Step aside Megalodon, this fossil is raising quite a stink!
Carcharocles angustidens tooth in phosphate nodule, pictured from collections at the Charleston Museum, Charleston, SC (PV 55.105.25).
It starts with an S and it ends with a T, and it comes out of you and it comes out of me. I know what you’re thinking if you call it that, but be scientific and call it scat!
- The Scat Rap (Doug Elliot; Bullfrogs on your Mind)
Have you heard the stink lately? One particular fossil has made near-viral fame on the internet to the tune of "Ancient shark tooth and bite marks found in crocodile poo," and other sensational titles: (Charlotte Observer, Newsweek, Daily Mail, EarthTouch News, IB Times) as thousands of people have discovered the defecatory nature of our Earth's fossil history.
The specimen: a tooth of the giant white shark, Carcharocles angustidens, embedded in a nodule claimed to be a fossilized piece of dung from an Oligocene crocodilian. (Pictured here.)
All of this dirty talk got us here at CFA (located in the town where the specimen was discovered) to wonder about the nature of the recent discovery, and its place on the stinky stage of fossilized poo. Is it truly a coprolite? What are examples of other coprolites found? And how can science help us determine poop from, well... not poop!
A LOOK into the literature
Fossilized feces, or coprolites (from the Greek kopros=dung, lithos=rock) as they're known to paleontologists, are not a new discovery. If anything, these fossils are among some of the oldest collected, with Mary Anning and William Buckland studying the existence of these "bezoar stones," or "fossil fir cones" as they had previously been described. In 1835, Anning and Buckland properly identified the masses as remains that had passed through the intestines of reptiles and fish from the Jurassic Period, over 176 million years ago (Ma), and not fossilized pine cones or masses from the digestive tract, as commonly assumed at the time. [Britannica]
Over time, coprolites became recognized more frequently, and added to the list of trace fossil that can be found in fossiliferous deposits.
More recently, Milan et al. (2012) conducted a study of a crocodilian coprolite with a partially exposed fish vertebra and described feces of crocodiles as alternating layers of dense and less dense material, laid down in the original way it was produced in the intestines of the consumer. The overall shape of crocodilian feces is elongate, cylindrical, and occasionally flattened (in cross section), with concavo-convex units segmenting the entire piece. Coprolites belonging to fish (sharks included, as they are cartilaginous fish) are spiral shaped, and those specifically of sharks are known to be "heteropolar" spirals, meaning the spirals are more compacted on one end.
Upon scanning and analysis of the coprolite with included fish vertebra, Milan et al. concluded that it belonged to a marine turtle or possibly large bony fish or shark, based on one distinguishing feature: the fish vertebra was entire. The digestive tract of crocodilians contains high concentrations of hydrochloric acid, which readily dissolves and decalcifies all bone before secretion; whereas the gastric acid in turtles is not nearly as strong, leaving fish bones and even mollusks intact upon excretion.
In another 2012 paper entitled "Crocodylian scatology - a look into morphology, internal architecture, inter- and intraspecific variation and prey remains in extant crocodylian feces," J. Milan analyzed the remains of extant crocodiles. In the study, published in a bulletin from the New Mexico Museum of Natural History & Science, Milan goes on to describe external features of crocodilian feces such as longitudinal banding and cylindrical tapering, and internal features such as the alternation of dense "clay-like" material and undigested animal remains. Milan event went so far as to screen the scat for all animal remains, but not a trace of bone or scales was detected, even with a mesh size of 0.122 millimeters! (That's about the width of a thick human hair!) [Milan, J. 2012.]
A ROCK by any other name...is still a rock
Paleontologists give rocks that take on the appearance of fossils the name of pseudofossils. "So," you ask yourself: "are there pseudofossils of feces? What if something is shaped like poo, but doesn't fit the bill of a coprolite?" Why, we're glad you asked...
If you're a resident of Lewis or Cowlitz County in western Washington State, you may be familiar with this pseudocoprolite:
"But wait!" You say, "How do we know these are really just rocks? They LOOK exactly like a big pile of poop!"
