Acid preparation discoveries: earbones filled with fecal pellets

 

One of Terry Iversen's discoveries, a small toothed mysticete skull (eye socket to the left) sitting in an acid bath a few years ago.

I've been acid prepping fossils from the Pacific Northwest for about ten years now, and even in the most soluble rocks, not quite everything dissolves. Fossils sent to me by Jim Goedert, which we have livingly dubbed "whale mail", hail mostly from the the Makah Formation (early Oligocene, ~30-33 Ma) and the Pysht Formation ("middle" Oligocene, ~26-28 Ma). These two units are a calcareous siltstone - and while a lot of the rock dissolves away completely, there is always a bit of a muddy residue that remains. Clay minerals, after all, are insoluble. In some cases, some interesting structures can be left behind by the acid, which is harsh, chemically speaking, but extremely gentle physically speaking. Delicate latticework like micro-burrows can be left behind in rare circumstances. But, what I keep on finding are clusters of tiny little fecal pellets!

Sounds like Sh*t 

The tympanic bulla of Terry's baby simocetid specimen. The anterior part of the bulla fell off during acid preparation, and I took advantage of the missing part to photograph the pellets prior to reassembling it.

These clusters are usually no more than about a centimeter in diameter, and the pellets themselves are usually sort of egg-shaped but slightly elongated - roughly about 1-1.5 mm long and just slightly less than a millimeter in diameter. I've found these in over a half dozen specimens, and occasionally these clusters are just out in the matrix and might get scrubbed away by my tooth brush - after all, they are only slightly less soft than the insoluble clay I intermittently rinse and brush off of one of these fossils during an acid bath. However, they tend to stay in good shape in protected areas that don't get scrubbed hard - right up against nooks and crannies of the skull or vertebrae, inside the pulp cavities of incompletely formed teeth of juvenile dolphins, and, most recently, inside earbones! Last fall and earlier this year I prepared a lovely partial skull of a very young simocetid dolphin (probably Olympicetus) that was discovered by Washington collector Terry Iversen and given to me for study and preparation. He had discovered the corresponding halves of this concretion at different times, and matched them up based on the matching breaks on each half. I guarantee you'll be hearing about this exciting specimen again in the future.

Two clusters of fecal pellets inside the tympanic bulla of an Oligocene simocetid dolphin.

There happen to be fecal pellet clusters inside both tympanic bullae, within the tympanic cavity - this is essentially the same structure as your middle ear cavity, and the middle ear ossicles (stapes, incus, malleus) bridge the auditory gap between the tympanic bulla and the inner ear. It gives a bit of a new meaning to "that sounds like sh*t".

 The tympanic bulla of Iversen's simocetid after preparation and reassembly was completed.  

Fecal pellets expelled from a burrow, most likely from a Carolina ghost shrimp, Callichirus major. Folly Beach, SC.

Who dealt it? 

In all seriousness though, who is responsible for these cute little poops? Cylindrical to ovoid fecal pellets (which are differentiated from coprolites only based on size) are almost certain to represent some kind of burrowing invertebrate. Beach bums (like myself) with a habit of looking down might notice what I call "forbidden chocolate sprinkles" - 1-3 mm long sprinkle-like pellets ejected from sandy burrows on the beach at low tide, often forming a little mound or halo around a burrow. If you're lucky, you can even watch a little "eruption" of water from the burrow with these pellets flowing out and onto the beach - telltale evidence of someone cleaning house below. In these cases, the organism responsible is usually identified as a callianassoid ghost shrimp; the photo above is likely evidence of a Carolina ghost shrimp. If you notice, these are strictly cylindrical with flat or "chopped" ends - quite unlike the egg-shaped pellets I have found.

A single fecal pellet - Coprulus oblongus - imaged using a scanning electron microscope or SEM. From Godfrey et al. (2022).

Trace fossils are given binomial names, which can cause some confusion as trace fossil naming (ichnotaxonomy) is based entirely upon superficial similarities and differences as the practice is entirely descriptive. The major tenet of ichnology, or the study of trace fossils, is that 1) the same individual organism can make many different traces during its life and 2) many different species can make the same trace. That being said, the shape of these pellets clearly identify them as Coprulus oblongus - a fecal pellet type that goes back to the Cambrian period. Modern groups that produce similar pellets in the ichnofamily Coprulidae include polychaete worms, acorn worms, gastropods, and tunicates. In a paper I'll mention shortly, the presence of these undisturbed clusters of fecal pellets within relatively tight, difficult to access spaces like the inside of a tympanic bulla probably rules out ghost shrimp, which are, as adults, too large to fit into a space like this. Godfrey et al. (2022) suggested that similar finds from the Miocene of Maryland are perhaps more consistent with some kind of polychaete worm.

