades could move laterally in a scissor-like motion, possibly serving to grasp either in defense of couplation. The paired, incurved posterior spines of earwigs are similar, but nothing like this structure appears in any other eurypterid.”
In addition to M. ohioensis, M. shideleri, M. williamsae, M. welchi, and M. alveolatus have been described, though none have been based on specimens nearly as complete. With the exception of the latter species, which is known from the upper Ordovician of Virginia, Megalograptus is restricted to the Cincinnati Arch region.
On a final note, while traditional reconstructions of eurypterids depict them inhabiting irregular marine environments, exploration of exquisitely-preserved Cincinnatian deposits has revealed Megalograptus in association with trilobites, brachiopods, bryozoans, crinoids, and molluscs, among other creatures. It’s therefore entirely possible that the rarity of these amazing arthropods due to their highly-degradable exoskeletons is grossly misleading and that the group was far more common and widespread than many paleontologists had previously imagined.
May the fossil record continue to enchant us all!