Wrong! These are concretions composed of limonite and goethite (pronounced GER-tight), iron-rich minerals, and are perhaps some of the most convincing pseudocoprolites out there! The telltale sign that these strange rocks are just that -- rocks -- can be seen if one is broken open to reveal the hollow, layered chambers (inset magnified view, above). Unlike layering in coprolites, the formation of these concretions leaves empty layers of air between mineralized deposits.
By definition, a concretion is "a hard, compact mass of mineral material formed when minerals in water are deposited about a nucleus (such as a leaf or shell or other particle) forming a rounded mass whose composition is usually different from the surrounding rock." [Kansas Geological Society]
Trickily enough, some concretions and other geologic features can mimic similar external morphology that an untrained eye would attribute to a coprolite. The following images are of various pseudocoprolites, geologic features, and fossiliferous nodules.
Above: Concretion with weathered limonite and goethite visible on the interior. (ChM specimen.)
Above: Concretion with weathered limonite and goethite visible on the interior. (ChM specimen.)
Above: Concretion with weathered limonite and goethite visible on the interior. (ChM specimen.)
Above: Phosphorite with embedded ammonite. Locality - France. (ChM specimen.)
Above: Concretion with weathered limonite and goethite. (ChM specimen.)
Above: Phosphate nodule with embedded Carcharocles angustidens tooth, exhibiting phosphatic steinkerns (internal casts) of bivalves. Locality - Charleston. (ChM specimen, PV 55.105.25.)
Above: Phosphorite. Locality - France. (ChM specimen.)
A CONVERSATION with the experts
To uncover more scatological information, CFA headed to the Charleston Museum and the Mace Brown Museum of Natural History to speak with experts there about coprolites, and get the scoop on fossil poop. (Tap or hover over images for captions.)
Charleston Fossil Adventures, LLC (CFA): Good morning, gentleman. Thank you for agreeing to be a part of this interview today. Before we begin, could you please let our readers know your names, affiliations, and respective positions?
Matthew Gibson (MLG): My name is Matthew Gibson. I'm the Curator of Natural History at the Charleston Museum (ChM).
Robert Boessenecker (RWB): My name is Robert Boessenecker, PhD, Adjunct Lecturer, Department of Geology and Environmental Geosciences, College of Charleston.
CFA: How many years of experience in paleontology do you hold?
MLG: I have 13 years of experience in paleontology. My undergraduate degree was with Johnathan Geisler at Georgia Southern University. I worked on whale material and similar vertebrates to what we have here in Charleston. (Shark teeth, for example.) My graduate degree is from East Tennessee State University; I have a Master's Degree in Paleontology from there.
RWB: Formal experience, 15 years.
CFA: What is a coprolite?
MLG: Well simply, a coprolite is fossilized dung. All of the biotic material has been replaced with minerals present in the layer it was buried in.
RWB: A coprolite is fossilized feces. Typically, identification is based upon external morphology -- the shape of the sample. If it's not shaped like poo, it's probably not!
CFA: What do you look for in a specimen to determine it is a coprolite?
MLG: I would look for the shape. So, basically, if you've seen a carnivore dung sample, they're elongate -- almost torpedo-shaped. You usually see striations along the sides. Where it breaks, you may find bones or other materials to show it was from a carnivore. (Herbivore coprolites are a little trickier.) Carnivore coprolites are pretty much what you'd expect to see if you went out and collected dog poo...it pretty much looks like that, except it's fossilized!
RWB: It's difficult to identify coprolites because they can come in many shapes. They are usually cut open and examined under a microscope. Without [that], it's usually difficult to identify a coprolite. The biggest thing is to look at the shape; everybody knows what their own looks like... Most coprolites are going to have some sort of fusiform shape. That's not always true, because sometimes you'll have critters like rabbits, wombats, and most herbivorous mammals where it breaks apart in clumps. But most carnivore [feces] -- which make up the majority of the coprolite record -- have a fusiform shape. In other words, it will be tapered at both ends; sometimes they'll be blunt, sometimes they can be long and 'rope-like' or cylindrical, but generally, it has to be passed through a small hole so there's going to be some sort of fusiform, teardrop, or cylindrical shape.
