Paleontology: The scientific study of the fossil record.
Course introduction and basic issues
This sounds simple, but a wide range of practical sub-disciplines and scientific objectives are covered in this definition including:
- Biostratigraphy: the use of fossils in the identification and dating of sedimentary rock units.
- Paleoenvronmental reconstruction: the application of fossils to the study of the ancient environments in which they were deposited. (Crucial to practical endeavors like petroleum geology and the study of environmental change.)
- Systematics: the study of the evolutionary patterns, relationships, and classification of organisms.
- Biogeography: the study of the distribution of organisms in space.
Yet when the public thinks of paleontology, they typically envision yet another discipline:
Paleobiological Reconstruction: the hypothetical or speculative reconstruction of the life form and behavior of fossil organisms. This course is an investigation of the methods that paleobiologists use to develop and scientifically constrain these reconstructions.
It is no wonder that humans are fascinated by extinct animals. The past two decades have seen the discovery of such wonders as:
How do we attempt to understand these for what they are?
- Two carnivorous bipedal dinosaur species larger than T. rex
- Freshwater fish with wrists and ankles but no fingers or toes.
- Aquatic sloths
- Whales that could walk on land
- Pterosaurs with crests as long as their bodies
- The earliest known vertebrates
At a first-order approximation, paleobiologists use the comparative method - the comparison of extinct life forms to living ones to develop hypotheses of their life styles. Such comparisons take two kinds of similarity into account:
- Homology: Features are homologous in different organisms when they are derived from the same structure in in those organism's last common ancestor. E.G.: The humerus (upper arm bone) of a human, badger, bat, and blue whale are homologous because all are derived from the humerus of the last common ancestor of placental mammals.
- Analogy: Features are analogous when they perform similar functions, regardless of their derivations. Thus, the wing surfaces of bats, birds, and dragonflies are analogous even though they are derived from evolutionarily distinct structures made of different materials. Apparent analogies are particularly compelling when they occur in homologous structures.
Structures that appear functionally analogous are our starting point for the development of hypotheses about life-style. E.G. numerous anatomical characters demonstrate that the extant timber wolf
and the extinct native North American dire wolf
They differ mostly in that the dire wolf's skull and teeth were more robust - a little closer to what one sees in spotted hyaenas or other bone-crushing animals.
Thus, we speculate that the dire wolf was generally similar to the timber wolf, but better adapted to bone crushing. Perceiving such similarities, like the perception of any pattern in a set of data, is a creative act. Ironically, of all paleontological methods, this one probably has the greatest tendency to push the limits of the proper Scientific Method. Humans are pattern-matching organisms and never short on imagination. What happens when we try to interpret the fossils of an animal with no obvious modern analog?
A Cretaceous Period lambeosaurine ornithopod dinosaur. Like other ornithopods, it had a deep, laterally compressed torso and tail. The vertebral column of both was stiffened by ossified tendons. The comparison of the deep flat tail to those of swimming vertebrates, combined with the early 20th century conviction that large dinosaurs would have had trouble supporting their weight on land gave rise to reconstructions of Parasaurolophus as an aquatic creature. The idiosyncratic crest must have been a snorkel.
This was a falsifiable hypothesis. More thoughtful biomechanical analyses of the vertebral column showed that the trunk and tail were inflexible from side to side. Furthermore, the tail vertebrae of actual aquatic reptiles like crocodilians generally had long lateral extensions to give axial muscles better leverage.
||Worse, although the crest connected to the pharynx and nasal cavity though an elaborate system of passages, no specimen of the "snorkel" actually had a hole in the top to admit air. Eventually a revised interpretation of Parasaurolophus as a land animal emerged.
Still, its crest stimulates speculation, including.
Interesting ideas, all based on analogy with living creatures, chosen to make two points:
- Species and breeding status signal
- According to David Weishampel of Penn State, resonating chambers for vocalizations. Weishampel modelled these passages in PVC pipe, creating an instrument that produced sound in a low register similar to that of living elephants.
- According to creationist publicist Duane Gish, holding chambers for reactive chemicals that, when exhaled, would explode, giving rise to legends of fire-breathing leviathans. (Cf. bombardier beetle)
- Many ideas about the life habits of ancient creatures push or exceed the limits of falsifiability.
- Speculation can range from high to low degrees of plausibility.
BSCI392 has two purposes:
- To familiarize you with the varieties of data used by paleobiologists to infer the physical and behavioral characteristics of extinct animals.
- To teach you the concepts and methods we use to constrain those inferences and, wherever possible, express them as falsifiable hypotheses.
By the time we are done, you will not only have considerable knowledge of paleobiology, but will be able to make sense of the mix of data, falsifiable hypotheses, and pure speculation you encounter in the media; and speak about it with some real authority.
A starting point - The major constraints on the imagination:
- Modeling: Hypotheses of function based on analogy can be tested using physical or mathematical models. Indeed, models can provide the basis for new inferences.
- Geologic context: The rock record constrains hypotheses of life habit by indicating, absolutely, where the creature died, and usually the type of environment where it lived. Trace fossils may preserve a record of the organism's actual behavior in its environment.
- Ontogeny: Any reconstruction of an organism or its life habits must account for it at every life stage. An adult form that cannot be generated by developmental change is impossible.
- Phylogeny: Organisms are the product of evolution. Thus, knowledge of their position in the tree of life and the identity of their close relatives constrains our interpretation.
- Ecological context: Hypotheses of organismal life habit that don't fit coherently into the fossil community in which it is found are likely to be wrong.
The wonder is that, with a combination of background knowledge of living animals, imagination, honest skepticism, it is possible to develop reasonable hypotheses regarding even very enigmatic fossils.