In November I published a rather massive monograph on the fossil dolphin Xenorophus - it's my longest publication to date (166 pages!), and my third published monograph - and one of two published in 2023* (check out my blog summary of my other 2023 monograph on Coronodon here, here, and here). This series of posts will be in four parts: 1) an introduction to Xenorophus sloanii and the family Xenorophidae; 2) new specimens of Xenorophus sloanii; 3) the new species Xenorophus simplicidens; and 4) the paleobiology of these dolphins and some of what we've learned about them.
*I am very tired.
The Discovery of Xenorophus sloanii
Paleontology in Charleston had an early start - likely owing to the combination of the plantation economy (lots of digging activity) and some of the earliest correctly identified fossil vertebrates from North America in general were found by enslaved Africans on the Stono Plantation in the mid 18th century and identified by them as elephant teeth - later revised to mammoth teeth. During the antebellum years, other finds made by the enslaved and slaveowners include the holotype skull of the Oligocene dolphin Agorophius pygmaeus and the Eocene basilosaurid Dorudon serratus. After the Civil War, the phosphate mining boom resulted in a proliferation of fossil discoveries and widespread stratigraphic confusion that took another century to untangle (see my earlier blog post on the Ashley Phosphate Beds here).
The holotype skull of Xenorophus sloanii, collected from the Ingleside Mining Pit near Ladson, South Carolina, sometime before Kellogg's (1923) publication.
My own photograph of Xenorophus sloanii - from a 2016 visit to USNM collections with Morgan Churchill and Sarah - just after our colleague Dr. Rachel Racicot's wedding a few miles away!
The mining boom had died down by 1890 or so, but a few marl pits continued operating into the early 20th century. One such pit, the Ingleside Mining Pit*, yielded an unusual cetacean skull sometime before 1923 and was acquired by Earl Sloan, the state geologist, who subsequently donated to the Smithsonian for Remington Kellogg to study. Kellogg named the skull Xenorophus sloanii** in 1923 after Sloan just three years before Sloan passed away. The specimen was clearly from the "Cooper Marl" - now known as the Ashley Formation. Kellogg recognized that this specimen represented an early odontocete, which he referred to as a "dolphin" and noted archaic features such as double rooted multicuspate teeth and retention of an intertemporal constriction. However, he also noted that this skull had a very unusual lacrimal bone, and premaxilla. The lacrimal is the bone that houses the tear duct in terrestrial mammals; in most marine mammals the duct is lost and the bone just forms the anterior part of the orbit. In most dolphins, it is relatively small and forms the anteriormost part of the orbit - but is also fused to the jugal bone, stretched into a delicate bone the thickness of a toothpick. Instead, in Xenorophus, the lacrimal is large and triangular and covers the entire front half of the orbit - but on the dorsal side. In most other odontocetes, the dorsal part of the orbit is typically formed by ascending process of the maxilla. Xenorophus also has an ascending process of the maxilla - clearly making it an odontocete. The premaxilla, on the other hand, is also quite strange. In a normal odontocete, the premaxilla forms the tip of the snout, bears a few teeth, forms the middle of the snout (housing the mesorostral groove/gutter) and then wraps around the left and right sides of the blowhole. The premaxilla does all of these things in Xenorophus - but also invades the entire part of the skull between the eye socket and the blowhole and has a large, posteriorly expanding cone-shaped body that is actually exposed ventrally just behind the eye socket where the frontal bone has a large ring-shaped window in it. Normally the premaxilla is a thin plate that lies atop the maxilla around the nares - but in this case, the opposite is true: the maxilla instead lies above the premaxilla. The premaxilla basically occupies a space in the skull normally 'built' by the frontal. All of this is just bonkers-ass crazy, even for dolphins, which already have bizarre skulls. But we're not done: the inflated part of the premaxilla here is also osteosclerotic - extremely dense.
*Coincidentally, on the property of the Ingleside Plantation, the plantation of Francis Holmes - former curator of the College of Charleston Museum and the guy who started the phosphate mining boom - and discoverer of Agorophius.
**Commonly misspelled sloani, but the correct spelling by Kellogg shows it as sloanii.
The holotype maxilla fragment of Squalodon pelagius - as figured by Leidy (1869).
Other Early Discoveries of Xenorophidae
Famed American paleontologist Joseph Leidy was a contemporary of Francis Holmes, who sent a small rostrum fragment with a single tooth (found along the Ashley River) to Leidy. Leidy published the specimen in his 1869 monograph, naming it Squalodon pelagius. Leidy almost certainly used the genus Squalodon here as a wastebasket owing to the occurrence of double-rooted triangular teeth. He also noted the presence of what we now call embrasure pits - pits to accommodate the opposing teeth. These pits are unique to the family Xenorophidae, and I think this specimen compares quite well with Echovenator and Albertocetus (see below for more on these taxa). Unfortunately, the specimen is now lost.
The mysterious skull of Archaeodelphis patrius, from Miller (1921) - my scan of a copy of Ed Mitchell's personal copy of the paper with his labeling and highly distinctive writing - which he had provided to Ewan Fordyce (long before all that Llanocetus nonsense).
Another curious specimen was 'discovered' in the collections of the Museum of Comparative Zoology at Harvard around World War 1 and named Archaeodelphis patrius by prominent mammalogist Grover M. Allen. This specimen consists of a braincase somewhat resembling Xenorophus, but with an orbit that is only partially covered by the ascending process of the maxilla - uniquely 'primitive' amongst all fossil odontocetes.* Like Xenorophus, this specimen possesses an intertemporal constriction and a relatively untelescoped skull. However, it doesn't quite have the same sort of highly inflated premaxilla. Archaeodelphis patrius has been needing redescription and reinterpretation for a century - and for the past half century, was locked away in a loan cabinet - while many researchers have considered it one of the most important fossil odontocetes ever discovered. OK, but here's the rub: we have no idea where the damned thing is from or how old it is. In general, the skull "looks" like an Oligocene dolphin - only Oligocene dolphins are as archaic in their morphology. A similar specimen at Charleston Museum, ChM PV 4746, was collected from the Chandler Bridge Formation of South Carolina - but the morphology is an inexact match. Unfortunately, during 100 years of collecting, not a single perfect match has been found - which is admittedly a little bit of a problem, as most of the taxa we have here - named or unnamed - are known by at least a couple of specimens (however, ChM PV 4746 is an exception and is still a singleton). Still, Charleston seems likely for several key reasons: it's the only place in the southeastern USA where fossils of archaic odontocetes were being routinely found in the 19th century. The skull was likely part of the collection Agassiz was studying and at some point divorced from its documentation (and never labeled). Holmes and Leidy never wrote about such a specimen - but perhaps it was discovered late in the 19th century (post-Leidy?) by someone in Charleston and whoever committed the information to memory either died or forgot it, and the skull sat in MCZ collections for some time prior to being 'discovered'. Finding a new Archaeodelphis would really be something.
*I very strongly suspect that the holotype of Archaeodelphis is a juvenile, however, and some of this may simply be ontogenetic recapitulation of archaic to derived features.
The holotype skull of Albertocetus meffordorum, whitened with ammonium chloride - the skull is nearly black in color. It is much closer in morphology to Xenorophus than it is to Echovenator or Cotylocara. From Uhen (2008).
Size comparison of Albertocetus and Xenorophus. Modified from Uhen (2008).
Skull, endocast, earbones, vertebrae, and skeletal reconstruction of Albertocetus meffordorum - images from Boessenecker et al. (2017B).
A North Carolina xenorophid: Albertocetus
Onslow Beach in coastal North Carolina has long been a location where fossils have been found strewn along the water line. In the early 2000s, several dolphin-bearing concretions were discovered and sent to the Smithsonian. I saw a talk by Mark Uhen in 2006 at the Neoceti symposium at the SVP meeting in Ottawa (my second SVP ever!) reporting these archaic specimens - and in 2008, Uhen published a paper naming Albertocetus meffordorum, the first xenorophid named in 90 years. Like specimens from the famous "Emlong collection" at the Smithsonian, this specimen had to be carefully prepared out of a hard calcareous nodule using air scribes. Albertocetus has a similar facial region to Xenorophus, and a braincase that looks - well, a lot like an archaeocete whale. Critically, it also preserved the first good earbones for the Xenorophidae - a periotic is preserved in Archaeodelphis, but it was not well-figured. Later, in 2017, I reported new specimens of Albertocetus from the Ashley Formation of Charleston - demonstrating that this taxon lived in both the Charleston and Salisbury embayments during the Oligocene - hardly a surprise. These specimens also preserved earbones removed from the skull and associated postcrania.
The skull of xenorophid Cotylocara macei from the Chandler Bridge Formation of South Carolina, from Geisler et al. (2014).
Xenorophids and the origin of Echolocation: Cotylocara and Echovenator
Shortly thereafter witnessed a flurry of papers on xenorophid dolphins from Charleston, South Carolina. The first of these made it into the journal Nature - likely to be the highest profile publication to ever come out of the College of Charleston (very curious). The study, published by Jonathan Geisler et al. (2014), named the new genus and species Cotylocara macei - the holotype of which, CCNHM 101, is the first published specimen from my old institution, the Mace Brown Museum of Natural History at the College of Charleston. Cotylocara macei is clearly a xenorophid, though it has a much narrower snout than Xenorophus, and a wider, dish-shaped facial region. It also has a somewhat more telescoped skull - barely having an intertemporal constriction at all, with the facial bones thrusted posteriorly and overriding most of the anterior part of the braincase. However, it is distinctive in possessing a large pit behind the blowhole, which Geisler et al. named the postnarial fossa. Additionally, the specimen has very well-preserved antorbital fossae, noted previously but not interpreted. These large facial fossae are further lined with low-density bone similar to that lining the air-filled sinuses of extant odontocetes. This suggests that Cotylocara had considerable air-filled sinuses in its facial region, likely connected to diverticulae in the highly modified blowhole and indicating the ability to produce high frequency sounds used in echolocation. However, CT scans of the cochlea of Cotylocara were not informative at the time to confirm whether or not these sounds could be heard.
Life restoration of Echovenator sandersi - by Alberto Gennari.
Comparison of periotics of Albertocetus from the Ashley Formation of Charleston SC (A-B) and Belgrade Formation of NC (C-D; collected Gary Grimsley in 2016), the North Carolina Echovenator periotic studied by Park et al. (E-F; collected J. Mefford, Onslow Beach NC), and a periotic of cf. Echovenator I collected from Belgrade Quarry in 2017 (G-H) - from Boessenecker et al. (2017B).
High frequency hearing was later confirmed in xenorophid dolphins by two studies that came out in parallel: Park et al. (2016), who scanned an isolated periotic of a xenorophid from Onslow Beach (North Carolina - same site as Albertocetus), and Churchill et al. (2016), who reported another xenorophid, Echovenator sandersi, from the same rock unit as Cotylocara - the Chandler Bridge Formation of South Carolina. The North Carolina periotic had evidence of adaptations for hearing high frequency sounds, but was unidentifiable (at the time) past the family level. The beautifully preserved skull of Echovenator sandersi, on the other hand, had some similar, but more subtle facial sinus fossae like Cotylocara, and well-preserved periotic bones. Morphometric analysis of the scans indicated that Echovenator could hear high frequency sounds; between these two genera, xenorophids in general were clearly able to echolocate. Ironically, Park et al. (2016) and Churchill et al. (2016) were unaware that the other team were working on the same taxon: a year later, I published an article reporting the periotic morphology of Albertocetus, and in it reidentified the North Carolina specimen as Echovenator sp. owing to numerous shared features.
Rostral proportions through time in Odontoceti - Inermorostrum is already one of the most brevirostrine odontocetes, and it evolved within just a few million years of the mysticete-odontocete split. From Boessenecker et al. (2017A).
