Tuesday, 9 June 2015

Predation traces in the fossil record

Traditional palaeontology as we think of it consists of finding bones, shells, etc., and describing them and mounting the skeletons in a museum to look at. However, most of the actual science is looking at things like behaviour. From looking at bones, how can we infer the animal’s behaviour? And more than that, how can we figure out things like predation and interactions between different animals?

Of course, your average bone isn’t going to tell you this kind of information, but bones with bite traces can start to give us these hints. Bite traces on bones can tell us that the animal was attacked, and in what way. If the bone shows evidence of healing, then the animal was obviously attacked while it was still alive and survived the attack. However, if there is no evidence of healing (and this is substantially more common), then the animal was dead. Whether or not it was a fatal blow where the animal was attacked and died, or whether it was dead for some time before being chewed on can be harder to tell.

Bite traces on Late Cretaceous dinosaur bones showing
serrated marks. From Jacobsen and Bromley (2009).
Majungatholus tooth showing denticles and bite traces
showing denticle marks. From Rogers et al. (2003).
Different kinds of bites can leave different traces on the bone, as well as different kinds of teeth. Fine detailed analysis can help us understand exactly how these marks were made, and by what kind of animal. For example, teeth with denticles (small tooth-like projections) can often leave drag traces from the denticles on the bone after biting and dragging, which can only be made by denticles or serrated teeth. Many theropod dinosaurs have denticles, including tyrannosaurs, dromeosaurs, troodontids, etc. Conversely, many crocodilians do not have denticles or serrated tooth, but rather have a simple cone-shaped tooth, so the lack of serration traces can suggest this kind of predator (but does not necessarily mean that). Additionally, different bite traces can indicate different behaviours such as gnawing. Mammalian gnawing leaves very distinct traces on the bones that are not produced by other means. This has been seen in Late Cretaceous dinosaur bones that were gnawed on by multituberculate mammals. Bite traces can range from punctures (when the tooth breaks through the bone cortex) and pits (a single vertical bite with no cortical breakage), to scores and drags, caused when the animal bits and drags its teeth across the bone.
Multituberculate gnaw traces on several Late
Cretaceous bones. From Longrich and
Ryan (2010). 

Unfortunately, determining the exact predator can be extremely difficult, if not impossible in many cases. Generally we can narrow it down to “theropod”, “crocodile”, “mammal”, or other broad categories like that. If you’re in an area where there are very few theropod predators for example, than you can make a reasonable assumption that that is what caused it. Or if you have other evidence, like for example numerous shed teeth from a tyrannosaur like Albertosaurus, then it’s not unreasonable to assume that bite traces may be due to Albertosaurus.

However, there are cases where the predator can be identified. One example of where the predator is clear is a beautifully preserved azhdarchid pterosaur from Alberta. The animal consists of a partial skeleton (7 bones to be exact) with wing, leg, and vertebrae present. The coolest part of this is that one of the long bones has several bite traces on the shaft on one end (Currie and Jacobsen 1995). This alone would not be enough to identify the cultprit. However, conveniently, it also has a partial tooth still embedded in the bone. This tooth can be identified as a dromaeosaurid tooth. The only dromaeosaurid known from this time in this area of the world is Saurornitholestes, which is pretty well known from teeth and a few skeletal remains in Dinosaur Provincial Park, Alberta, where this pterosaur was found. It’s a pretty cool specimen, especially considering how rare pterosaur remains are in Alberta. To find one with bite traces and a tooth is pretty cool! Teeth are not infrequently embedded in bone, and this has happened in other pterosaur remains, as well as dinosaurs and many other extinct animals.

Azhdarchid pterosaur long bone with tooth embedded (right side, bottom of the bone). Image by Liz Martin. 
Close up of pterosaur bone with tooth emedded and bite traces visible. Image by Liz Martin
The nature of the bite can also tell us about the nature of the animal making the traces. Most bite traces found in the fossil record are typical of scavenging. They show no evidence of healing, and are often found in areas that wouldn't typically be covered in bites if it were something like live inter or intra-specific competition such as the ends of bones. However, there are also bite traces in the fossil record that show evidence of healing. A tyrannosaur (Daspletosaurus) shows evidence of several healed bites on its skull, leading the authors to believe this was some kind of intra-specific competition with other Daspletosaurus (Hone and Tanke 2015).

Examples of dermestid mandible marks on
Jurassic Camptosaurus bones. From Britt
et al. (2008).
Of course predation traces are not restricted to vertebrates. They are commonly found on things like ammonites, which were often predated on by mosasaurs in the Cretaceous oceans. And of course predation traces or scars are not limited to being caused by vertebrates. Many invertebrates are capable of scarring bones and shells. Dermestid beetles are well known today for decomposing flesh and cleaning of skeletons, but they can also leave traces on the bones, and have been found in dinosaur fossils. Molluscs are known for using their "thorny tongue" or radula to scrape away shells in order to get inside the shell at the animal living inside. These bore-holes are common in modern shells and frequently seen in the fossil record as well. Sometimes these borings are stopped partway through the shell, and considered "unsuccessful", while they are often termed "successful" as the hole goes through the shell to the unsuspecting clam or oyster within.

In addition to predation traces, there are also several other kinds of marks that can be found on a specimen, including trample traces, transport marks (abrasion, etc.), and other kinds of breakage indicators. This leads to the field of taphonomy, which is basically everything that has happened to an animal from the time it dies to when it is discovered by a palaeontologist. These things tell us about the environment it lived in and aspects of its preservation, and is much to wide of a topic to discuss here. Maybe next time!

Determining the different marks or traces on fossil bones, where they came from, and what other animal may have caused them can be extremely difficult, despite the fact that these marks can be extremely common in the fossil record.

NOTE: Since posting this, Lothar Vallon has pointed out that there is a specific scientific definition for the use of marks vs. trace, in case anyone is wondering why I use trace in most places and mark in others. You can see his comment below!

Britt, BB, et al. 2008. A suite of dermestid beetle traces on dinosaur bone from the Upper Jurassic Morrison Formation, Wyoming, USA. Ichnos 15: 59-71.
Currie, PJ, and Jacobsen, AR. 1995. An azhdarchid pterosaur eaten by a velociraptorine theropod. Canadian Journal of Earth Sciences 32: 922-925.
Hone, DWE, and Tanke, DH. 2015. Pre- and postmortem tyrannosaurus bite marks on the remains of Daspletosaurus (Tyrannosaurinae: Theropoda) from Dinosaur Provincial Park, Alberta, Canada. PeerJ 3: e885.
Jacobsen, AR, and Bromley, RG. 2009. New ichnotaxa based on tooth impressions on dinosaur and whale bones. Geological Quarterly 53: 373-382.
Longrich, NR, and Ryan, MJ. 2010. Mammalian tooth marks on the bones of dinosaurs and other Late Cretaceous vertebrates. Palaeontology 53: 703-709.
Rogers, RR, et al. 2003. Cannibalism in the Madagascan dinosaur Majungatholus atopus. Nature 422: 515-518.