Welcome to Part 2 of my series on Xenorophus. To read Part 1, click here. Part 3 is on its way!
We've been in the middle of buying a new house - which is very exciting! And, also, exhausting. We're preparing for our move in 2.5 months to sunny San Diego - so things are a bit hectic, and I haven't had as much time to devote to blogging. I had intended to write this post over a month ago, but the Valenictus paper dropped - early - and I had to write that one instead!
ChM PV 4823, what is now the Xenorophus simplicidens paratype specimen - on display at Charleston Museum.
A quick note: type specimens for beginners
You're going to see 'holotype', 'paratype', and 'referred specimen' thrown about a lot in this post. This blog is directed towards an informed audience comfortable with at least semi-technical language. In some cases, precise wording exists because it would take too damn long to describe the concept in layman's terms. I also firmly believe that most people are smart enough to not need their hand held. So, to define these terms:
Holotype: the single best specimen known at the time when a new species is named. All future identifications need to based on comparisons and observations of the holotype specimen. You can only designate type specimens once - when the species is being named.
Paratype: often the second best specimen of a new species. In many cases the paratype will be designated as it preserves some bit of anatomy better than the holotype, even though it might be a bit worse for wear.
Referred specimen: all other specimens identified as (or 'referred' to) a particular species, either in the initial study, or in all followup studies. Remember a type specimen can only be named once - if you find a much more complete skull later on, it cannot be designated a holotype since one already exists. You've got yourself a referred specimen.
New discoveries
Over the past 30 years, a number of new specimens of Xenorophus had turned up - many of these ended up in Charleston Museum collections (ChM), and several collected since the late 1990s ended up at the Mace Brown Museum of Natural History (CCNHM). All told, when I started this project, there were over ten specimens including skulls - a phenomenal sample! Jonathan Geisler made it clear from an early stage that there was likely to be more than one species. Al Sanders had considered most of the new specimens to represent a new species - which he wanted to call Xenorophus simplicidens (an unpublished manuscript name which we ultimately used in recognition and honor of his work - keep reading). The most complete specimen that Al and Jonathan examined back in the early 2000s when Jonathan was working on his dissertation research (eventually published as Geisler and Sanders, 2003) was ChM PV 4823, a nearly complete skull collected from an unusual bed of the Chandler Bridge Formation near the Edisto River. Two additional braincases from the same stratigraphic unit were also present in ChM collections - ChM PV 4266 and 4822. Two additional specimens including a mostly complete juvenile skull, ChM PV 7677, and a somewhat deformed skull with much of the vertebral column, ChM PV 5022, had been collected from the Ashley Formation. A few partial mandibles with teeth, also from the Ashley Formation, were curated at ChM.
All of the skulls of Xenorophus from the Ashley Formation in lateral view. At the beginning of this project I was unsure if any were conspecific with the Xenorophus sloanii holotype specimen (USNM 11049, upper right). We ended up concluding that these are all one species (see below). From Boessenecker and Geisler (2023).
The well-preserved basicranium of CCNHM 168, Xenorophus sloanii. From Boessenecker and Geisler (2023).
The basicranium and in situ periotic bone (inner ear bone) of CCNHM 1077, Xenorophus sloanii. From Boessenecker and Geisler (2023).
Mandibles of Xenorophus sloanii that are associated with the key skulls reported in the paper. From Boessenecker and Geisler (2023).
