"Prorichthofenia": brachiopod horn corals

April Fools' Day not only falls on a weekend this year, but Easter, so I wouldn't be surprised if the typical crop of spoof news articles and blog entries is thinner due to other time commitments. Not that I mind too much; the whole thing gets old, year after year. Still, with April 1 in mind, here are some fossils that aren't what they seem: brachiopods from the Permian of Texas that put a lot of effort into being horn corals.

We'll open the story with Benjamin Franklin Shumard. We've met up with Shumard a few times before, including at Fort Snelling, on the old Santa Fe Trail, and near Grey Cloud Island. For this occasion, it's the late 1850s and he's the State Geologist of Texas, looking at a fresh batch of Permian fossils from the Guadalupe Mountains on the New Mexico–Texas border. Shumard hadn't collected the fossils himself; instead, his brother George Shumard collected them while acting as a geologist for an expedition under Captain John Pope. Pope, incidentally, went on to command in the Civil War; at the end of August 1862, he may well have wished he was back in the Guadalupe Mountains.

Shumard described about 25 species of invertebrates from the Guadalupe Mountains in 1858 and 1859, marking the beginning of study of the classic Guadalupian fauna. The Guadalupe Mountains and their Permian fossils are in fact so outstanding that they have lent their name to the Guadalupian Stage of the Permian. It's a bit of an inauspicious start, though, as Shumard did not illustrate any of the species or give any detailed locality information. Furthermore, none of the fossils are extant. The incarnation of the Texas Geological Survey that Shumard organized fell apart in 1860 amid political infighting and his collections and maps, left in Austin, were all but completely lost when the survey's facilities were converted to making percussion caps for the Civil War, with any straggling fossils lost to the state capitol fire in 1881 (Hill 1887). The absence of figures, at least for the Guadalupian fossils, was corrected by George Girty in 1908, via a lavishly illustrated monograph on the fossils of the Guadalupe Mountains and related areas (you can read it section by section here, or download the whole thing as a pdf here; the former doesn't require as much of a commitment, but the latter has better scans of the figures).

Among Shumard's species was a brachiopod he named Crania permiana in 1859. Over a long paragraph he described it as a large brachiopod with an enlarged subconical upper valve, about 1.5 inches tall and 1 inch in diameter, with concentric lines, and a variety of brachiopod minutiae that I am choosing to omit. Almost 50 years later Girty was confident that Shumard's species was represented in a new group of specimens from the Delaware, Glass, and Guadalupe mountains, and provided a new description as well as moving the species to a different genus, Richthofenia, in the family Richthofeniidae. He also illustrated a number of specimens, which give a much better idea of the animal than the text alone. An additional taxonomic change was made by King (1930), who made Crania permiana the type species of new genus Prorichthofenia. However, the loss of Shumard's specimens, combined with larger collections and other advances, led Cooper and Grant (1969, 1975) to decide to leave Shumard's species alone and rather extensively rework the taxonomy of "Prorichthofenia". For the sake of convenience, I'll refer to the brachiopods in question as "Prorichthofenia", with the note that this should be understood as representing several genera and species from the Permian of Texas and New Mexico.

A clip from Girty (1908) (plate 14) showing several views of a specimen from Capitan Peak.

What Shumard and Girty had were brachiopods in which one valve had grown into an elongated conical structure, and the other had turned into a sort of hinged-trash-can-lid, although not on top of the conical valve but recessed within; in other words, brachiopods that looked very much like a moderately sized horn coral with an inner cover. "Prorichthofenia" was not the only brachiopod to have done this, or even the only bivalved animal: for example, in the Late Jurassic and Cretaceous, a lineage of bivalve mollusks known as rudists hit upon the same idea of one big conical valve and one lid valve. The rudists, which appeared long after the extinction of rugose corals, formed reef-like accumulations in the Cretaceous before running into the Cretaceous–Paleogene extinction. (Of course, for both richthofeniaceans and rudists, we can throw out the general rules for shell symmetry.)

