Sunday, October 24, 2010

SVP 2010: barnacle and sea lion taphonomy

Hey Folks, I know it's been a while (I feel like I say this a lot...), but I've been really, really busy. The semester has been really busy - I've been working on four different papers since I got back to school - but postings will be much more frequent for the remainder of fall. The entire month of September was spent writing a new manuscript, and getting revisions done for a manuscript that is most likely accepted for publication in the Journal of Vertebrate Paleontology. The last week of September and first week of October were spent coding characters for a poster I had with my good buddy Morgan Churchill and his advisor Mark Clementz (on pinniped phylogeny), my own poster, and two art projects I was involved with for two of the Romer Prize Presentations at SVP (by MOR/MSU students Denver Fowler and John Scannella, who between the two of them requested 9 or 10 drawings of ceratopsid skulls).Barnacles!!! (not that you've actually read about them yet). Isn't that cool?!

Six days before SVP started, I finally got the chance to work on my own poster, titled "Barnacle colonization of Middle Pleistocene Sea Lion (Carnivora:Pinnipedia) bones elucidate the biostratinomy of a fossil marine mammal". And no.... this is still not part of my thesis, although it is very similar to what I am doing. And actually, getting the poster done and now recapping on it have really helped me get reinvigorated about my thesis; I consider any time spent thinking about taphonomy is more or less time spent toward my thesis. Because this week I am starting to devote 100% of my research time to my thesis, more of my future posts this fall will be taphonomy related (and specifically marine vertebrate taphonomy, one of the most understudied and underpublished aspects of taphonomy).
But enough about my thesis. It's time for barnacle-on-bone action.

My poster from SVP (locality information is blocked out).

I had my poster presentation on Monday, day two of SVP, which was comforting because marine mammal talks and posters are usually the very last day of SVP, and during the afternoon at that (late afternoon/evening if you give a poster!). Anyway, a couple of years ago (June 2008) I was prospecting a marine Pleistocene locality in Oregon and found a couple of bones in concretions - no big whoop. Because of that, I didn't do anything with the bones for about two months, until I set 'em down in my parent's driveway and sprayed them down with a garden hose. As the soft outer rind of sediment began to wash away, I saw little white things I quickly identified as minuscule barnacles. Due to the library of papers on marine vertebrate taphonomy I've amassed for my thesis, I did a quick mental search, and quickly realized that these had never been reported as encrusting bone before. This made me really excited.

A beautiful sea lion femur with barnacles only encrusting one side.

A less cursory glance at the literature shows that these associations have indeed been recorded before in the literature, but never figured. Two examples have been figured, but either the photos are so god-awful or the item in question probably isn't even a barnacle. Otherwise, it's been mentioned as a sentence (or a part of one) before here and there. There are plenty of examples of modern barnacles encrusting old bones: I see that almost on a daily basis in the intertidal zone in Santa Cruz. To clarify, these are fossil barnacles.

The specimen where I first saw the barnacles.

So, into the nitty gritty. There are 1400+ barnacles distributed over three ones (the femur had ~200, and one of the two vertebrae is hogging 700+ to itself). They appear to be of two size classes: big ones (4-7mm with a few outliers) and small ones (~1-2mm). The big ones do not occur on any articular surfaces or in the neural canal, and predominantly only on the neural arch (i.e. few on the centra). The small ones occur pretty much everywhere, and in many cases are growing on the bigger ones. This indicates at least two colonization events. Dr. R. Van Syoc at California Academy of Sciences was gracious enough to identify the barnacles for me over the summer; they appear to be cf. Hesperibalanus hesperius. It is a species that is still extant, and is a prolific fouler of shells.

Closeup of the neural arch (left) and transverse process of the vertebrae (right).

Another interesting tidbit is the presence of barnacle attachment scars: here they are on the bone as oval-polygonal shaped traces, that are only excavated along the periphery. This has been reported before for rocks and invertebrate shells, but not bone.

So, what else can we infer? Because we know the ontogeny of modern Hesperibalanus hesperius, we can reasonably infer a gross estimation of minimum time averaging (i.e exposure time on the seafloor), given the barnacle size (basal plate diameter). The modern species only has a lifespan of 7 months, and seeing as none of these are full grown, they probably only represent 2-4 months. This is, however, assuming that barnacle basal plate diameter is constant: it is well documented that crowded barnacles only achieve a certain diameter until their basal plates are all touching and cramped (such as many on these specimens). So, it is possible that some of these were full grown, and so a time of ~ 2-7 months or so, ballpark.

The lack of any barnacles on some surfaces suggest that the surface was uninhabitable due to being smothered in sediment, or by the adherance of tissue. The femur lacks barnacles on the distal condyles, strongly suggesting the retention of articular cartilage there. Also, the femur only has barnacles on its posterior side, suggesting that the anterior side lay in the substrate and was never flipped. On the contrary, the vertebrae have barnacles more evenly distributed, so that it would be difficult not to smother barnacles at any position the bone was situated at. This suggests that the fossil was rolled around on the seafloor frequently enough to keep individual barnacles from smothering, very similar to the balanulith concept recently published in Palaios and Palaeontology. Anyway, there's much more to type about, but you'll have to wait for the paper, lest I retype everything on here.

On another note, I just got a paper on taphonomy (unrelated to this, but slightly related to my thesis) accepted in publication for Palaios, so you may be able to hear about that sometime early next year, perhaps.


Alton Dooley said...

Cool post!

You may be interested to know that we have a mysticete vertebra from the Yorktown Formation covered in barnacles like that (I have a bryozoan-covered radius from the same site). But I don't think I've ever seen a barnacle on a bone from the Calvert Formation, even though barnacles are known from the unit.

Never really thought about it before...

Robert Boessenecker said...

I've heard of barnacle encrusted specimens from the Yorktown, and from other units - but again, no one has bothered to publish on them. Given the few numbers of cases I've heard of, I've concluded that occurrences like this are really rare. Then again, the high energy environment required for the barnacles to live also increases the chances that the barnacles will be damaged and detached.

Meredith said...

I also read somewhere about barnacles at Sharktooth Hill (but I'm sure you know that) - which I noticed after seeing your poster. Looking forward to more taphonomy posts!

Robert Boessenecker said...

Hey Meredith,

Pyenson et al.(2009) mentioned in a single sentence a couple of partial, possible barnacle attachment scars on mysticete bones... and that was about all they mentioned or reported on.

Definitely expect more taphonomically related posts - there have been some real eyebrow-raising papers in taphonomy that have come out recently, and I don't mean that in a good way.

J. Velez-Juarbe said...

Your poster was really cool and it was nice to meet you at the SVP! Reading this post reminded me that I have some sirenian vertebrae that are partially encrusted with barnacles. They are back in Puerto Rico, but the next time I go there I'll take a careful look at them!

Robert Boessenecker said...

Thanks Jorge! It was good to finally meet you! I'm very intrigued by your sirenian barnacle associations! Interested in writing that up??

J. Velez-Juarbe said...

That would be good! Once I go to Puerto Rico latter this year I'll look at the specimens. Right now I'm preparing some other sirenian material and its got oysters growing around and on them.