Here's a very strange fossil from the Chengjiang Lagerstätte, an early Cambrian fossil bed from 525 million years ago. It's a collection of Waptia-like arthropods, nothing unusual there; these are ancient creatures that look rather like headless shrimp. What's weird about it is the way the individuals are locked together in a daisy chain, with the telson (tail piece) of each individual stuck into the carapace of the animal behind. It's not just a fluke, either — they have 22 fossil chains, and just one animal all by its lonesome.
(Click for larger image)
Waptia-like arthropod, Lower Cambrian, Haikou, Yunnan. (A) Individual with twisted abdomen, part of chain, Yunnan Key Laboratory for Palaeontology, YKLP 11020a. (B) Chain, about 20 individuals, various dorsoventral-lateral orientations, composite image (joined at cpt/p arrow), YKLP 11020a and YKLP 11020b. (C) Individual linked to carapace behind, lateral view, part of chain of nine individuals, YKLP 11021. (D) Isolated individual, subventral view, YKLP 11019. (E to G) Reconstruction shown in dorsal, ventral, and right lateral views, respectively. Scale bars in (A), (C), and (D) indicate 1 mm; in (B) and (E) to (G), 5 mm. b, s, and t indicate bent, stretched, and telescoped individuals, respectively; cpt, counterpart; f, facing direction; p, part; and tw, twisted.
They do not look like animals that were constrained in a burrow, or that were crawling over the surface. Rather, they had been swimming together in a chain at death, and the whole chain fell to the sea bed, bending and kinking but still remaining firmly locked together.
Why were they doing this? My first thought was of sex; everyone knows how dragonflies and damselflies lock together for mating, but of course that would predict pairs of individuals, not 20 at a time. It also reminded me of the Drosophila mutant fruitless, in which male flies court other male flies, and they spontaneously form conga lines in the culture bottles. That's also unlikely, since that kind of behavior doesn't lead to a consistent pattern of successful reproduction, but maybe if these animals were hermaphroditic, it might work. It's not a behavior that any modern arthropods show, however.
The authors consider the possibility it is a feeding strategy, but that's even worse: they're locked basically mouth to anus, which would mean the fellow at the end of the line gets a very unpleasant diet. They conclude that the most likely explanation is that this represents a migratory behavior, perhaps involved in daily vertical migration. It may have been that strings of these animals would link up and paddle together to move to new feeding sites, where they separated and dispersed until the time came to move elsewhere.
Hou X-G, Siveter DJ, Aldridge RJ, Siveter DJ (2008) Collective Behavior in an Early Cambrian Arthropod. Science 322(5899):224.