First fossils of phocoenid porpoises from the Atlantic Coastal Plain, USA

Note: a shorter version of this post already appeared on the Mace Brown Museum of Natural History blog.

Porpoises belong to the modern family Phocoenidae, and are one of the less diverse ‘families’ of modern echolocating whales (Odontoceti), with six species in three genera. They are all typically quite small, and have a near worldwide distribution; the harbor porpoise itself (Phocoena phocoena) lives along most northern hemisphere coastlines in temperate latitudes; other porpoises have a more limited distribution, with some living in small embayments, like the critically endangered Vaquita (Phocoena sinus). Modern phocoenids have short snouts, and big lips for suction feeding; their teeth are quite distinctive and are spatulate rather than conical like most other odontocetes.

A modern harbor porpoise, Phocoena phocoena (from Scholastic.com)

Porpoises have a diverse fossil record, and extend back to the late Miocene of California and Japan; fossils indicate they were more diverse and disparate than they are today (disparity = ecological diversity). Phocoenids seem to be the dominant small odontocetes from upper Miocene and Pliocene deposits of California and Japan, with few examples of modern oceanic dolphins (Delphinidae), suggesting some significant and rapid faunal change between the Pliocene and modern day. Today, delphinids are the most diverse and numerically abundant small odontocetes in almost every ocean basin. In the latest Miocene and Pliocene of the North Pacific, however, there were delphinid-like porpoises with long snouts and approaching the size of bottlenose dolphins, as well as smaller harbor porpoise like species; and, of course, bizarre species like the ‘skimmer porpoise’ Semirostrum ceruttii (Racicot et al., 2014). Certain species may have been present in both California and Japan, in a specialized province with little communication with the North Atlantic or south Pacific (Boessenecker, 2013).

 

Life restoration of the skimmer porpoise Semirostrum ceruttii (by R.W. Boessenecker)

The North Atlantic Pliocene odontocete record is mostly known from fossil assemblages from North Carolina, Belgium, and the Netherlands. These fossil assemblages are dominated by delphinids including many modern genera. Phocoenids, however, are rare, and known only from two specimens from the Pliocene of Belgium apparently belonging to different genera – Septemtriocetus bosselaersi, and Brabocetus gigaseorum (both named for the discoverers of each skull). These fossils post-date the opening of the Bering strait, and likely represent an early dispersal through the Arctic from the North Pacific, along with the extinct walrus Ontocetus emmonsi. However, no fossils of phocoenids have ever been discovered in the more richly sampled Pliocene deposits of the Atlantic coastal plain of North America. This includes the famous Yorktown Formation at the Lee Creek Mine in North Carolina, arguably the Pliocene marine fossil assemblage we know the most about worldwide; no porpoises at all (Whitmore and Kaltenbach, 2008). The Yorktown assemblage is dominated by ‘modernized’ cetaceans that inhabit the North Atlantic, whereas most of the cetaceans from Pliocene deposits in the North Pacific belong to extinct genera, families, or gener/families no longer inhabiting the North Pacific  - again, indicating faunal endemism different from the modern fauna.

 

Skull of the early Pliocene phocoenid Brabocetus from Belgium (from Colpaert et al., 2016).

Recently, friend of the museum and donor Ashby Gale and his fossil tour clients (Ashby runs a local business taking visitors to publicly accessible fossil localities) discovered the first fossil phocoenid from the Western North Atlantic (to my knowledge). It is an isolated earbone collected from Folly Beach, South Carolina; the first was discovered last fall, and a second specimen was just found last week and generously donated by his client Don Pendergast. Ashby informs his clients of the scientific significance of the specimens they find and as a result many donations have been made to the museum (thanks!). Bedrock exposures do not exist anywhere on the barrier island; in fact, the fossil-bearing deposit is located offshore, and most of the fossils were accidentally dumped on the beach as part of a beach renourishment project where seafloor sand was added to the beach as a means to curb barrier island erosion; a slurry of seawater and sand is pumped from the shelf to the beach by means of a large pipe. This was last done in 2014, and it introduced a ton of fossils and large sandy limestone blocks onto the beach; Folly Beach residents quickly became vocal about the rough debris. Fossil collectors rejoiced, however, as fossils may now be found quite regularly. 

