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Constructional morphology and morphodynamics

Constructional morphology is one of the seminal concepts of 20th century palaeobiology, and has inspired the publication of hundreds of scientific papers. In a low-key announcement of a research project at Tübingen University, Adolf Seilacher (1970) introduced constructional morphology as a practical framework for analysing the morphology of organisms. He proposed that this morphology results from the interplay of three aspects, or factors: phylogenetic tradition, construction, and function. That paper contains a diagram (often reproduced in subsequent literature) with the three aspects located at the vertices of a triangle. The triangle diagram is not meant to be used as a quantitative graph, but only as a visual metaphor of the triple-aspect approach. Knowledge of this framework was subsequently spread among American palaeontologists largely by David M Raup.

Phylogenetic tradition includes genes presumably shared by a taxon, albeit not necessarily expressed in all its species. This aspect can help to explain parallel evolution within a group. Construction deals with growth, morphogenesis and self-assembly of an organism. Function is seen as an adaptive strategy, a synergic set of functional characters. The three aspects are often treated as constraints that limit the evolutionary choices available to a taxon.

Other definitions of constructional morphology exist, for example a group of largely German palaeontologists subscribes to a somewhat different concept. The term is also used, in unrelated ways, in linguistics and geology. This discussion deals only with Seilacher’s definition.

Constructional morphology has been applied to fossil and extant species, albeit in the latter case largely by palaeontologists. Thus, these studies tend to concentrate on skeletal parts likely to be preserved in fossils, although the conceptual framework has no such limitation.

A common feature of papers in constructional morphology is attention to extreme adaptations and life histories, on the assumption (often justified by the results) that unusual sets of adaptations may be easier to interpret in evolutionary and functional terms. This attention to unusual case histories is sometimes dismissed as ‘storytelling’ by its critics.

Other common patterns in constructional morphology studies are the analysis of convergent evolution, particularly easy to recognize in extreme adaptations, and the comparative analysis of multiple taxonomic groups. I think these applications prove that constructional morphology can transcend storytelling and provide conclusions of general significance. For instance, these studies provide strong evidence that the number of possible adaptive strategies, or peaks in the adaptive landscape, is finite and small (see Adaptive landscape and genetic algorithms). Furthermore, some elements of constructional morphology have been applied in bionics.

It has been occasionally proposed to add numerous other aspects to Seilacher’s original framework. However, multiple aspects make the categorization of an organism’s features more difficult and subjective, without appearing to provide significant advantages. But Seilacher subsequently introduced biological morphodynamics as an apparent successor to constructional morphology, adding a fourth aspect — the environment immediately surrounding the organism — to the conceptual framework and, predictably, a tetrahedral diagram with the four aspects as vertices. In a private conversation (between 1995 and 2000), Seilacher explained morphodynamics as largely a way to avoid conflict with scientists keen to argue that their interpretation of constructional morphology was more correct. Nonetheless, I found morphodynamics fruitful, especially in sessile organisms permanently committed to their immediate environment.


References

Seilacher, A (1970). Arbeitskonzept zur Konstruktions-Morphologie. Lethaia, 3, 393–396, DOI 10.1111/j.1502-3931.1970.tb00830.x.

Seilacher, A and A Gishlick (2014). Morphodynamics. CRC Press. 551 p.

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