Were Efficient Burrowers Responsible for the Disappearance of Ediacarans?

Enrico Savazzi
Hagelgränd 8, 75646 Uppsala, Sweden.
(email: enrico5@savazzi.net)

Number of Words: 474
Number of References: 1 
Number of Figures: 0 
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Running header text: Efficient Burrowers and the Ediacarans

A current explanation of the replacement of Ediacaran communities with “modern” ones is based on the evolution of several groups of invertebrates capable of efficient movement on and/or burrowing in soft sediments, which effectively caused the disappearance of microbial mats from most environments. Along similar lines, it has been proposed that the demise of several sessile, or sluggish crawling/burrowing invertebrates in the Lower to Middle Palaeozoic was caused by the evolution of more efficient burrowers (the “biological bulldozers” of Thayer 1979). Often, the taxa that lost out had body-plans that were inherently less suitable to mobile life styles.
Inferring the burrowing efficiency of lower Palaeozoic invertebrates is fraught with difficulties, principally because of the lack of closely related Recent forms. As an alternative approach, the presence of morphologic skeletal features that are specialised for a burrowing function can be used to infer efficient burrowing habits, even in the lack of close Recent analogues or relatives. 
Burrowing sculptures (i.e., external relief patterns that display a morphological and frictional asymmetry, thus facilitating penetration through the software and reducing backslippage) are such a feature. They are especially suitable for this task because they have been studied in a broad range of Recent and fossil invertebrates, are relatively well understood, and cannot easily be confused with characters providing unrelated functions. 
A partial survey of burrowing sculptures in Lower Palaeozoic invertebrates shows that lingulate brachiopods and trilobites evolved specialized burrowing sculptures already in the Cambrian. Bivalves and mitrates followed in the [?]Ordovician-Silurian, and other groups only after the end of the Palaeozoic. This pattern is further complicated by the fact that only a minority of lingulate brachiopods and part of the trilobites evolved burrowing sculptures. Lingulate brachiopods and mitrates probably became themselves victims of more efficient biological bulldozers afterwards, and its survivors afterwards inhabited marginal, low-energy environments and had no burrowing sculptures. It appears that the functional morphology of these characters in trilobites is not fully understood.
While the absence of burrowing sculptures does not equate with poor burrowing performance, their presence is a strong indicator of fast, efficient and/or frequent burrowing. This inference can be extended also to closely related taxa that lack burrowing sculptures but are otherwise similar in general shape. Additionally, the evolution of burrowing habits probably preceded that of burrowing sculptures.
Lingulate brachiopods and trilobites (and, to a lesser extent, mitrates) certainly are among the organisms responsible for a marked increase in bioturbation in the Lower Palaeozoic. However, the hiatus between the decline of microbial mat communities and the evolution of invertebrates with specialised burrowing sculptures suggests that the former event likely was initiated by earlier, less efficient burrowers lacking burrowing sculptures, and that the latter evolved subsequently, as the result of increased competition among actively moving infaunal organisms.

Reference
Thayer, C.W. 1979. Biological bulldozers and the evolution of marine benthic communities. Science, 203, 458-461.