CFA: What species are represented by coprolites in the South Carolina Lowcountry?
MLG: I know we have crocodilian coprolites, there have been some tales of shark coprolites. I have not come across any in our collections; all specimens we have are crocodilian or thought to be crocodilian. There may be some turtle mixed in, too. I will admit, many of our coprolite samples are 'TBD' (To Be Determined) -- we know they are coprolites, but we're not certain what animal left them.
RWB: Identifying the producer of a coprolite is very, very difficult. Coprolites are trace fossils, and people who study trace fossils always stress the difficulty of linking a trace fossil with a producer in the fossil record. This usually requires matching the anatomy. That's easier for footprints where you have the skeleton of a foot and you can match it to the size [of the track]. If they're  the same age (from the same rock unit) then that's a reasonable hypothesis to be made. The most slam dunk examples are  trilobites that are found on a [rock] plate, where the tracks lead up to the dead animal, itself. So, you literally are looking at the fossilized final steps of that individual. Coprolites satisfy neither of those [requirements]; they are usually found in isolation from the trace maker, and we have no idea what the rectal anatomy of most fossil species is like.
CFA: Is it reasonable to compare the rectal anatomy of modern species within the same group and try to extrapolate that to the fossil species?
RWB: Yes, that's called extant phylogenic bracket. That's basically using modern species to make inferences about close relatives in the past. Generally speaking, if we see something in the rock record that looks like 'crocodile turd,' it's likely to be one. Or, something that at least had colorectal anatomy similar to a crocodile, and there's many candidates that may have approximated that. But we don't know, since we don't have the soft tissue preservation (we likely never will).
CFA: Have coprolites been found with vertebrate remains still present?
MLG: In our collections we have very few samples with vertebrate material in [the coprolite], but what we do have is a little bit of fish material: small fish vertebrae, though fairly broken apart.
RWB: Yes, in rare cases shark teeth have been found. I believe there are a few examples of coprolites found with crocodilian teeth inside, and the teeth have lost most of their enamel, owing to stomach acid. There are some cases, but it depends upon the group in question. [Shark vs. croc vs. fish...] Coprolite specialists would call whatever is found within a coprolite an inclusion, and the short answer is yes. Fragments of prey items whether it's plankton or shells or bone fragments or teeth or bits of wood, inclusions like that are widely known. But, you can often have coprolites with inclusions that you can't identify to a particular species or genus.
CFA: Have coprolites been found with invertebrate remains still present?
MLG: I'm not aware of any in our collections. I could see some invertebrate remains -- very fragmented shells, things like that -- [being possible]. Invertebrate remains are pretty susceptible to acidic solution, so it would be very difficult to preserve those.
RWB: Yes. The short answer is it's typical in species that eat invertebrates. There have been studies of shell ducks in Europe which poop out 'shell hash' and that could in theory fossilize. I've always kept my eyes peeled for little pellet-like pods of small finely-milled shell fragments (which could in theory be some kind of bird like those I mentioned) or some kind of durophagous -- shell eating -- fish. There are plenty of examples of [durophagy]. Parrotfish, for example. I've not found one yet. I've been aware of coprolites and what to spot for 15 years, and on the west coast (i.e. California) I've only found two to three convincing, plausible examples. I'm not saying they are, I'm saying they might be. Everything else I've seen would not fit the bill.
CFA: Based on the photos circulating the internet, what is your professional opinion on the purported coprolite found here in Charleston?
