Functional morphology, ontogeny and evolution of mantis shrimp‐like predators in the Cambrian

Abstract

Abstract:We redescribe the morphology of Yohoia tenuis (Chelicerata sensu lato) from the Cambrian Burgess Shale Lagerstätte. The morphology of the most anterior, prominent, so-called great appendage changes throughout ontogeny. While its principal morphology remains unaltered, the length ratios of certain parts of the great appendage change significantly. Furthermore, it possesses a special jack-knifing mechanism, i.e. an elbow joint: the articulation between the distal one of the two peduncle elements and the most proximal of the four spine-bearing claw elements. This morphology might have enabled the animal to hunt like a modern spearer-type mantis shrimp, an analogy enhanced by the similarly large and protruding eyes. For comparison, details of specimens of selected other great-appendage arthropods from the Lower Cambrian Chengjiang Lagerstätte have been investigated using fluorescence microscopy. This revealed that the morphology of the great appendage of Y. tenuis is much like that of the Chengjiang species Fortiforceps foliosa and Jianfengia multisegmentalis. The morphology of the great appendage of the latter is even more similar to the morphology developed in early developmental stages of Y. tenuis, while the morphology of the great appendage of F. foliosa is more similar to that of later developmental stages of Y. tenuis. The arrangement of the elbow joint supports the view that the great appendage evolved into the chelicera of Chelicerata sensu stricto, as similar joints are found in various ingroup taxa such as Xiphosura, Opiliones or Palpigradi. With this, it also supports the interpretation of the great appendage to be homologous with the first appendage of other arthropods.

New records of Burgess Shale-type taxa from the middle Cambrian of Utah

“…Moreover, rather than forming a fringe they are almost completely covered by the tergopleurae. Thus, in the new species, the exopods are shorter than in Y. tenuis (see Haug et al, 2012, fig. 9).…”

“…Length of tergites 3-5 (l1 of Haug et al, 2012) is 4.2 mm, length of the head shield (l2 of Haug et al, 2012) is 4.6 mm. Plotting l1/l2 (Fig.…”

“…Anteriorly, however, the head region has rotated longitudinally. Previous reconstructions of Yohoia (Whittington, 1974;Haug et al, 2012) show the eyes to be near the top of the head shield, adjacent to the base of the lateral area of the head shield. In this specimen, the eye position is similarly adjacent to the base of the lateral area of the head shield, but the head shield extends further dorsally from the eye than would be the case if it were strictly a lateral compression.…”

“…Plotting l1/l2 (Fig. 2) on the graph of similar measurements for Y. tenuis (Haug et al, 2012, fig. 2) shows Y. utahana to lie well beyond the scatter of points, at more than twice the mean for both measurements of the type species.…”

“…Remarks.-Terminology follows Haug et al (2012). It is not straightforward differentiating morphology from abiogenic marks in the matrix, such as with a scattering of darker patches, particularly near the anterior of the specimen.…”