Meanwhile, a bunch of specimens were collected in the late 1990s and 2000s that ended up in CCNHM collections. These included two nearly complete skulls: CCNHM 104, the "Bailey" specimen, and CCNHM 168, the "Jaap" specimen - the latter is a virtually complete skull, missing only the right orbit, and also preserves left and right mandibles and some postcrania. Both of these specimens were from the Ashley Formation as well. A juvenile skull, CCNHM 5995, also from the Ashley Formation, was collected just before I got to Charleston. A few months after I arrived, Mace and I went to visit Mark Havenstein - a College of Charleston alumnus - and he agreed to donate a bunch of fossil cetaceans from his collection which he had earmarked for museums. Among these was a series of blocks including a massive block of limestone with a very large posterior skull and articulated mandibles; the rostrum had been taken off by a bulldozer. We had a nearly complete postcranial skeleton for this specimen - unfortunately, missing the flippers, of course.* Preparation of this specimen took about two years in fits and starts, and unfortunately an undergraduate taking a preparation class for credit destroyed much of the postcrania by waiting until finals week to get back to working on it, and pulled out many fragments from the blocks, did not write down where they went, and then just left all of the separated fragments in a pile and left for summer vacation. Moral of the story - never assign fossil preparation projects to undergraduate students for credit. I spent a lot of time searching desperately through the postcranial fragments for potential bits of vestigial pelvic elements - never found any.
*Flipper/forelimb elements are, frustratingly, not known for a single specimen of any xenorophid.
The Aylor specimen of Xenorophus, CCNHM 8720, after we pried off all of the globbed on plaster. This rather ugly looking lump of bone ended up becoming the Xenorophus simplicidens holotype.
Preparator Shelley Copeland removing some of the chewing gum from a tooth of the Aylor Xenorophus.
Teeth emerging from the siltstone.
The palate of the Aylor Xenorophus starting to take shape.
A selfie Shelley took from her apartment during lockdown! This is about the first week of April, 2020. I'm glad we managed to get it to work - fossil prep is not the easiest 'work from home' opportunity for a student.
A year or two later (fall 2018), I was alerted to the existence of an unprepared skeleton in a couple of blocks that some local amateur collectors had found and were trying to sell for an exorbitant amount of money. They even brought it by the museum, and were confused when I explained that I was a college professor and didn't have any money, and that the museum didn't have an operating budget (not a dime; "you're lucky the school doesn't charge the museum rent" was the prevailing quote from superiors at the time). I told them, honestly - local blue collar workers like them probably made more money than I did, and that I couldn't help them. We would gladly take a donation though, and set them up with an appraiser for the purposes of getting a tax writeoff. They declined. It was a shame, because it was a real beautiful specimen - they had even gotten it CT scanned at a local hospital; nearly complete skull, with at least one earbone, partial postcranial skeleton, and much of one mandible. Eventually I forgot about this interaction or at least mentally suppressed it until Sarah got a phone call about a year later - early December, 2019 - and a local attorney (David Aylor*) was wanting to donate a specimen. A brief verbal description of the fossil on speakerphone, with the two of us listening in eagerly - suggested that it was the same specimen. We politely asked for some photos of the specimen to be emailed to the museum - and a few hours later, these photos confirmed that the specimen was the very same. We set up an appointment to visit David's house on the Ashley River - a gorgeous view! David signed the deed of gift, and we loaded as much of the specimen as we could into the trunk of our Honda accord and carefully drove it back to the museum, just a couple miles away - but there are speed bumps, and of course, lots of potholes (a South Carolina staple!). When we returned to get the skull block, we were horrified to learn that David was gone, and the two young gentlemen who told us they'd be back in one hour when we returned - had also vanished. We tried calling David's phone - no response, car gone, doors locked. The specimen was in his garage space, with an open door - but it weight like 200 lbs, way too much for Sarah and I to get by ourselves. We waited for about a half hour. Fortunately, his neighbor was walking by with his golden retriever and I politely explained the pickle we were in (it was also approaching 5pm, and I had to be back in the classroom for a 6pm lab). I gave him my business card to explain I wasn't a crazy person and offered to show the deed of gift, but he said it wasn't necessary - and agreed to help lift the fossil into the car, meanwhile muttering something about those two gardeners working for him as well and not being terribly reliable, which connected some dots (sounded like it was beer-thirty). We lifted the fossil, sitting on a plywood sheet, onto an old ass radio flyer toy wagon with all flat tires, and pulled the pathetic contraption to the back of the car, taking care not to let the fossil get jostled - and managed to make the last lift into the car. Once back at the college, we drafted some students into lifting it onto a cart and placed it safe and sound in the museum - and I hurried off to teach, just in time!
*Sadly, David Aylor passed away in January 2023.
