Tuesday, December 17, 2013

A "new" publication: sea lions, barnacles, and taphonomy - what can encrusting invertebrates tell us?

Earlier this year I had two papers published which I never really had time to talk about on here. The first of them was published in the July issue of the Journal of Paleontology, and regards fossil barnacles preserved on sea lion bones from the Pleistocene of Oregon. The second – published a bit later in Acta Palaeontologica Polonica – is about fossil globicephaline dolphins from the Purisima Formation of Northern California.

This paper (viewable online here at Bioone.org) has its origins in June 2008. I had just completed my last semester of my undergrad geology program, and had planned an ambitious week of fieldwork along the coast of Oregon and Northern California. Some of these localities included the Wildcat Group, Moonstone Beach Formation, Falor Formation, St. George Formation, and the Port Orford Formation. I arrived at the first locality, located in Curry County, Oregon, after 16 hour drive to Eugene from Montana, and a short drive out to the coast. I knew of the locality from a paper by Larry Barnes on a new genus and species of sea lion, which had been collected by my idol Douglas Emlong about thirty years prior. I had also read the 1979 Ph.D. thesis of UC Berkeley student Barry Roth, who reviewed an impressive number of Plio-Pleistocene mollusk fossil localities in Northern California and Oregon.

Sunset on the southern Oregon coast.

Upon arriving in the area, I located a campsite and set up a tent for the night – I had planned on spending the night and doing some fieldwork the next day. I trekked down a mile and a half of windswept, desolate coastline; the wind was so strong, I felt pretty exhausted by the time I reached the locality – each step was an effort. Not being used to fossil localites outside of central California, I was expecting a fairly rich fossil site – Emlong had collected a skull, mandible, and a bunch of other bones from here, after all. At many fossil localities in Central California, one can identify dozens of exposed fossil bones within a half hour of arriving. At this locality, it was not so rich; after a few minutes, I did find a complete mandibular cartilage of a skate (Raja), and about ten minutes later, identified a pair of associated bones sticking out of the cliff; I photographed the locality, wrapped the specimens up in newspaper, and secured them in my pack; I was hoping for more. These only appeared to be vertebrae, after all. The fog was getting quite a bit thicker, with visibility down to about fifty feet; I thought it would begin raining soon, so I moved on, deciding to write my notes down later that evening. I headed up into a couple of gullies, and at one point sank nearly up to one hip in a curiously quicksand like mixture of mud and cobbles (it was, in fact, a recently deposited debris flow formed by late Pleistocene cobbles mixed with early Pleistocene muds). Shortly thereafter, it began raining: just great; by the time I made it back to the car I was drenched. After making a phone call to inquire about the weather in Curry County and further south, I found out that it was supposed to be sunny and warm further south in Humboldt County – my next stop. So, I went and packed up my tent, and continued south, escaping the crap weather.

Heading back through the dunes to the car with a full pack, and unbeknownst to me at the time, sea lion vertebrae completely coated with fossil barnacles (May, 2008).

A week later after some largely unfruitful visits to Humboldt County fossil sites (aside from collecting a rare sea otter tooth – Enhydra macrodonta), I was back in the bay area, and unwrapped nature’s fossil presents. I took the lumps of bone-bearing rock and sat them in the driveway, and blasted them with a water hose; after a few minutes, it became obvious that I had a pinniped vertebra. After another minute, I saw numerous barnacles – which appeared to be attached directly to the bone surface! The same held true for the other bone; their closeness in the rock (~20 cm) suggested that they were associated (i.e. originated from the same individual carcass). After a brief search online, I realized that this was fairly significant and that noone had really adequately documented this sort of occurrence before. Encrusting organisms are assumed to be common, but are rarely recorded in the literature; the occasional mentions they receive rarely pass the “Oh, interesting, there’s barnacles on this bone” stage, and few papers have ever inspected the taphonomic implications of encrusting invertebrates in detail. In contrast, documentation of encrusting organisms on invertebrate fossils is excellent, demonstrating that it’s probably a bias against taphonomic research by fellow vertebrate paleontologists (perusing back issues of PPP, Palaios, and Lethaia easily gives the impression that vertebrate paleontologists contribute only about 10% - or less – of taphonomic research in comparison to invertebrate paleontologists).

Dick Hilton (Sierra College) looking for Pleistocene marine mammal fossils (July, 2009). All those little white specks on the ledge are tiny fossil bivalves.

In 2009 I returned to the locality with Dick Hilton, and collected a sea lion femur; at the time I couldn’t see any barnacles, and when I returned to Montana that fall I placed it into a light acid bath as a demonstration for my friend Ash’s fossil preparation class. After a minute or so, I peeked into the violently fizzing bath and saw white flecks poking out of the calcareous sediment; I pulled it out and saw that indeed, this bone was totally encrusted with barnacles, too! After realizing this, I decided that enough was enough, and I spent about three weeks completing prep work over at Museum of the Rockies on the three specimens. Normally, concretions from that locality would have prepared well in acid, but barnacles are composed of the same mineral that made up the cement in the rock – calcium carbonate – which is highly soluble even in weak acetic acid. I’m glad that I pulled the bone out – otherwise I would have checked up on it a day later, and who knows how many barnacles would have been lost. Once the specimens were prepared, I began writing up a manuscript detailing the find and the implications. Although a first draft was completed in 2010, it took a back seat to my master’s thesis (arguably a more important endeavor). After arriving here in NZ last year, I realized that it would only take about a week to complete all the necessary tasks to bring the manuscript to being submission-ready – and accordingly I quickly wrapped the project up and got it submitted (to a separate journal, to which it was eventually rejected by a reviewer with a number of inappropriate and largely irrelevant arguments).