MLG: Based on phosphate samples I've seen collected in the area, it looks like a phosphate nodule with a shark tooth in it. We have a couple phosphate nodules in our collections that also have a shark tooth in them. They're not embedded to the level that this tooth is, but honestly, it's just the way the phosphate forms around the tooth: for some [nodules] the tooth is right underneath, others, the tooth is hanging off the side... It really just depends on how the tooth contacted the phosphate as the nodule formed. I don't think it's out of the realm of possibility that this is just a phosphate nodule that formed around a shark tooth.
RWB: I have many thoughts on that piece: 1) Nobody knows what the purported coprolite is made out of because it hasn't been sampled, and nobody's looked at it under a microscope. That necessitates some degree of destructive sampling, and I know it's in a private collection and I'm assuming a lot of money was paid for it, so nobody is interested in cutting it up with a rock saw and making a microscope slide out of it. 2) Second, and most importantly, it doesn't look like a coprolite. It's rock-shaped and irregular, it has an irregular surface texture, and most coprolites are kind of smooth. I've never seen modern dung that has that surface texture. Some of the pits in it could be from 'boring bivalves' like piddock clams. There are cases of coprolites that are already lithified and subsequently have piddock clams bore into them. Piddock clams require a hard substrate, and coprolites in a phosphatic bone bed, along with phosphate nodules and occasionally early mineralized bone -- all of those are hard substrates that piddock clams can bore into. I'm not necessarily convinced all of those pits [on the specimen] are boring clams, as some of the holes don't have smooth walls with a sharp, circular opening. Some of them look like the naturally-occurring pits that appear in phosphate nodules. The specimen altogether looks like a phosphate nodule. It looks like a small concretion. Phosphate nodules are also made of calcium phosphate, just like coprolites. They can often have shark teeth embedded in them. In the Charleston area, they can also have molds of solitary corals. Our species of coral is called Balanophyllia and it gets up to 1.5 cm wide to about 1 cm tall, and calcium carbonate (the mineral that makes up shells and limestone) dissolves during periods of time when phosphogenesis is occurring. So if you make phosphate nodules, you end up dissolving a fair bit of calcium carbonate, and any shell material inside that phosphate nodule gets dissolved out, so you end up with cavities inside. 3) The shape, the pits, the size, all of these look like the hundred of thousands of phosphate nodules you see lining the shorelines of our local rivers in Charleston.
CFA: Is it credible that a shark would a) bite a pile of dung from a crocodile, and b) if so, could a tooth dislodge in the excrement?
MLG: That seems incredibly odd for that to happen. I have been to talks where people have suggested some coprophagy among sharks. ..... [Describing the scenario leading to the purported specimen.] What you have here is a croc that's near the surface [of the water], with a shark below. The croc uses the bathroom, the coprolite drops, and for some reason the shark wants to investigate it. ..... I suppose, in the realm of possibility, yes, possible. Why the tooth would break off? It would have had to be hanging on by a thread. (It's not like dung is hard.) So the fact that the shark took an investigative bite -- which is usually light -- to lightly tap this dung to break off its tooth but not split the dung in half? I don't even understand how that would happen.
RWB: First, sharks do test bite, and they will bite inanimate objects. There is a pretty solid example of a true shark-bitten coprolite from the Miocene of Maryland in the Calvert Formation. In that case, the authors of the study took latex and poured it into the shark bite and pulled it out. Sure enough, the tooth marks look exactly like a tiger shark tooth. In that published case, I believe it. There are also some pretty good examples of coprolites bitten by gar fish -- which have large needle-like teeth and tiny needle-like teeth, so you'll see this pattern of large and tiny tooth marks coming at opposite directions, and that
example was published just about a year ago. So it's possible, however, poo as most people recognize, is pretty gushy; as such, the pit left in the Maryland example was where the tooth went in and came out, and left a mold of the tooth. The only time a coprolite would be hard enough to break a tooth would be when it's too brittle to actually leave the same kind of tooth mark. I suppose anything's possible, but the problem is the entire tooth is embedded in the nodule, including the root. How do you break off a tooth but have the part of the tooth that's still connected to your head embedded in the coprolite? The only way that could happen would be when the coprolite is still soft, and in that case, it's not going to behave in a manner where the tooth is dislodged. We do have some rare examples of teeth breaking off in bone, and these are cases of scavenging. In every case, only the tip of the shark tooth is stuck in, and there's a little bit of deformation of the bone around the tooth. It's essentially like a nail made out of material more brittle than metal, and you hammer the nail in, snap it off, and in the process of doing that, damage a little bit of wood. Anyone that's worked with wood knows what I'm talking about: the forces involved in shark teeth biting into bone -- especially weaker bone like whale bone that's more porous like a sponge -- end up treating the bone like a nail and wood. You end up with a little bit of surface deformation around the tooth. In every single case, however, only the tip is snapped off. For all of these reasons, [the claim] sounds kind of ridiculous.
CFA: It appears that the viral specimen has some marks and features present, other than the shark tooth. What do you make of those?
MLG: From what I can tell compared to the phosphate nodules we have here, they're just impressions from invertebrates that were also in contact when this phosphate nodule was forming. Like I said before, invertebrate material is highly susceptible to acidic liquids, so at some point those shells just dissolved away. Shark teeth are more resistant [than calcium carbonate] so it didn't. These are actually pseudo-impressions that are making it seem like they're bite marks, but they're really not. I get the feeling if you got some clay and pushed it into those impressions, you'd find you've just made a copy of a shell.
RWB: One of the two features is a purported bite mark of a shark. It's supposed to be (I assume) the same species of shark tooth, Carcharocles angustidens, that ended up embedded in the nodule. A typical bite mark from Carcharocles or Carcharodon (the megatooth and great white sharks) will have a wedge-shaped cross section, especially for Carcharocles because the teeth are relatively wide. The edge of the tooth is also straight; it doesn't curve in or out of the mouth. There's two ways these sharks will make tooth marks: 1) the bite will glance away at the bone and will leave a series of parallel ridges on the bone from each serration, and 2) the other way is a linear groove. In those cases, it's the serrations moving in the same direction as the edge of the tooth. In other words, how you saw away at a piece of wood,
Above: Bivalve mold displaying longitudinal ribbing. (ChM specimen, PV 55.105.25)
in which case, you don't really get serration marks. The purported tooth mark in this specimen has none of these features. It is concavo-convex, not wedge-shaped, aka it curves in one direction. It's also very thin, and extends into the matrix of the specimen several millimeters. The hole -- judging from the photographs -- is only about one or two millimeters across, and instead of being wedge-shaped (shallow and deeply widening toward the outside with a distinct groove in the middle), this one is kind of flat like if you took a piece of paper and curled it. Furthermore, on the convex side are a series of ridges which have been misinterpreted as the grooves caused by the serrations [on the shark tooth]. Any undergraduate geology student who has spent more than three or four hours in the field learning sedimentary rocks and basic fossil identification will tell you that [mark] is a textbook case of a bivalve mold. (Bivalves are clams. Many bivalves, especially scallops -- which are common in the Ashley Formation, the widespread formation here that produces many of these phosphate nodules -- have ornamentation on the outside from the hinge that extends to the outside edge of the shell in a radial pattern. When you see this in cross section, it looks exactly like the purported bite mark on this specimen.) You can get several types of bivalve molds: you can get internal molds where both shells were together, but if you only see one shell, you either typically get a
longitudinal section where you see it curve more and more strongly towards the hinge. Or, if you're looking at it and the shell is pointing directly out of the cliff/nodule, you'll get those ridges in cross section, and that's basically what we've got here. So, phosphate cement adhered all of this sandstone around the shell, and then the calcite of the shell was subsequently dissolved away because of the reducing environment that characterizes phosphogenesis.