The palate and dentition of the Aylor Xenorophus, almost at completion!
We cracked open the jacket a few days later - it was a mess. No
burlap, three inches of plaster just globbed on over a layer of muddy
sand that had been applied like play-dough over the bone, but separated
from the bone by a single layer of tin foil. The silty sand had since
dried, and hardened to nearly like rock; it was Chandler Bridge
Formation matrix, which is soft as hell when wet, but hardens again when
dry. So the plane of weakness was at the foil layer - the silty sand
rock and plaster tended to come off in big chunks. Fortunately - and I
don't say this lightly - either the plaster was so damn hot, or the
muddy layer that had been applied contracted significantly when it dried
out - there were some large cracks in the plaster. We couldn't saw
through it because the plaster was too thick, so we had to actually
chisel off the plaster in places where there were no contraction cracks.
When we finally got through to the fossil, about a half dozen teeth had
been coated with wads of chewing gum (trident cinnamon gum by my
reckoning), which had since bonded to the teeth completely. My first
fossil preparator, hired through funds I raised on my own, spent several
days just carefully rehydrating the disgusting chewing gum and scraping
it from the tooth enamel. Shelley continued on the skull block in
January when classes resumed, and had the palate and upper dentition in
pretty good shape by March 2020 - and you know what happens next - the
Covid-19 pandemic. Shelley was in desparate need of work, so after I
returned from our poorly timed spring break vacation to Turks and
Caicos* I got permission to enter the museum and assemble a 'fossil
preparation from home' kit and dropped off the detached rostrum block
from the same specimen along with vinac, brushes, toothbrushes, dental
picks, and even plaster and burlap if she needed to make a support
cradle for it.
After Shelley graduated, Sam started out as our next preparator - and we flipped the jacket in summer 2021, and got most of the dorsal side prepped out - even if a few pieces were broken when a bunch of high school students were admitted to the laboratory by someone else.
After Sam graduated, Megan Dia volunteered a couple days a week during spring 2022 - here we are trimming off excess plaster from our temporary jacket.
And lastly, Ana Sillsbury finished preparation of the specimen before SHE graduated! Sarah and I put the specimen out on exhibit in summer 2022.
Over the next two years, I had three additional preparators work on the specimen - Sam Czwalina, Megan Dia, and Ana Sillsbury - with the finishing touches done to the delicate mandible in Fall 2022. At this stage, I had already written about half of the eventual Xenorophus monograph, and at this stage I had planned on focusing on specimens from the Ashley Formation, and a separate paper later on Chandler Bridge specimens (more on that reasoning later). The prolonged preparation time of this specimen stretched my credulity, but at some stage I realized a few things that made me comfortable with including the specimen - chief among these was my inability to get the manuscript finalized well ahead of preparation. Once I realized the specimen would be completed in another couple of months, I got to work - starting with description of the skull, and then mandible, as soon as it was prepared and curated. With the cradle made for the mandible, all of the Xenorophus material was finally ready for study - after eight years of observations and waiting.
What defines Xenorophus?
In the prior post I surveyed xenorophid diversity - it's pretty clear what defines some of the oddballs like Inermorostrum (small size, short rostrum, toothlessness) and Cotylocara (deep postnarial fossa with median ridge, loss of the intertemporal constriction) but what about Xenorophus? At the outset, the taxon was basically restricted to the holotype - which admittedly is a bit of a weird specimen. I'll talk about the weirdness of the holotype skull below and what it means for species in Xenorophus, but for now, just focus on what features these specimens share with it and what makes the genus Xenorophus.