A fossil fish skull belonging to a stargazer (Astroscopus) from the Miocene of Maryland - filled with Coprulus fecal pellets. From Godfrey et al. (2022). 

Sh*t for brains?

A few years ago a puzzling specimen was reported by my colleague Stephen Godfrey, who published a neurocranium (braincase) of a fossil fish (a stargazer, Astroscopus) from the upper Miocene St. Mary's Formation of Calvert Cliffs in Maryland. Stephen has been publishing a series of wonderful short papers on some real taphonomic curiosities - but this is perhaps the strangest. The neurocranium appears to be completely filled to the brim with pellets identifiable as Coprulus oblongus. Judging from the photos above, you can tell that something narrow and flexible that could wriggle through small spaces would have been needed to produce fecal pellets like this inside the braincase! They note, however, that this is not the first example of clusters of fecal pellets inside other fossils - while it is the first reported inside a vertebrate, there are many older examples noted in the literature, including within echinoderms, cephalopods, gastropods, trilobites, and brachiopods.    

Why do we seem to preferentially find these fecal pellet clusters in close quarters like this? Are these evidence of scavengers feeding on ancient dolphins and fish? Is this evidence that these critters were deliberately hiding inside of tight, protected spaces? These ideas are of course certainly possible. Many polychaetes are simply "deposit feeders" - they burrow through the sediment and ingest organic-rich sediment, metabolizing the organic debris and passing the sediment (in the form of these pellets, which appear to basically just be mud shaped into pellets). It's certainly possible that these polychaetes were consuming part of the carcass during the "bonanza" phase of a whale or fish fall on the sea floor. The sheer volume of pellets inside the stargazer skull might suggest prolonged habitation of the braincase.

 

Clusters of fecal pellets preserved in association with clam shells and in some phosphatic nodules. From Godfrey et al. (2022).

However, there is a third option. I have definitely found just as many fecal clusters out in the siltstone concretions surrounding these little cetaceans from the Olympic Peninsula as I have inside the protected cavities of the skull (and teeth). These generally don't survive acid preparation, and I know that I have had to sacrifice some of these clusters to expose the bone - and in cases where leaving the clusters of pellets intact would not negatively affect interpreting the anatomy of the fossil dolphin, I've tried to preserve them in place. In other cases, the pellets have been found in close association with other fossils - including some clam fossils, or within small concretions; see the above examples figured by Godfrey et al. (2022). 

I think it is possible, if not likely, that such pellet accumulations are probably common throughout rock layers, but we might only pay attention to them or be able to even identify them in the field in rare occasions - either when they form an accumulation with a fossil we're interested in and already looking for - or in a small concretion large enough to spot at a brisk pace along a shoreline. "Hey, look, what's this weird thing?" The sheer majority of fossil collectors, even in a place like Calvert Cliffs where they are relatively informed, are just out looking for shark teeth. There's no problem with that. But, many collectors just aren't interested in even well-preserved mollusks. I am probably not paying enough attention to mollusks and ought to do better! 

On a similar level, these pellets likely disaggregate unless they're mineralized somehow, or, perhaps ironically, preserved within a cavity of another fossil that a fossil preparator is unlikely to empty out. Fecal pellets in a flat lying bed of sandstone or in a vertical cliff are unlikely to be identified let alone collected for study - and I'll cite myself as an example. I'll see something neat, that isn't quite within the scope of my studies, and say "oh neat!", snap a cell phone picture, and continue my search for marine mammal fossils.

In sum, I think that the way these things tend to erode like the siltstone and sandstone that often hosts these pellets means that we might only be paying attention to them under unusual circumstances. 

References 

Godfrey, S.J., A. Collareta, J.R. Nance. 2022. Coprolites from calvert cliffs: Miocene fecal pellets and burrowed crocodilian droppings from the Chesapeake Group of Maryland, U.S.A. Rivista Italiana di Paleontologia e Stratigrafia 128:69-70.