A xenorophid that really sucked: Inermorostrum
One of my first projects at CCNHM was to report this adorable little skull clearly representing an adult dwarf xenorophid. This skull has a vertex with similar skull sutures to Albertocetus, and shared with it and Xenorophus a flat vertex - in other words, lacking the postnarial fossa of Echovenator and Cotylocara. While the small size is distinctive, that pales in comparison to the business end of the skull: the snout is quite short - approximately 1/3 of the length of Cotylocara if you scale it to the width of the snout - and completely toothless. That is not to say the teeth fell out - there are no tooth sockets, either - indicating that the species was completely toothless. We named this new genus and species Inermorostrum xenops - the genus name meaning 'weaponless snout' and the species name meaning strange face, hearkening back to Xenorophus itself and the family. This little dolphin was from the Ashley Formation, a contemporary of Albertocetus and Xenorophus and about 28-30 million years in age; only a single referred specimen exists, a partial vertex in Charleston Museum collections. Analyses of rostrum length evolution through time indicated that longirostry (long snouts) and brevirostry (short snouts) evolved many times in parallel within the Odontoceti, and that xenorophids evolved short snouts very, very early in odontocete evolution, shortly after diverging from the baleen whales. What good is a short, toothless mouth? Reduction of the dentition has occurred in many modern odontocetes including sperm whales (Physeroidea), beaked whales (Ziphiidae), Risso's dolphin (Grampus griseus), and the narwhal (Monodon monoceros). In all such cases, the few teeth that are retained are generally reserved for combat and not used in biting prey. Most also happen to have short snouts. Many toothy dolphins with short snouts also exist. What these all share is the ability to suction feed for soft-bodied prey like squid and fish. The loss of teeth accomplishes two things: streamlines the mouth to improve the flow of water, and in the case of those species that feed on squid, decreases hard points that suckers can attach to easily. A short snout makes for a smaller mouth, which if paired with large lips, increases suction forces as well. The tiny size of Inermorostrum means that it was also a shallow, rather than deep diver: this rules out deep dives for squid. Perhaps Inermorostrum was coastal in its distribution (like most small odontocetes today) and foraged on the shallow sea floor for soft bodied prey like sea cucumbers, octopus, and worms.
In sum, many new xenorophids have been discovered since Kellogg named Xenorophus sloanii in 1923 - but that's obviously not all that's been uncovered. Next up in Part 2, I'll review the new specimens of this new species and some of what we've learned.
References
Allen, G.M. A new fossil cetacean. Bulletin of the Museum of Comparative Zoology 1921, 65, 3-14.
Allen, J.A. Note on squalodont remains from Charleston, S.C. Bulletin of the American Museum of Natural History 1887, 12, 35-39.
Boessenecker, R.W.; Fraser, D.; Churchill, M.; Geisler, J.H. A toothless dwarf dolphin (Odontoceti: Xenorophidae) points to explosive feeding diversification of modern whales (Neoceti). Proceedings of the Royal Society B 2017A, 284, 20170531
Boessenecker, R.W.; Ahmed, E.; Geisler, J.H. New records of the dolphin Albertocetus meffordorum (Odontoceti: Xenorophidae) from the lower Oligocene of South Carolina: encephalization, sensory anatomy, postcranial morphology, and ontogeny of early odontocetes. PLoS ONE 2017B, 12, e0186476.
Churchill, M.; Martinez-Caceres, M.; Muizon, C.d.; Mnieckowski, J.; Geisler, J.H. The origin of high-frequency hearing in whales. Current Biology 2016, 26, 2144-2149.
Geisler, J.H.; Colbert, M.W.; Carew, J.L. A new fossil species supports an early origin for toothed whale echolocation. Nature 2014, 508, 383-386.
Kellogg, R. Description of an apparently new toothed cetacean from South Carolina. Smithsonian Miscellaneous Collections 1923, 76, 1-7.
Park, T.; Fitzgerald, E.M.G.; Evans, A.R. Ultrasonic hearing and echolocation in the earliest toothed whales. Biology Letters 2016, 12, 20160060. Uhen, M.D. A new Xenorophus-like odontocete cetacean from the Oligocene of North Carolina and a discussion of the basal odontocete radiation. Journal of Systematic Palaeontology 2008, 6, 433-452.
No comments:
Post a Comment