The second claim is very absurd: the feature is supposed to be a dermal denticle that was
dissolved away, and it's an 'undetermined/unstated species of ray or skate' that this dermal denticle was supposed to come from. The problem is there's no batoid (ray or skate) that has a dermal denticle that's shaped like that or about that size. Instead, what we're seeing is a circumferential mold and then a series of radial lines that are also dissolved away, and this is very clearly a solitary coral. It's so distinctive, you can actually identify the genus of coral; it's called Balanophyllia, which is really common in phosphate nodules from the Charleston area. We have Balanophyllia stuck in phosphate nodules that have developed around dolphin skulls, right here in the [College of Charleston Natural History Museum (CCNHM)] collections. I, personally, have collected at least three or four concretions without specifically looking for Balanophyllia where this coral was present. One concretion I found had a vertebra of a ray right next to a Balanophyllia mold, from a river bank right here in Charleston. Lastly, what also doesn't make sense, dermal denticles are made of the same mineral as shark teeth, and actually, shark teeth have more enamel on them (which is more readily dissolved away) so the hypothesis that the dermal denticle dissolved away because of stomach acid but the tooth did not doesn't make sense. Also, that's not how poo works: stomach acid takes place in the stomach and not in the colon and rectum. In order for that hypothesis to work, this coprolite would have to be eaten twice.
Above: Balanophyllia, far left circle; bivalve cavities, right two circles. (ChM specimen, PV 55.105.25)
Above: Bryozoan mold. (ChM specimen.)
Above: Balanophyllia mold. (ChM specimen.)
Above: Bivalve mold with radial banding from hinge to outer edge. (ChM specimen.)
Above: Bivalve mold with longitudinal banding. (ChM specimen.)
CFA: Most of the SC Lowcountry’s fossil remains are preserved through a process called “phosphogenesis.” Can you please describe this process?
MLG: Basically, leaching causes the biological materials to be pulled out of fossils as minerals replace them. We have a high number of phosphate deposits [in Charleston], so phosphate is replacing the biological material of the fossil. As this happens, these [fossils] are basically phosphate copies of what was originally there. So if you go chemically test some of these fossils, they're going to come back as phosphate. If you test phosphate, it's going to come back as phosphate. So, it's really hard to do chemical analysis of a coprolite, and to say, "Oh, it's got the materials I would expect in a coprolite." If you test phosphate it's going to come back the same way, because chemically they're the same. In fact, phosphate forming here was such a big part in the history of the mining industry in Charleston, we actually have records of a lot of these fossils that came out of the phosphate mines. You can kind of understand how those sediment layers yielding marine fossils (like a shark tooth, like a coprolite) paints the picture that there's a possibility that this is coprolite because it's phosphate, but you've actually got some circular reasoning going on. It's phosphate because it's in a layer where phosphate forms: these fossils have a similar composition as phosphate because they [mineralized] in layers of phosphate.
CFA: Dr. Boessenecker, your paper entitled “Comparative Taphonomy, Taphofacies, and Bonebeds of the Mio-Pliocene Purisima Formation, Central California: Strong Physical Control on Marine Vertebrate Preservation in Shallow Marine Settings” focused on phosphogenesis as a large part of the study. What can you say about this process and fossils collected from the Charleston embayment?
RWB: The broad overarching comment is basically that phosphogenesis is a really interesting process that deals with the alteration of poor water chemistry just below the sea floor and it's a process that eliminates calcium carbonate and preferentially concentrates phosphatic material. So, you'll often get phosphate nodules, true coprolites, bones, and teeth all in the same deposit. In some cases, phosphogenesis has made bone beds that are actually called coprolite beds where it's just a bone bed and 90% of the biogenic particles are all coprolites. There's examples from England that are like that, from the Triassic, I believe. In the Charleston area, we have the first ever economically significant phosphate deposits. They're not as extreme as phosphate deposits in Florida, but they were the first to be economically mined right after the Civil War.
CFA: Any closing remarks that you would like to add about coprolites, phosphate, or scientific processes?
MLG: [Phosphogenesis] is still a topic I'm reading up on, ... but as I've worked here and seen how common [phosphogenesis] is (as opposed to my work with silicate [fossils] in Gray, Tennessee), I'm not surprised the fossils take on that same chemical characteristic; I'd be more surprised if they didn't. So chemical analyses of things like coprolites is a little dubious. You need to be careful with that.
