Bone-eating zombie worms, part 4: more on bird bones, and Osedax colonizes whale teeth

As covered in a previous post, Kiel and colleagues (2011) recently reported on Osedax traces in Oligocene marine bird bones (Plotopteridae) from Washington, implying that by the late Paleogene Osedax was adapted towards using bones of many types of marine vertebrates (birds and cetaceans). This has direct implications for the divergence time of Osedax - two dates have been determined by molecular divergence dating: Eocene (coinciding with the radiation of pelagic cetaceans), or Cretaceous (a period with numerous large marine reptiles). Kiel et al. indicated that medium sized birds spanned the K/Pg interval and were some of the only marine tetrapods with available bony substrate during certain areas in this time interval. Fish bone, of course, is always going to be more plentiful in the marine record - but fish bones are generally small, and at the time of publication, Rouse et al. (2011) had not yet published their experimental colonization of fish bones by Osedax. Later that year, Higgs et al. (2011) indicated that the bimodal size of boreholes and lack of distinct individual cavities could suggest that Kiel et al. (2011) had not really reported Osedax colonizing bone, but that the traces on bird bones may actually be sponge borings - sponges form a trace called Entobia, which looks (on typical substrate like mollusk shells) like a bunch of tiny pinholes; the sponge inhabits the cavity, using the numerous tiny pinholes for inhalent papillae, and a few larger pinholes for the exhalent papillae.* Additionally, the borehole density reported by Kiel et al. (2011) was far denser than reported on experimentally colonized whale bones, which Higgs et al. (2011) further identified as evidence that the Oligocene bird bones were not in fact Osedax-bored, but sponge-bored.

Close up shot of the bone surface, showing the two sizes of pinholes in the Oligocene plotopterid. Are they the sponge trace Entobia, or the Osedax trace Osspecus? From Kiel et al. (2011).

Steffen Kiel and colleagues followed up their previous discoveries with a new paper in Paläontologische Zeitschrift, reporting in more Oligocene marine vertebrates bored by Osedax: fish bones and whale teeth. Altogether, it's not all too surprising: we have borings in a wide array of modern and fossil critters already. We already have modern fish bones - the really cool thing, in my opinion, is that Osedax will consume teeth. At the moment, I'm wrapping up a large manuscript which does include a little blurb and a figure showing possible Osedax borings in a dusignathine walrus tooth - which means I've got to add a new reference to my paper. But that's a story for another day.

Osedax-bored fish bone, from the Oligocene Makah Formation of Washington state. From Kiel et al. (2012).

The fossils hail from various Oligocene deep water rock units which are already known to produce whale fall faunas, cold seep assemblages, and wood falls. These include the Makah and Lincoln Creek Formations of the northern coast of the Olympic peninsula, as well as the (also) Pysht Formation just across the river from beautiful Astoria, Oregon. Coauthor Jim Goedert and his wife Gail have prospected the rugged coastline of the Pacific Northwest for decades, finding marine mammals, sharks, and marine birds. He's described Paleogene pelagornithids from the area, and named a plotopterid (Phocavis). The toothed mysticete Chonecetus goedertorum from Washington was named for them (they collected the holotype in 1984), and Gail also found what would later become the holotype of Pteronarctos goedertae in Oregon.

Archaic mysticete teeth from the Oligocene Pysht Formation of Washington with Osedax borings. From Kiel et al. (2012).

Arguably the most surprising finding of Kiel et al's new study is the pervasive boring of whale teeth. These teeth were all found in a partial, highly corroded mandible; the authors did not specify what kind of corrosion, but it was in all likelihood bioerosion (and Osedax related at that). They didn't identify what kind of mysticete the teeth belonged to - published aetiocetids from the Oligocene of Oregon and Washington have simpler teeth, but I have seen aetiocetid teeth in USNM collections similar to these. In several specimens, the crown was heavily bioeroded by Osedax, and the root just below the crown was as well. They argued that the loss of the crown in some cases was caused by scavenging invertebrates (possibly crustaceans) accidentally damaging the root while trying to eat the Osedax worms. Similar crustacean-mediated destruction of bone has been observed in modern whale falls. (As a total aside, it's mid summer here in New Zealand, and both my wife and I got pinched on the toes by shore crabs while in the water.)

CT images of the Osedax borings within the whale teeth, with individual borings shown in yellow in 3D below. From Kiel et al. (2012).

Kiel et al. (2012) also took the opportunity to respond to some of the comments by Higgs and colleagues about the bird bone traces. Kiel et al. pointed out that in numerous modern specimens, they observed comparatively dense Osedax borehole clustering, so extreme borehole density does not invalidate the their identification. Kiel et al. (2012) also point out that there were mollusk shells present along with the bird bones, and the shells were not bored; shells are the typical substrate for such boring sponges. As a taphonomist, I should note that this argument doesn't necessarily hold sway: most marine assemblages are time averaged, and because vertebrate and mollusk remains are of different chemistry, size, and have different soft tissue anatomy and production rates, they are subjected to different taphonomic pathways. In other words, it is certainly possible that the period of modification was different for the bones and shells. To make that more clear: the whale fossil could have been sitting on the seafloor exposed for a long period of time, allowing Osedax to colonize; towards the end of the pause in sedimentation, some mollusk shells are washed in and buried too quickly for the shells to have also been colonized. Or, the mollusks could have been burrowing taxa. Back on topic: Kiel et al. (2012) concluded that these fossils demonstrate that Osedax has been a generalist bone-consumer for over thirty million years, which strikes another blow to the "Osedax as a cetacean bone specialist" hypothesis.

*I'm no expert on Poriferan anatomy, but I can only assume that the papillae are homologous to the large openings through which water is pumped in and out.

Don't forget to see the other posts in this series:
 
Bone-eating zombie worms, part 3: Osedax consume more than cetacean bones

Bone-eating zombie worms, part 2: the discovery of fossil Osedax traces

Bone-eating zombie worms, part 1: whale falls and taphonomy

References cited:

Higgs, N.D., C.T.S. Little, A.G. Glover, T.G. Dahlgren, C. R. Smith, and S. Dominici. 2011. Evidence of Osedax worm borings in Pliocene (~3 Ma) whale bone from the Mediterranean. Historical Biology 24:269-277.

Kiel, S., Kahl, W. A. and Goedert, J. L. 2010 Osedax borings in fossil marine bird bones. Naturwissenschaften 55:51–55.

Kiel, S., Kahl, W. A. and Goedert, J. L. 2012. Traces of the bone-eating annelid Osedax in Oligocene whale teeth and fish bones. Paläontologische Zeitschrift DOI 10.1007/s12542-012-0158-9

Rouse, G.W., Goffredi, S.K., Johnson, S.B., and R.C. Vrijenhoek. 2011. Not whale-fall specialists, Osedax worms also consume fishbones. Biology Letters 7:736-739.