Friday, February 9, 2018

An embarrassingly enormous dermal denticle from South Carolina; or, what the hell is Ceratoptera unios?




The rivers, creeks, and sand pits in the Charleston area of South Carolina are one of the world’s premier destinations for collecting shark teeth. With the closure of the Lee Creek Mine in Aurora, NC, in 2011 to private collectors, Charleston has reclaimed its position as the world’s capital for collecting the much coveted giant teeth of Carcharocles megalodon. Shark fossils have been on naturalist’s radar since the early 1800s, with early monographs by College of Charleston professor Robert W. Gibbes (later Surgeon General for the state of South Carolina during the Civil War). People flock from all over the east coast to visit Charleston and collect shark teeth – whether by walking creeks and beaches, digging in quarries and construction sites, or braving the murky waters of our rivers with a dive mask and scuba tank. Teeth are nice and beautiful, and understandably fawned over – but many other puzzles are often ignored or left by the wayside. As a scientist, I really enjoy puzzles – whether literal puzzles consisting of a broken fossil needing some glue, or a confusing fossil requiring some mental gymnastics.

https://upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Denticules_cutan%C3%A9s_du_requin_citron_Negaprion_brevirostris_vus_au_microscope_%C3%A9lectronique_%C3%A0_balayage.jpg/220px-Denticules_cutan%C3%A9s_du_requin_citron_Negaprion_brevirostris_vus_au_microscope_%C3%A9lectronique_%C3%A0_balayage.jpg

Typical placoid dermal denticles of a shark (from Wikipedia).

Dermal denticles are interesting to me as I started finding some on the west coast that nobody had ever really published on; they looked like little donuts with a thumbtack in the middle. Turns out they were a dermal buckler of a skate, similar (but not identical) to a modern Raja clavata; two bucklers would be present in each individual. This was my first foray into identifying fossils nobody had ever really seen or reported on before – and my first experience with ‘groping in the dark’ (as my Ph.D. adviser calls such endeavors). I found that few dermal denticles are ever reported on in the literature, as they are often difficult to identify. Most dermal denticles are tiny and studies of seafloor sediments with concentrations of denticles have utilized quantitative methods normally reserved for planktonic microfossils to examine changes in marine vertebrate abundance through the Cenozoic (Sibert & Norris, 2015, PNAS: 112:28:8537-8542). http://www.pnas.org/content/112/28/8537 Aside from this, however, denticles are usually forgotten about as curiosities.

 
The remarkable holotype dermal denticle of Ceratoptera unios, from Leidy, 1876, Pliocene?, Ashley Phosphate Beds, South Carolina

One such curiosity is the case of “Ceratoptera unios” – a species named by none other than preeminent American paleontologist Joseph Leidy in 1877 in a monograph on fossil vertebrates from the Ashley Phosphate Beds of Charleston, South Carolina. “Ceratoptera” – horn-wing – is now of course a junior synonym of the genus Manta – manta rays. Leidy examined this enormous dermal denticle, measuring 82 mm long, 54 mm wide, and 36 mm in thickness. There’s no way around it: this thing is a massive hunk of… osteodentine, I would assume? It is oval in shape with a flat bottom, mound-like, with a triangular enameloid scale that narrows towards the central apex of the denticle. Leidy, judging from the embarrassingly large size of this thing, named it as a new species and identified as a caudal stinger of a manta ray.
 

An actual caudal stinger of a manta ray, Manta birostris (or Manta hynei), from the Pliocene Yorktown Formation, Lee Creek Mine, North Carolina, Rita McDaniel (CCNHM collections).