First and foremost, while the new specimens are large - indeed, the largest known xenorophids - the X. sloanii holotype is also large in comparison to other xenorophids, though not quite as big as some of the large specimens like CCNHM 1077, 168, and ChM PV 5022. This readily distinguishes these specimens from Inermorostrum, Albertocetus, and Echovenator. These specimens have a well-developed intertemporal region with a sagittal crest, have a flat plateau of the frontal bone between the eye sockets (lacking a postnarial fossa), and large teeth set into a wide rostrum - clearly differentiating these specimens from Cotylocara and an unnamed species represented by ChM PV 2758*. One thing that sets Xenorophus apart from all other xenorophids, aside from Albertocetus - are its small, rectangular nasals. They're somewhat more elongated in others, and rather than possessing a median ridge as in Cotylocara, they frequently have a median groove or furrow. The wide rostrum is very distinctive, being proportionally wider than in all other xenos. It's also got the absolutely and proportionally largest teeth of any xenorophid. Most critically, however, it has some unusual patterns of asymmetry - I'll cover these more in the third post, but for the purpose of defining Xenorophus - it has asymmetrical palatines with the right palatine being wider than the left and extending further forward by nearly a centimeter in most specimens.
*Somewhat ironically, this specimen was initially interpreted as a referred specimen of Xenorophus by Whitmore and Sanders (1977) as it was the first xenorophid discovered in over 50 years - but later realized by Sanders that there were in fact many xenorophids, and that specimen ended up being somewhat more distantly related to Xenorophus sloanii than others (such as those discussed in this post).
How many species of Xenorophus?
Regardless of what these specimens share with the type specimen of Xenorophus, they all look a bit different from it. Recall above - I mentioned that Al Sanders initially didn't think any of these specimens represented Xenorophus sloanii. I thought that the X. sloanii holotype was pretty weird looking - what exactly made it weird? A few things. First, it's got big teeth: the teeth are absolutely (and especially proportionally) large, fairly cuspate, and rugose - perhaps more rugose than other specimens, and larger and more ornate/cuspate than many others including the Jaap specimen (CCNHM 168), Havenstein specimen (CCNHM 1077), and the Crowfield ditch specimen (ChM PV 5022). However, CCNHM 168 and ChM PV 5022 didn't have the posteriormost 'molars' preserved, perhaps tainting our observations. Next up, the X. sloanii holotype has some pretty dinky nasal bones - they're very short and nearly rectangular. Lastly, and this is the one that started to make things click for me: it's small. Still bigger than Albertocetus and Echovenator, but quite a bit smaller than any other specimen. I did not appreciate this until I saw the specimen in person in May 2016 right after my colleague Dr. Rachel Racicot's wedding (like true scientists, Morgan Churchill, Sarah, and I bundled a wedding with a research visit to the Smithsonian).
The small size of the X. sloanii holotype finally clicked when I thought about the spacing between the teeth. Are the teeth absolutely large, or just proportionally large? Why are the teeth so crowded? An then it struck me one afternoon at home during Covid: it's weird and small with proportionally large teeth because it's a baby! Well, not a baby, but a juvenile. You see, xenorophids probably only had one set of teeth - like modern odontocetes. When mammalian permanent teeth erupt, they're usually in half-grown individuals, and so look comically big. My background in fossil pinnipeds gave me additional important background: in juvenile pinnipeds, the teeth erupt and are overlapping and crowded, eventually being pulled apart in a much larger adult mandible as it grew larger and larger - and because the teeth do not change size, the gaps between them grow during growth! The same was true for Xenorophus. After I connected these dots, I decided to measure tooth size and count the cusps on them:
The teeth of the X. sloanii holotype are certainly the largest out of the sample, at least on this chart (tooth length X number of distal accessory cusps) - but not by much, only by a millimeter or so. They also overlap in terms of cusp counts with specimens like CCNHM 168. Further, the teeth of CCNHM 104 are probably actually larger: the measure nearly the same length, despite being highly worn down. So, the holotype's teeth are on the periphery of variation in Xenorophus - but fall within the range of variation. What about the nasal bones? Nasals are short in most odontocetes, but long in the largest Xenorophus specimens. I had a hunch that the nasal bones might lengthen during growth - I demonstrated a lengthening of all rostral bones including the nasals during my PhD research on eomysticetid whales, and in particular, in the species Waharoa ruwhenua. Juvenile mammals typically have shorter, stubbier, 'cuter' rostra than the adults. Sure enough, the smallest specimens like the holotype have the shortest nasal bones. The largest specimens - like CCNHM 1077, ChM PV 5022, and CCNHM 168 - have the longest nasals. Other specimens that are somewhat larger - referred to conveniently as subadults - have intermediate length nasals (CCNHM 104, ChM PV 7677). One of these in particular, ChM PV 7677, has nasals that are only slightly longer - and critically, are starting to grow this posterolateral splint that really gets long and accounts for most of the length in the old, large specimens.
