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:
- Tradition: William Paley's Natural Theology, 1802, asserted the perfection of the design of creation and held it as evidence of an intelligent creator.
- Natural Selection: The discovery of evolution by natural selection held out a mechanistic alternative, the inexorable effects of natural selection operating over time on every part of the organism which, in principle, should drive it toward an optimum morphology for its environment.
- Observation: Quite often, we see organisms with disparate phylogenetic origins converge on the same morphology. E.G.: Sessile Benthic suspension feeders (right).
This only seems explainable in terms of the primacy of natural selection.
However, 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:
- Biomechanics: The in vivo study of the structural and dynamic attributes of living creatures, involving direct observations and measurements of living organisms:
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.
Parameters of gastropod coiling from Palaeos
- Theoretical morphology: The investigation of theoretical parameters of and constraints on the diversity of morphologies.
Comparison of bird and pterosaur flight adaptations from Witton and Habib, 2010
- Paleontological functional morphology: The application of known biomechanical constraints and the comparative method to extinct organisms.
As paleontologists, we are primarily concerned with the last two.
From Blackwell Publishing
- Measurements of significant anatomical features:
- David Raup, 1966 2D ammonoid curves
- David Raup's 1967 3D marginal accretion growth curves
- Gatesy and Middleton, 1997 on theropod hindlimbs.
- Composite measurements such as principle components derived from multivariate statistical analyses, E.G. Butler et. al, 2007 morphospace in Anolis lizards.
Murex pecten from Wikipedia
- D = distance of aperture from axis of coiling
- W = rate of expansion of aperture
- T = rate of translation of aperture along axis of coiling
Raup's mollusk adaptive landscape
- Functional explanations for the specific observed pattern (structural integrity, drag, prevent external shells from adopting this form whereas an internal shell is not so constrained.)
- In a general sense, we are reminded of the Adaptive Landscape metaphor of the geneticist and new synthesis protagonist Sewell Wright. In this metaphor:
- Species or alleles migrate under the influence of natural selection toward local optima in the adaptive landscape.
- Species at one local optimum would stay put unless:
- environmental conditions changed - i.e. the landscape changed shape
- some non-adaptive evolutionary process (genetic drift, pleiotropy, etc.) drove the species from its optimum.
(A Tom Lehrer song inspired by Wright's exploits.)
The spandrels of San Marco
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 (1979) 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 in classic Medieval churches. (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.
The panda's thumb from Athro Limited
- Structural constraints: By their basic anatomy and margin accretion growth strategy, adult mollusks must retain the skeleton they had as juveniles, even if they would be better off not having to carry it around.
- Evolutionary heritage: Organisms must work with what they inherit. Indeed the variety of ways in which they have addressed biomechanical problems - each in some way suboptimal but all good enough - testifies to that fact. E.G.: rapid swimming in vertebrates and cephalopods. Rapid undulations of the body is more energetically efficient than cephalopod "jet-propulsion," in which they must expend energy hauling their reaction mass (water) around inside their mantles before expelling it; yet there squids are, the victims of their heritage.
- Pleiotropy: Some features may be suboptimal because they are coded in the same gene that codes for a highly beneficial trait. Consider aggression and genitalia in hyenas.
- Structures may have more than one function: In that case, their form represents a compromise between adaptive trends for each function. Consider wydah tails. The male must use this structure both to attract a mate, and in flight whereas the female only needs to fly with hers.
- Limits to genetic diversity: Just because a given change is adaptive doesn't mean that a species has the genetic material with which to effect it.
The result is that few structures are adaptively optimal, as in the classic panda's thumb. (Another structure constrained by evolutionary history.)
Testing functional hypotheses:So how can we test a functional hypothesis for a morphology? Prothero provides a consensus list of methods:
- Define and diagnose the adaptation. This enables you to identify creatures that may have it
- Employ phylogenetic information to determine whether the "adaptive feature" is actually a derived evolutionary state.
- Consider whether the feature is a "spandrel" - an inevitable artifact of other features.
- In a paleontological setting, propose a hypothesis to explain the structure's function. (Once the above issues have been addressed.) This can then be tested using:
- The comparative method.
Quetzalcoatlus model from Wie Ornithopter Fliegen
- Physical models both simple and complex (Quetzalcoatlus ornithopter, right)
Finite element model of Smilodon bite force from McHenry et al., 2007.
- Mathematical and computer models (E.G. McHenry et al., 2007).
Egg-shape morphospace from Deeming and Ruta, 2014
- Mapping onto theoretical morphospace, with reference to the distribution there of well-understood organisms.
Panthera atox and Panthera leo from Cryptomundo
- The comparative method.
Canis dirus - Canis lupus comparison from Prehistoric Wildlife.com
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 (right) 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, this contention can be evaluated with any of the methods listed above.
Alas, so do other, ecologically dissimilar reptiles, including vertical clingers and leapers like Boyd's forest dragon. In this light, speculations on Eudibamus' locomotion seem reckless.
Parasaurolophus from It's Nature
Consider Parasaurolophus, 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 generating the energy to support their weight on land gave rise to reconstructions of Parasaurolophus as an aquatic creature. The idiosyncratic crest must, therefore, have been a snorkel.
This, at least, 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.
- Species and breeding status signal
- According to Weishampel, 1997 resonating chambers in the nasal passages facilitated 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)
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 ever being rigorously tested.
- It requires a biological material to exceed its known strength. Thus, Farlow et. al, 1995 demonstrated that an adult Tyrannosaurus rex could not withstand a fall from a standing position.
- Requires muscles to exceed the known limits of their energy output.
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 the maximum possible proportion of overall body mass, regardless of size. Thus, they cannot be scaled up allometrically. Huge birds that would require more muscle than this are forbidden. 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:Teratornis merriami must employ some kind of soaring strategy, at least some of the time. Thus, we are confident that a 40 kg. Pteranodon would also have to resort to soaring because of the same constraints.
Indeed, mapping pterosaurs onto theoretical morphospace (see Witton and Habib, 2010, for example) might support this scenario, provided our morphological assumptions are correct and in agreement.
- David S. Berman, Robert R. Reisz, Diane Scott, Amy C. Henrici, Stuart S. Sumida, Thomas Martens. 2000. Early Permian Bipedal Reptile. Science 290,(5493): 969-972.
- D. Charles Deeming and Marcello Ruta. 2014. Egg shape changes at the theropod-bird transition, and a morphometric study of amniote eggs. Royal Society Open Science 1(3): 140311.
- James O. Farlow, Matt B. Smith, and John M. Robinson. 1995. Body mass, bone "strength indicator," and cursorial potential of Tyrannosaurus rex. Journal of Vertebrate Paleontology 15(4): 713-725.
- Gatesy, S.M. and Middleton, K.M. 1997. Bipedalism, flight and the evolution of theropod locomotor diversity. Journal of Vertebrate Paleontology 17(2):308-329.
- S. J. Gould and R. C. Lewontin. 1979. The Spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist program. Proceedings of the Royal Society of London B 207:581-598.
- Colim McHenry, Stephen Wroe, Philip Clausen, and Eleanor Cunningham. 2007. Smilodon fatalis revealed by high-resolution 3D computer simulation. Proceedings of the National Academy of Sciences 104(41): 16010-5.
- Paley, William. 1802. Natural Theology; or, Evidences of the Existence and Attributes of the Deity. Collected from the Appearances of Nature. Darwin Online
- David M. Raup. 1966. Geometric Analysis of Shell Coiling: Coiling in Ammonoids. Journal of Paleontology Vol. 41(1): 43-65.
- Blaire Van Valkenburgh. 1988. Trophic diversity iin past and present guilds of large predatory mammals. Paleobiology 14(2): 155-173.
- David B. Weishampel. 1997. Dinosaurian cacophony - Inferring function in extinct organisms. BioScience 47(3): 150-159.
- Mark Witton and Michael Habib. 2010. On the Size and Flight Diversity of Giant Pterosaurs, the Use of Birds as Pterosaur Analogues and Comments on Pterosaur Flightlessness. PlosOne, November 2010.