Another clip from Girty (1908) (plate 31), this time showing silicified specimens from the Glass Mountains, later named Cyclacantharia kingorum in Cooper and Grant (1969).

Apart from the lid, "Prorichthofenia" and its relatives would have visibly differed from your typical horn coral by having spines. As productid brachiopods, they had numerous thin spines; in the clip from Girty's Plate 31, you can see fine perforations in the conical valve, which would have housed spines. Spines would have helped to anchor the tip of the conical valve in the substrate, keeping it upright. Spines also projected from the interior of the conical valve and from the lid valve, forming a complex mesh over the opening in some species.

The unusual anatomy of "Prorichthofenia" has attracted some interesting ideas. Rudwick (1961) proposed that the thin lid valve was not a protective structure, but was flapped up and down to drive a feeding current, with the spines on the inside of the conical valve and the underside of the lid valve screening out large particles while capturing smaller particles, perhaps reducing or even eliminating the need for a typical brachiopod lophophore organ. As you might suspect, this flapping model proved controversial. It was pretty much done in by the publication of a specimen in which another more typical brachiopod was found trapped between the lid and a mesh of spines on the inside of the conical valve. This second brachiopod had arrived there as a larva and grown up into an adult while the "Prorichthofenia" also lived on; clearly the lid did not need to be able to open widely for the "Prorichthofenia" to feed (Grant 1975). Instead of a flapper, "Prorichthofenia" may have been similar to other brachiopods, opening the lid to expose the lophophore and closing it as a protective measure (Cowen 1983).

Or was "Prorichthofenia" that similar? One final twist: befitting an animal that has adopted at least part of a coral-like lifestyle, it has been suggested that "Prorichthofenia" and similar brachiopods had symbiotic zooxanthellae living in exposed mantle tissue in the conical valve and on the lid valve (Cowen 1970, 1983).

References

Cooper, G. A., and R. E. Grant. 1969. New Permian brachiopods from West Texas. Smithsonian Contributions to Paleobiology 1.

Cooper, G. A., and R. E. Grant. 1975. Permian brachiopods of West Texas, III. Smithsonian Contributions to Paleobiology 19.

Cowen, R. 1970. Analogies between the Recent bivalve Tridacna and the fossil brachiopods Lyttoniacea and Richthofeniacea. Palaeogeography, Palaeoclimatology, Palaeoecology 8(4):329–344.

Cowen, R. 1983. Algal symbiosis and its recognition in the fossil record. Pages 431–478 in M. J. S. Tevesz and P. L. McCall, editors. Biotic interactions in Recent and fossil benthic communities. Plenum Press, New York, New York.

Girty, G. H. 1908. The Guadalupian fauna. U.S. Geological Survey, Washington, D.C. Professional Paper 58.

Grant, R. E. 1975. Methods and conclusions in functional analysis: a reply. Lethaia 8:31-33.

Hill, R. T. 1887. The present condition of knowledge of the geology of Texas. U.S. Geological Survey, Washington, D.C. Bulletin 45.

King, R. E. 1930. The geology of the Glass Mountains, Texas. Part II: Faunal summary and correlation of the Permian formations with description of Brachiopoda. The University of Texas Bulletin 3042.

Rudwick, M. J. S. 1961. The feeding mechanism of the Permian brachiopod Prorichthofenia. Palaeontology 3(4):450–471.

Shumard, B. F. 1858. Notice of new fossils from the Permian strata of Texas and New Mexico, collected by Dr. George G. Shumard, geologist of the United States Government Expedition for obtaining water by means of artisan wells along the 32d parallel, under the direction of Capt. John Pope, U. S. Corps Top. Eng. Transactions of the Academy of Science of St. Louis 1:290–297.

Shumard, B. F. 1859. Notice of fossils from the Permian strata of Texas and New Mexico, obtained by the United States Expedition under Capt. John Pope, for boring artisan wells along the 32d parallel, with descriptions of new species from these strata and the coal measures of that region. Transactions of the Academy of Science of St. Louis 1(3d):387–403.