Musical accompaniment: Complete Works of Scott Joplin played by Richard Zimmerman.
This week I review 2 papers on fossil invertebrates.
Paul A. Selden, José A. Corronca, Mario A. Hünicken. 2005. The true identity of the supposed giant fossil spider Megarachne. Biology Letters 1:44-48.
In 1980, Argentine palaeontologist Mario Hünicken described Megarachne servinei, a Permo-Carboniferous1 fossil, as the largest spider that had ever lived. Its leg-span2 was estimated to be 50 cm. This paper, with Hünicken as a coauthor, revises the identity of this fossil as a “bizarre eurypterid”, a water scorpion (Eurypterida). The animal apparently lived in a lake environment.
With the disqualification of Megarachne, the largest spider title returns to the extant tarantula Theraposa leblondi, whose leg-span, according to the authors, is 30 cm. Water scorpions (along with giant earthworms) and the question of what evolutionary factors may limit the sizes of terrestrial invertebrates were the subjects of a previous post of mine.
1. I suppose this means it was from the boundary of the Permian and the Carboniferous, which would make it about 290 million years old. A geologic time scale is available here.
2. If my understanding is correct, there is apparently no standard way to measure spider leg-span; it can be measured from the tip of leg 1 to the tip of leg 4 diagonally or on the same side.
Di-Ying Huang, Jun-Yuan Chen, Jean Vannier, J. I. Saiz Salinas. 2004. Early Cambrian sipunculan worms from southwest China. Proceedings of the Royal Society: Biological Sciences 271:1671-1676.
Sipunculans (peanut worms) form a small phylum (Sipuncula) of non-segmented marine worms. According to this paper there are about 150 species of sipunculans, but according to the University of California Museum of Paleontology website, there are about 320 species.
This paper describes 3 fossil sipunculans of Lower Cambrian age (about 520 million years old). The fossils were identified as sipunculans based on their morphological similarities to extant sipunculans. But then, the authors say that the fossil forms have "striking similarities" to modern sipunculans, and conclude that “most typical features of extant sipunculans have undergone only limited changes since the Early Cambrian, thus indicating a possible evolutionary stasis over the past 520 Myr.” Isn't this a circular argument or am I missing something? The point is if there were Lower Cambrian sipunculans that were very different than extant species, would they be identified unequivocally as sipunculans?