Unfortunately for our dear professor Leidy, it doesn’t resemble an actual Manta Ray tail spine. Those are similarly sized, with a less compact tissue that resembles cancellous bone, and they are lozenge-shaped. Rather, smaller denticles found in Pliocene sediments like the Yorktown Formation share the same overall shape but only attain sizes of 2-3 cm at most – maybe ¼ to 1/3 the size of the Ceratoptera unios holotype. In their landmark paper on fossil sharks and fish from the Lee Creek Mine, Purdy et al. (2001) reported other dermal denticles matching the modern thornback stingray Dasyatis centroura and identified all large dermal denticles as this species – including Ceratoptera unios. However, the only modern specimens they figured are disc-shaped Frisbee like denticles with radial ridges and a tiny central cone, or set of cones – the type resembling “Ceratoptera unios”, as far as I can tell, has not been figured in a modern specimen. There is likely more of the literature than I am aware of, of course. There’s still not much known about the Ceratoptera unios type specimen, either; Leidy does not provide any useful locality data.


 Ashby Gale's enormous dermal denticle of "Ceratoptera unios", from the early Pleistocene Waccamaw Formation of South Carolina. CCNHM collections.

Last month I returned with my friend Ashby Gale and his partner Tabytha Walls to a sand pit near Summerville SC I’ve been doing research at since November, which preserves fossils from the Plio-Pleistocene Waccamaw Formation. Ashby, almost reluctantly, picked up something to double check that it was not a bone or something important, and after he looked at it, remarked “Golly!” as he typically does; my exclamations were rated R and best not repeated here. He had found only the second specimen (to my knowledge) of a giant dermal denticle matching the anatomy of Ceratoptera unios. He gladly donated the specimen to our museum (CCNHM, Mace Brown Museum of Natural History) and now I am puzzling over it. The specimen is almost as massive as the holotype, measuring nearly seven centimeters in length and over four centimeters wide. It’s also surprisingly heavy. We’re talking about a really old and probably very large ray.




Smaller denticles resembling Ceratoptera unios from the Pliocene Yorktown Formation, Lee Creek Mine, North Carolina, Rita McDaniel (CCNHM collections).

We may not know anything further about the relationships and identification of Ceratoptera unios, but this at least helps us better interpret the age and probable stratigraphic origin of the holotype. We’re unsure of the original locality, since Leidy’s locality was simply “Ashley River Phosphate Beds”. This isn't a single stratum, and poor geologic notes were recorded during the reconstruction era; most discoveries were made in phosphate strip mines which were worked by recently freed slaves - the phosphate mining industry, though short lived in South Carolina, was an important crutch in getting Charleston back onto its feet after the Civil War. I live right off of the Ashley River, and have studied a lot of maps and poked my head into a lot of holes – and on the banks of the rivers, we tend to find the following “clusters” of fossils: Oligocene marine vertebrates (common), early middle Miocene marine vertebrates (rare), Pliocene marine vertebrates (common-uncommon), and Pleistocene terrestrial mammals (uncommon); these likely originate from the Ashley & Chandler Bridge formations, Marks Head Formation, Goose Creek Limestone, and Wando Formation (respectively). Most of these end up on the shorelines of the Ashley River from old river dredgings where they used a bucket rather than pipe dredging; these dredgings were dumped on the river banks, and have been slowly washing out for the past century. 

The Waccamaw Formation at our quarry is early Pleistocene in age with abundant reworked Pliocene fossils, and rare Oligo-Miocene fossils; most of the autochthonous and parautochthonous fossils from the Waccamaw are sharks and rays that can be found in the slightly older Pliocene Goose Creek Limestone. Based on Ashby’s discovery, a pretty strong case can be made for the Ceratoptera unios type specimen originating from the Goose Creek Limestone. In fact, most of the fossils on our local beaches are likely from the same unit (e.g. Folly Beach, one of my favorite collecting spots here).

 Are Ceratoptera unios and Dasyatis centroura synonyms? Maybe – although for once, we don’t need any more fossils; we need more dissections and comparative studies of dermal elements to be published by ichthyologists!

References

J. Leidy. 1877. Description of vertebrate remains, chiefly from the phosphate beds of South Carolina. Journal of the Academy of Natural Sciences of Philadelphia 8:209-260

R. W. Purdy, V. P. Schneider, S. P. Applegate, J. H. McLellan, R. L. Meyer and B. H. Slaughter. 2001. The Neogene sharks, rays, and bony fishes from Lee Creek Mine, Aurora, North Carolina. Smithsonian Contributions to Paleobiology 90:71-202