Nasal growth in Xenorophus. From Boessenecker and Geisler (2023).
Xenorophid biochronology in the South Carolina. From Boessenecker and Geisler (2023).
The holotype skull block of Xenorophus simplicidens with the skull in dorsal view. CCNHM 8720. From Boessenecker and Geisler (2023).
And another view with just the skull highlighted. From Boessenecker and Geisler (2023).
The exceptionally nice mandible of Xenorophus simplicidens, CCNHM 8720, and a view of the postcanine teeth. From Boessenecker and Geisler (2023).
The Ashley and Chandler Bridge formations are about five million years
apart: the Ashley dates to 28-30 myo and the Chandler Bridge to 24.5 or
so. In this context, I typically think that it's unlikely to find the
same species in each unit, and that stratigraphically separated species
are likely. In two cases, I've found good evidence for species
longevity: Ankylorhiza tiedemani, and Albertocetus meffordorum -
at least based on published material; there are more specimens out
there that may, of course, lead me or others to reinterpret these
hypotheses. In this context, I started looking for any evidence of 1)
difference between the Ashley and Chandler Bridge samples of Xenorophus and
2) any evidence of two, rather than one, species in either rock unit. I
failed to find any evidence of multiple species, and noticed a number
of differences between the Chandler Bridge and Ashley Formation
specimens. Specimens from the Ashley Formation tended to be larger, with
longer nasals, and somewhat more complex teeth - CCNHM 8720 and ChM PV
4823 in particular have fewer accessory cusps and one less double rooted
tooth position than the Ashley Formation sample, as well as less rugose
enamel throughout the dentition. Al Sanders chose the name Xenorophus simplicidens
largely based on ChM PV 4823 - the Edisto river specimen - though the
teeth are perhaps less extremely simplified than he let on privately.
Nonetheless, we duplicated most of Al's observations in CCNHM 8720 (the
Aylor Xenorophus). While CCNHM 8720 had a bit of a squashed and
incompletely exposed skull, it does have two things the admittedly nicer
skull of ChM PV 4823 lacked: a virtually complete upper dentition, a
mandible, and a nearly complete lower dentition. So, we honored Al Sanders with using the species Xenorophus simplicidens for the species from the Chandler Bridge Formation with somewhat simpler teeth. We also found that the Chandler Bridge Formation skulls are somewhat smaller and less robust than Xenorophus sloanii. We chose ChM PV 4823 as a paratype specimen, since it is a very nice virtually complete skull though it critically lacks many of the teeth preserved in CCNHM 8720, which we chose as the holotype.
All told, this sample demonstrated that 1) there are two, stratigraphically separated species of Xenorophus and that all specimens from the Ashley Formation belong to Xenorophus sloanii; 2) all specimens from the Chandler Bridge Formation belong to the new species Xenorophus simplicidens; 3) there are some interesting ontogenetic trends including the lengthening of the rostrum and nasal bones, and other changes I'll talk about in part 3!
References
Boessenecker, R.W., and Geisler, J.H. 2023. New skeletons of the ancient dolphin Xenorophus sloanii and Xenorophus simplicidens sp. nov. (Mammalia, Cetacea) from the Oligocene of South Carolina and the ontogeny, functional anatomy, asymmetry, pathology, and evolution of the earliest Odontoceti. Diversity 15:1154.
Geisler, J.H., and Sanders, A.E. 2003. Morphological evidence for the phylogeny of Cetacea. Journal of Mammalian Evolution 10:23–129.
1 comment:
Very informative and entertaining. Love the history part, glad that key specimen made it back to you. But chewing gum, really? Anyway, I love the Oligocene "dolphins".
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