RWB: What you can say about phosphogenesis in the Charleston area is the rock layers here are thin, there are many time-rich horizons and bone beds and erosional surfaces throughout the record here, and those time-rich surfaces are usually mantled in phosphate nodules. Phosphate nodules record periods of time where there is very little sedimentation (and probably upwelling) so phosphorous is being brought up from the deep sea. Most importantly, unusual sub-seafloor water chemistry is present and the fact that we have extremely rich phosphate beds here in Charleston is the same reason why we have such a wonderful and extensive fossil record here. The fossil record of Charleston is unfathomably immense, and there's literally fossils coming out of the ground everywhere; you can't walk anywhere in the Lowcountry without tripping over fossils or phosphate nodules, and the two often go hand in hand.
CFA: Thank you, gentlemen!
Our deepest thanks for M. Gibson and R. Boessenecker's cooperation with being interviewed and sharing their time, intellect, and collections with us. The sharing of knowledge for scientific advancement is invaluable during this age of fast-paced "clickbait" articles, and for that sharing, we are grateful.
The following are a few scientific articles that cover topics such as phosphogenesis and coprolites in further detail for those whose appetites are only just wet after this introduction.
A. Allison, Peter & Pye, Kenneth. (1994). Early Diagenetic Mineralization and Fossil Preservation in Modern Carbonate Concretions. PALAIOS. 9. 561. 10.2307/3515128.
Baturin, G.N. & Dubinchuk, V.G. The composition of phosphatized bones in recent sediments. Lithology and Mineral Resources, 2003, Volume 38, Number 3, Page 265
Boessenecker RW, Perry FA, Schmitt JG (2014) Comparative Taphonomy, Taphofacies, and Bonebeds of the Mio-Pliocene Purisima Formation, Central California: Strong Physical Control on Marine Vertebrate Preservation in Shallow Marine Settings. PLoS ONE 9(3): e91419. https://doi.org/10.1371/journal.pone.0091419
Diedrich, Cajus & Felker, Horst. (2012). Middle Eocene shark coprolites from shallow marine and deltaic coasts of the pre-North Sea Basin in central Europe. 57. .
Eriksson, M.E., Lindgren, J., Chin, K. & Ma ̊nsby, U. 2011: Coprolite morphotypes from the Upper Cretaceous of Sweden: novel views on an ancient ecosystem and implications for coprolite taphonomy. Lethaia, Vol. 44, pp. 455–468.
Föllmi, Karl. (1996). The phosphorus cycle, phosphogenesis and marine phosphate-rich deposits. Earth-Science Reviews. 40. 55-124. 10.1016/0012-8252(95)00049-6.
Godfrey, S.J. & Billy T. Palmer (2015) Gar-Bitten Coprolite From South Carolina, USA, Ichnos: An International Journal for Plant and Animal Traces, 22:2, 103-108
Godfrey, S.J. & Smith, J.B. Naturwissenschaften (2010) Shark-bitten vertebrate coprolites from the Miocene of Maryland. The Science of Nature - Naturwissenschaften 97: 461. https://doi.org/10.1007/s00114-010-0659-x
Hunt, Adrian & Lucas, Spencer. (2012). Descriptive terminology of coprolites and Recent feces. New Mexico Museum of Natural History and Science. Bulletin 57. 153-160.
Hunt, Adrian & Lucas, Spencer. (2012). Classification of vertebrate coprolites and related trace fossils. New Mexico Museum of Natural History and Science. Bulletin 57. 137-146.
Lamboy, Michel & purnachandra Rao, Venigalla & Ahmed, Ezzat & Azzouzi, Nacer. (1994). Nanostructure and significance of fish coprolites in phosphorites. Marine Geology - MAR GEOLOGY. 120. 373-383. 10.1016/0025-3227(94)90068-X.