Thursday, November 22, 2012

US Research trip part 3 and 1/2 - fossil & modern mysticetes

Given my dissertation topic - fossil eomysticetids - I planned this trip so that I'd be able to see a lot of fossil mysticetes. Cenozoic marine sediments of the Atlantic coastal plain are densely fossiliferous, with several important 'classic' fossil localities: Bone Valley, Florida (late Miocene to Pliocene), Charleston, South Carolina (late Oligocene and Pliocene; Ashley, Chandler Bridge, Goose Creek Formations), Calvert Cliffs (early Miocene to late Miocene; Calvert, Choptank, St. Marys, Eastover Formations), Lee Creek/PCS Phosphate Mine, Aurora, North Carolina (middle Miocene, Pliocene; Pungo River Limestone, Yorktown Formation). With the exception of the South Carolina and Florida localities, all other strata are parts of the "Chesapeake Group" (for the uninitiated, an assemblage of formations that typically co-occur across a basin can be categorized into a group; groups can even be categorized into supergroups - such as the Proterozoic age Belt Supergroup of Montana.

In the early and mid 20th century, the preeminent paleocetologist Remington Kellogg described a large number of "cetotheres" from the Calvert and Choptank Formations of Maryland, including Aglaocetus patulus, Diorocetus hiatus, Halicetus ignotus, Parietobalaena palmeri, Pelocetus calvertensis, and Thinocetus arthritus. All of these specimens are at the USNM offsite facility at Suitland, Maryland - and I got to see most of them. Most of these are well preserved and morphologically informative - although some have been extensively reconstructed. For example, I examined the holotype of Diorocetus hiatus - and I was hard pressed to find any foramina or cranial sutures that appeared to be real. Much of the braincase was plaster or some other material that had painted the same color as the bone - there is a slight difference in color, but the telltale sign are the paintbrush striations, which contrast strongly with the natural bone texture. The holotype specimen has complete squamosals, a good vertex, and so on, but the rest of the braincase consists of non-overlapping fragments separated by wide expanses of sculpted and painted plaster. Because of issues like this, I don't feel terribly comfortable interpreting much about certain aspects of the anatomy of Diorocetus - which makes coding the taxon for phylogenetic analyses a big issue.

One of the few cetotheres named by none other than Edward Drinker Cope - Metopocetus durinasus, from either the Calvert Formation or the St. Mary's Formation (nobody really knows). Metopocetus has been considered to be a close relative of my favorite whale, Herpetocetus. Metopocetus was redescribed and reanalyzed by Kellogg in the 1960's, and more recently by Frank Whitmore and Larry Barnes.

Examination of modern mysticete skeletons is imperative for studying those of fossil mysticetes. It's even better when you can occasionally find modern crania sawn in half, to help make aspects of the internal osteology  more apparent. This is a sagittally sectioned skull of the minke whale, Balaenoptera acutorostrata.

 The heavily reconstructed skull of Diorocetus hiatus, named by Kellogg. The lighter brown areas are sculpted and painted; however, sculpted areas of the braincase are less obvious than the large patches on the rostrum.

Not exactly a mysticete - the petrosal (earbone) of the basilosaurid archaeocete Zygorhiza kochii from the Eocene of the southeastern US. Basilosaurids are important as they are typically used as outgroup comparisons for phylogenetic analyses of mysticetes; I've spent the last 7-8 months familiarizing myself with basilosaurid cranial anatomy, as I will include several in my Ph.D. analyses (Basilosaurus spp., Dorudon, Zygorhiza). This one in particular was ground away by Gerald Fleischer for his study of the evolution of the cetacean cochlea and hearing. We have a cast of this specimen here at Otago, but it was nice to see the original specimen.

The petrosal of the large Miocene balaenopterid, "Megaptera" miocaena; this same earbone morphotype (i.e. not necessarily the same species, but the same genus at least - although it needs a new genus, as it clearly doesn't belong in Megaptera) is also present in a number of other late Miocene and Pliocene rock units in California, including the Purisima Formation and Santa Margarita Sandstone (Santa Cruz County), San Mateo and San Diego Formations (San Diego County). I didn't look at the skull, but I saw the size of the storage jacket- I was not aware how enormous the skull is; it's basically nearly the size of a modern humpback whale.

The petrosal of Thinocetus arthritus, one of Kellogg's cetotheres or as some of us affectionately call them, "Kelloggitheres".

Even more earbones! A small part of the USNM's large collection of Balaenoptera physalus (Fin Whale) tympanic bullae. Samples like this help paleontologists interpret ranges of variation in modern cetaceans so that we may better interpret samples of fossil earbones.

 When available, fetal skulls are very useful for paleontologists. Little is known about ontogenetic skull changes in modern or fossil mysticetes - it's not that we don't have the data, it's just that few researchers have ever bothered studying it. However, ontogenetic information from modern mysticetes (and most modern taxa, for that matter) can inform us not only about the ontogeny of fossil mysticetes, but also lend critical data regarding possible growth changes related to paedomorphism/neoteny, as well as giving us an "ontogenetic polarity" for different features, a la "ontogeny recapitulates phylogeny" of Haeckel.

 One of the most important cetaceans within USNM collections - the holotype and only known specimen of Aetiocetus cotylalveus from the late Oligocene Yaquina Formation of Oregon (Yaquina, for those marine mammal paleontologists not in the know - is pronounced by local Oregonians as Ya-kwinna, not Yah-keena; I only found out after a trip to Oregon with Ray Troll and Kirk Johnson). Although originally described as an archaeocete in 1966, it was shortly thereafter correctly interpreted as an early toothed mysticete by Leigh Van Valen, who interpreted it as a morphological intermediate between basilosaurid archaeocetes and later toothless mysticetes. This was also the only fossil that my hero Douglas Emlong published upon, which he published and named at the age of 24 (and prior to that, had it collected and prepared).

Lastly, the ribs of Aetiocetus cotylalveus. One of these ribs was examined by Beatty and Dooley (2009) and found to be osteosclerotic (meaning thick cortical bone and reduced medullarity cavity) - unlike modern mysticetes which have osteoporotic ribs, but also unlike basilosaurid archaeocetes, which had osteosclerotic and pachyostotic ("inflated" bones, or outward growth/hypertrophy of cortical bone) ribs.

Next up: photos from the Smithsonian Air and Space Museum.

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