 These two periotics are very close in anatomy to extant harbor porpoises, Phocoena phocoena, and the one shown on top (below) is close enough that I would identify it as cf. Phocoena. In particular, the anterior process is delicate and tabular, and the pars cochlearis (hemispherical prominence housing the cochlea) is large and sort of low; the posterior process is oval-shaped, unlike the broader leaf or nearly square outline of the same feature in delphinid dolphins. The second one is perhaps not as close a match for Phocoena itself, and detailed further comparisons will be needed in order to make absolutely sure this is not some kind of kentriodontid-grade dolphin, since there are exposures of the upper lower Miocene Mark's Head Formation (~18 million years old), which is a lateral equivalent of the Pungo River Limestone  in North Carolina and the lower parts of the Calvert Formation (beds 2-9). Identifying delphinoid periotics is widely known to be a bit of a 'black art', and certain kentriodontids (archaic delphinoids, the common ancestors of oceanic dolphins, porpoises, the beluga and narwhal) have similar periotics to phocoenids and delphinids.

Newly collected earbones of a Phocoena-like porpoise from the Goose Creek Limestone, Folly Beach, South Carolina.

So how old are these earbones? In particular, vertebrate fossils seem to arise from four sources. Shark teeth appear to represent Oligocene, early Miocene, and Pliocene assemblages, with certain extinct species  reflecting each time period. These sources are probably the Ashley Formation (early Oligocene) which underlies most of the Charleston Embayment and produces rare Oligocene cetaceans and teeth of the megatooth shark chronospecies Carcharocles angustidens; the second is the lower Miocene Marks Head Formation, which produces rare sharks and marine mammals indicative of the late early Miocene (rugose teeth of archaic dolphins, earbones of early baleen whales like Parietobalaena, etc., the chronospecies Carcharocles chubutensis). The last, and most significant source, is the Pliocene Goose Creek Limestone; this unit produces most teeth including great white sharks (Carcharodon carcharias), the famous Carcharocles megalodon, and many marine mammals. Most dolphin earbones from Folly Beach either directly match species from the mostly contemporaneous Yorktown Formation. Several delphinid periotics, however, are close matches for the late Pliocene-early Pleistocene dolphin Astadelphis, known only from Italy (but also recorded in an early Pleistocene bonebed in the Waccamaw Formation further inland near Charleston). A younger unit is present, but is terrestrial and it yields abundant Pleistocene terrestrial vertebrates (turtles, horse, armadillos, etc.); it is an unlikely source for the periotics discovered on Folly Beach.

Ashby Gale’s discoveries are currently being worked on by CCNHM paleontologists, and one specimen is currently on loan to our colleague Dr. Rachel Racicot for CT scanning at Vanderbilt University. We hope to have these interesting (albeit fragmentary) records of porpoises published in the future!

References

R. W. Boessenecker. 2013. A new marine vertebrate assemblage from the Late Neogene Purisima Formation in Central California, part II: Pinnipeds and Cetaceans. Geodiversitas 35(4):815-939

W. Colpaert, M. Bosselaers, and O. Lambert. 2015. Out of the Pacific: A second fossil porpoise from the Pliocene of the North Sea Basin. Acata Paleontologica Polonica 60(1):1-10

R. A. Racicot, T. A. Deméré, B. L. Beatty and R. W. Boessenecker. 2014. Unique feeding morphology in a new prognathous extinct porpoise from the Pliocene of California. Current Biology 24:7:774-779 

F. C. Whitmore and J. A. Kaltenbach. 2008. Neogene Cetacea of the Lee Creek Phosphate Mine, North Carolina. Virginia Museum of Natural History Special Publication 14:181-269

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.

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