Fossils are divided into two broad categories: body fossils and trace fossils. As I tell students in my intro to paleontology class, a body fossil is (unsurprisingly) part of the body of an organism. It can be a bone, a tooth, a shell, a chunk of wood or leaf (all parts of the “body” of the plant), or any other number of miscellaneous parts and pieces. A trace fossil, on the other hand, is something left behind by the behavior of the organism. It might be a footprint, or burrow, or egg, or coprolite (fossil poo).
When people think of paleontology, they usually think of body fossils–dinosaur skeletons, fossil insects in amber, or trilobites on a slab of rock. Body fossils usually are the most useful for figuring out which plants and animals lived at a particular site, because these remains are often easily identifiable to species. Trace fossils, on the other hand, can be quite enigmatic. They are usually assigned to ichnogenera and ichnospecies, the trace fossil parallels to genera and species. Along with ichnofamilies and ichnoorders and ichno-everything-else, this whole classification system of ichnotaxa is called ichnotaxonomy. Unfortunately, ichnotaxonomy isn’t as clear-cut as classifications for body fossils. For the latter, one species gets one name. A Tyrannosaurus is always a big, toothy dinosaur. By contrast, one ichnotaxon can be made by a number of different, distantly related organisms. For example, the ichnogenus Thalassinoides is a trace that can be made by sea anemones, acorn worms, fish, or even crustaceans! As a result, many paleontologists tend to poo-poo trace fossils as so ambiguous as to be useless for most purposes.
Thankfully (and perhaps unsurprisingly), trace fossils often provide unique information about the resident animals in long-gone environments, as well as the nature of these environments. Some tracks and burrows are so distinct that they can come from only one type of critter doing one type of behavior–and every once in awhile, it’s something pretty cool!
The Maevarano Formation of northwestern Madagascar has yielded a huge sample of animals from the end of the Age of Dinosaurs, at least 66 million years old. Lots of nifty dinosaurs are represented, including the toothy carnivore Majungasaurus and the long-necked sauropod Rapetosaurus. There are also giant frogs (the appropriately-named Beelzebufo) and bizarre plant-eating crocodiles (Simosuchus), among others. Combined with careful geological investigations, it’s clear that prehistoric Madagascar was a fairly nasty place. Harsh dry seasons gave way to heavy rains that rolled thick and muddy debris flows across the coastal plain, burying the bones of recent victims of the inhospitable climate. This was an adequate place to be a dinosaur or land-savvy crocodile, but not a good place for most kinds of fish. In fact, fish fossils are fairly rare compared to those of other animals (particularly in the well-studied region around the village of Berivotra). During my walkabouts as a member of field crews working in Berivotra, I found lots of turtles and dinosaurs, frequent crocodiles, and scarce frogs, but few fish.
One particular type of fish was conspicuously absent from Late Cretaceous Madagascar–the lungfish. These close relatives of tetrapods (the four-limbed vertebrates including amphibians, lizards, dinosaurs, and mammals) are well-adapted for harsh environments. Lungfish can eke out an existence in shallow, dirty, and oxygen-poor waters (a rudimentary air-breathing lung is a handy trait in this case!), and thus fearlessly tread where other fish simply cannot. Furthermore, lungfish have extremely hard and durable tooth-plates that are a common find in many places. In fact, one site in northwestern Madagascar (perhaps 20 million years older than the rocks of the Maevarano Formation that we’ve just been talking about) was termed “Lungfish Hill” for obvious reasons. As a young graduate student, I remember being slightly thrilled at having found lungfish teeth there, and then being annoyed by not finding much else. In any case, the ~85 million year old occurrence of lungfish, combined with prime lungfish habitat around Berivotra 66 million years ago, made the lack of lungfish in the Maevarano Formation pretty puzzling. Had something happened during that 20 million year stretch to make the lungfish run away?
Trace fossils to the rescue! It turns out that paleontologists (me included) had just been looking in the wrong place for the wrong type of fossil. Barely 14 kilometers away from the fossil-rich rocks of Berivotra, a new discovery expanded our vision of Madagascar’s Mesozoic.
All will be revealed in Part 2.
Fishing Without a Fossil (Part 1) by PLOS Blogs Network, unless otherwise expressly stated, is licensed under a Creative Commons Attribution 4.0 International License.