[This essay was originally posted on February 24, 2011.]
Compared to other creatures of the Cambrian seas, Synophalos xynos seems rather plain. It was not a living pincushion like Wiwaxia, its body did not resemble a walking cactus like Diania, and it wasn’t a five-eyed, schnozzle-faced enigma like Opabinia. Next to these fantastic forms, Synophalos looks like little more than a peeled shrimp, but one thing made it remarkable. Over 525 million years ago, groups of Synophalos chained themselves together.
When first described by paleontologists Xian-Guang Hou, Derek Siveter, Richard Aldridge, and David Siveter in 2008, very little was known about this creature. It didn’t even have a name. Drawing a comparison with one of the fossils from the famous Burgess Shale site in Canada, Hou and colleagues simply called it a “Waptia-like arthropod”, though this animal was about 20 million years older and was found in the fossil-rich beds of Chengjiang, China. What made the creature stand out was that all the specimens found – with the exception of one – formed long chains of two to twenty individuals, with the tail of each animal inserted into the carapace of the one behind it. These animals were not simply lined up. Attached to each other, they had been swimming in these chains while alive.
Nothing quite like these arthropod chains had been found before. Processions of spiny lobsters walk along the sea bottom today, and in 2009 Andrzej Radwanski and co-authors described an approximately 370 million year old queue of blind trilobites, but these later arthropods did not physically attach themselves to each other like the Cambrian creatures. Why the Chengjiang arthropods created this chains was a mystery. Hou and co-authors proposed that the arthropods might have linked together to migrate up or down in the water column, but the exact function of the chains was really anyone’s guess.
The same team gave the critters their name – Synophalos xynos, roughly meaning “those who travel together under the sea” – in a more comprehensive Palaeontology paper in 2009. As originally described, these animals were very similar to Waptia from the Burgess Shale and Chuandianella from the Chengjiang deposits – Synophalos had a front carapace followed by 6-7 segments and a forked tail. Despite these anatomical similarities, however, such chains of individuals had not previously been seen among these arthropods. Instead, the chains of Synophalos resembled the mating chains of some gastropods such as slipper shells and sea hares, as well the free-floating chains of strange, squishy creatures called salps.
Feeding, mating, defense, migration – it would seem that Synophalos could have formed chains for any of these reasons, but Hou and co-authors tried to rule out some of these possibilities. For one thing, Synophalos probably did not chain together for some kind of feeding benefit because the oral opening of each creature would have been covered by the tail of the animal in front of it. (Hence the chains were probably temporary.) Likewise, while reproduction might be a reason for the chains, no arthropod is known to mate in such a way. Given the range of behaviors among living arthropods, the authors suggest that the chains might have something to do with migration – perhaps as a way to “increase the survival potential of the individual by making the larger unit appear more daunting to predators” – but there is no definitive supporting evidence for this idea.
Whatever the reason why these prehistoric arthropods linked up, though, the Synophalos individuals were clearly locked together very tightly. Had they not been, their bodies would have uncoupled and scattered when they fell to the sea floor prior to burial. Only one single individual was found – all the rest of the specimens formed chains of two to twenty, and some chains had clearly twisted and looped around when they reached the bottom. They were locked together in death just as they were in life.
Top Image: A long chain of twenty Synophalos individuals, all facing the same direction. The tip of the loop hit the seafloor first and then the rest of the chain twisted around as it fell. From Hou et al., 2009.
Hou, X., Siveter, D., Aldridge, R., & Siveter, D. (2008). Collective Behavior in an Early Cambrian Arthropod Science, 322 (5899), 224-224 DOI: 10.1126/science.1162794
XIAN-GUANG, H., SIVETER, D., ALDRIDGE, R., & SIVETER, D. (2009). A NEW ARTHROPOD IN CHAIN-LIKE ASSOCIATIONS FROM THE CHENGJIANG LAGERSTÄTTE (LOWER CAMBRIAN), YUNNAN, CHINA Palaeontology, 52 (4), 951-961 DOI: 10.1111/j.1475-4983.2009.00889.x
ANDRZEJ RADWAŃSKI, ADRIAN KIN, AND URSZULA RADWAŃSKA (2009). Queues of blind phacopid trilobites Trimerocephalus: A case of frozen behaviour of Early Famennian age from the Holy Cross Mountains, Central Poland Acta Geologica Polonica, 59 (4), 459-481