This only seems explainable in terms of the primacy of natural selection.
In general, functional morphology is the analysis of the mechanical and evolutionary relationship of anatomical form to organismal behavior and dynamics. As such the topic is of great interest to paleontology, holding out the hope that a sufficiently rigorous analysis of a fossil taxon's form might enable us to infer its behavior in detail.
Origins and biases: But why would we even expect there to be a connection between form and function? Our faith in this idea springs from three sources:
This only seems explainable in terms of the primacy of natural selection.
Moreover, When examined in detail, people calling themselves functional morphologists don't all do the same things. In fact, three broad categories of related but distinct research tend to get conflated under this term:
For more information on this, see Merck's lecture notes from BSCI392 on the biomechanics of:
Obviously, this only works directly on living organisms, however observations of them can indirectly illuminate interpretations of ancient ones.
As paleontologists, we are primarily concerned with the last two.
Theoretical vs Functional Morphology: The description, in abstract terms, of morphospace - the universe of possible organismal morphologies. Morphospace is, by necessity, an abstraction. Typically, the range of morphological variation in comparable organisms is reduced to two or three principle axes. these may be:
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Caveat:
The degree to which these favored measurements truly encompass morphological diversity is not knowable. Raup attempted to reduce coiled mollusk shell morphology to three parameters:
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Morphospace and adaptive landscapes:
Despite its limitations, Raup's initial work revealed a pattern that has been observed many times: The actual distribution of known organisms is often clustered within a specific region of theoretical morphospace. With Raup 1967, the distribution of known planispiral mollusks (ammonoids, primarily) clustered around two peaks (one evolute and one involute planispiral) and entirely avoided the region of morphospace in which successive whorls were not in physical contact. In contrast the spirula, with an internal shell sits well within this otherwise forbidden zone. Invokes two lines of thought:
(A Tom Lehrer song inspired by Wright's exploits.) |
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The Adaptationist programme: The adaptive landscape, and much of functional morphology, rests on the notion that adaptation under the influence of natural selection is the paramount determinant of morphology. For much of the 20th century, morphologists counted on this. Could they reasonably do so? Gould and Lewontine (1978) took aim at the notion, and the reflexive tendency of "pan-selectionist" researchers to offer adaptive secenarios for every aspect of morphology. They compared this to attempts to infer structural function to spandrels, sections of wall filling spaces between load-bearing components like arches and domes. (See the spandrels of San Marco right) In fact, spandrels are just space-fillers between functional elements. A biological comparison: the human chin - a functionless feature that is a developmental consequence of the reduction of the anterior tooth row.
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Non-adative influences on morphology include:
The result is that few structures are adaptively optimal, as in the classic panda's thumb. (Another structure constrained by evolutionary history.) |
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Testing functional hypotheses: So how can we test a functional hypothesis for a morphology? Prothero provides a consensus list of methods:
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The Comparative Method: 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 are closely related and generally similar. 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. Because we are dealing with an organism and with biomechanical functions that are well understood, the this contention can be evaluated with any of the methods listed above. |
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True biomechanical constraints: Perhaps the only absolute criterion for falsification of functional hypotheses. A hypothesis of function is definitively falsified if:
This allows us to make some inferences with confidence. Consider the maximum size of a flying bird: Selective pressure to develop maximum flight thrust is such that in most birds, the major flight muscles already occupy a maximum proportion of overall body mass, regardless of size. Thus, they cannot be scaled up allometrically. This places a limit on the overall size of an exclusively powered flyer of roughly 12 kg. - roughly the size of the largest powered flyers such as white pelican, kori bustard, mute swan. Any larger bird must employ some kind of soaring strategy, at least some of the time. We can be confident that a pterosaur larger than 12 kg. would also have to resort to soaring because of the same constraints. Indeed, mapping pterosaurs onto theoretical morphospace supports this scenario, provided our morphological assumptions are correct. |
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With less familiar creatures, or creatures that are less biomechanically constrained, we face greater difficulties. Consider the Permian reptile Eudibamus cursoris. Considered, largely on the basis of its small forelimb to hindlimb length ratio to be facultatively bipedal. (Indeed, bipedal lizards typically do show such a ratio.) Alas, so do other, ecologically dissimilar reptiles, including vertical clingers and leapers. |
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What happens when we try to interpret the fossils of an animal with no obvious modern analog?
A Cretaceous 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.
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Perceiving comparative 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. In the worst case, the result can be "Just so stories" - appealing speculation with no hope of being rigorously tested.
To Syllabus.