Aha! This is the sea lion femur at the time of discovery. Most of it was covered in a concretion and just a sliver of bone was sticking out.

During my visit last year to the Smithsonian, I examined some material (referred by Barnes et al. 2006 to various otariids) including a Zalophus californianus scapula collected from the same locality. The scapula lacked barnacles, but had a number of attachment scars preserved on it. I edited the manuscript again for submission, and included this “new” specimen in the paper.

The sea lion (Zalophus californianus) scapula which Emlong collected decades before. Image 3 shows the circular barnacle attachment scars.

Sorry for the long intro – there’s a bit of back story, but now that we’re done with that, what exactly were my findings? Three different specimens – two associated vertebrae, a femur, and a scapula – bore evidence of barnacle encrustation. The two vertebrae were encrusted with a total of 1400+ barnacles (I counted them all…), the femur with ~200 or so, and 15 on the scapula. Preserved evidence included actual attached barnacles, attachment scars with incised basal rings, and attachment scars visible only as changes in color (i.e. no physical etching was present). While the vertebrae had barnacles on nearly all surfaces, the femur and scapula only had barnacles preserved on one side. This led me to interpret that the vertebrae must have been overturned regularly, and that the femur and scapula were probably not overturned by currents during the period of barnacle colonization. More interestingly, the barnacles were of various sizes, and some of the smaller barnacles were encrusting larger ones: this implied that at least two phases of larval colonization were recorded in the assemblages. 

The two vertebrae I collected in May 2008, fully prepared as illustrated in the Journal of Paleontology article. 1-3, anterior thoracic vertebra; 4-6, posterior thoracic vertebra; 7, lateral side of neural spine of anterior thoraci; 8, same, but for posterior thoracic; 9, lateral side of centrum of posterior thoracic vertebra.

More important was the species identification of the barnacles. In summer 2010 I had dropped the barnacle encrusted specimens at California Academy of Sciences for barnacle specialist Robert (Bob) Van Syoc to make an identification. Bob identified the barnacles as Solidobalanus hesperius – a species which is still extant; this should not be immediately surprising, as the rock unit they were collected from is approximately 700,000 years in age (not late Pliocene, as erroneously identified by Barnes et al. 2006). The best part about finding modern species in the fossil record is that growth data often exist for them. In fact, one study – published in the early 1980’s in the Soviet Journal of Marine Biology (which, unfortunately, does not follow the lives and habits of Marxist mollusks) – reported age data for the species colonizing the Yesso or Yezo scallop (Patinopecten yessoensis, extant relative of the Pliocene dinner-plate scallop Patinopecten healeyi). These Russian authors found that Solidobalanus hesperius has a lifespan of about 6-7 months, during which it attains a basal shell diameter of about 15mm; over a 4-5 month period, they attain a basal shell diameter of 6-8mm. Most of the shells were smaller than this size range, but many were in the 6-10mm size range, so I used a conservative estimate of 4-7 months using those two endpoints.

As far as I’m aware, this is one of the first (if not the first) paleontologic studies to apply growth data from an invertebrate to determine a minimum period of exposure on the sea floor. I shouldn’t brag, because it’s not a novel concept: this method has been applied before to forensic cases where human remains were recovered from the seafloor with attached barnacles (e.g. Dennison et al, 2004). Regardless, we as taphonomists have a lot to learn from forensic taphonomists: there’s a lot more of them, they’ve been at it far longer than we have, and their work has to hold up in a court of law.

The femur collected in July 2009, fully prepared.

Another aspect of this study is that barnacle produced traces in addition to barnacles were identified on vertebrate remains. Barnacle attachment scars have been known for a long time (see Miller and Brown, 1979), and somewhat recently the circular, rather simple scars reported on these sea lion bones were given the name Anellusichnus circularis by Santos et al. (2005). This study confirmed that both physically etched scars, and scars in color only, may be formed on bone; further demonstrating the causal link between the barnacles and scars were some barnacles that were damaged during preparation with a partial shell remaining in the ring-shaped scar, uncovered by removal of the partially damaged barnacle. 

Lastly, this study demonstrates that encrusting organisms can act as a taphonomic window into the post-mortem interval. Ordinarily, we wouldn’t have any idea of what happened during this period, but in the case of these barnacle encrustations, we know that the bones were reoriented (or not, in the case of the femur and scapula), and were encrusted for a minimum of 4-7 months. The potential for encrusting organisms to yield more information about the post-mortem interval is of course much higher for other potential conditions of preservation (e.g. a more complete specimen with numerous types of encrusting invertebrates). All this goes to show that much more work on the taphonomy of marine vertebrates is warranted, as much more information can be squeezed from the fossil record – and with regards to marine vertebrates, we have barely scratched the surface.


BARNES, L. G., C. E. RAY, AND I. A. KORETSKY. 2006. A new Pliocene sea lion, Proterozetes ulysses (Mammalia: Otariidae) from Oregon, U.S.A., p. 57–77. In Z. Csiki (ed.), Mesozoic and Cenozoic Vertebrates and Paleoenvironments: Tributes to the Career of Prof. Dan Grigorescu, Bucharest, Romania.

DENNISON, K. J., J. A. KIESER, J. S. BUCKERIDGE, AND P. J. BISHOP. 2004. Post mortem cohabitation—shell growth as a measure of elapsed time: a case report. Forensic Science International, 139:249–254.

SANTOS, A., E. MAYORAL, AND F. MUNIZ. 2005. Bioerosion scars of acorn barnacles from the southwestern Iberian Peninsula, upper Neogene. Rivista Italiana di Paleontologia e Stratigrafia, 111:181–189.

No comments: