Do giant claws mean giant bodies? An alternative view on exaggerated scaling relationships.

With fascination we read a recent paper in Biology Letters, in which Braddy et al. (2008) report the find of a 46 cm long claw (chelicera) of the pterygote eurypterid Jaekelopterus rhenaniae from the Early Devonian Willwerath Lagerstätte of Germany, the largest claw ever found from arthropods. The authors conclude that this claw belonged to the largest arthropod fossil ever discovered and estimate the body length of the animal to be approximately 250 cm long based on extrapolations of scaling relationships of two other Silurian eurypterids (Acutiramus and Pterygotus). In the introduction, the authors state that the body size of eurypterids, of which only chelicerae or remnants thereof have been found, was estimated based on the relative proportions of body length and free ramus. The described fossil consists of ‘two disarticulated distal-most podomeres’, meaning that no other part of the body was discovered so far. The authors estimate the large size of the newly discovered J. rhenaniae, using the scaling relationships of smaller animals. With this they assume that all pterygotid eurypterids have the same isometric scaling relationship of their appendages, as Taylor (2001) demonstrated for chelipeds of extant decapods. But then the authors describe that ‘Compared with chelicerae of smaller Jaekelopterus specimens, the largest denticles, especially in the free ramus, demonstrate a positive allometric growth’. If the denticles show positive allometry (following the suggestion of Bush & Allman (2004), we will refer to this scaling relationship as hypermetry), then it is possible that not only the denticles but also the entire chelicerae had a hypermetric scaling relationship. In this context, it is striking that the largest isolated coxa discovered of the same species suggests a maximum body length of only 180 cm (Stormer 1936). The following two points should illustrate the caveats of estimating body size based on appendage size, assuming isometry without taking into account the quite common occurrence of exaggerated intraand interspecific scaling relationships in arthropod groups: