What Was Anomalocaris? The Cambrian Predator Assembled From Three Fossils
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For nearly a century the pieces of one animal sat in museum drawers under three different names. A segmented object was catalogued as a shrimp with its head missing. A circular fossil was filed as a jellyfish. A lumpy mass was recorded as a sea sponge. In 1985 a reexamination showed that all three were parts of a single creature, half a metre or more long, with a pair of grasping appendages under a pair of stalked eyes, and that it had been the largest hunter in the seas of its time.
Three animals that were one
The fossils come from the Burgess Shale of British Columbia, a deposit of extraordinarily fine preservation laid down about 508 million years ago, and from equivalent beds in China and Greenland. Joseph Whiteaves described the segmented piece in 1892 as the abdomen of a strange shrimp, from which the name meaning unlike other shrimps derives. Charles Walcott, who excavated the Burgess site from 1909, catalogued the circular fossil as a jellyfish and the lumpy one as a sponge. The three were found together often enough to be noticed, and were assumed to be an animal sitting on a sponge. Harry Whittington and Derek Briggs settled it in 1985, when a specimen was found with the shrimp-like pieces attached in pairs to the front of the lumpy body, and the jellyfish turned out to be the mouth, seen from below, a ring of thirty-two hard plates surrounding a central opening.
The animal itself
The reconstruction that emerged is unlike anything alive. The body is a flattened segmented trunk with a row of overlapping flexible flaps down each side, used to swim by passing a wave along them in the way a cuttlefish does, and a fan-shaped tail. At the front sit two large eyes on stalks, and beneath them a pair of jointed appendages covered in inward-pointing spines, used to grip prey and pass it back to the circular mouth. Most specimens are between thirty and sixty centimetres long, which sounds modest until it is compared with the rest of the fauna, since the typical Cambrian animal was a few centimetres and the largest thing in that world had never previously been anything like this size. One related species from Morocco, Aegirocassis, reached about two metres, and had converted the same body plan into a filter feeder.
The eyes
The most informative discovery came later. Isolated fossil eyes found in South Australia in 2011, and attributed to a member of this group, preserve the internal structure well enough to be counted, and the result reset the timeline for the evolution of vision:
- •Each eye contains more than sixteen thousand individual lenses, a figure exceeded among living arthropods only by dragonflies
- •That is far more than any other Cambrian animal is known to have had, including the trilobites
- •The eyes sit on stalks, giving a wide field of view, and are among the earliest complex eyes in the fossil record
- •Acute vision this early supports the argument that the arrival of eyes was itself a driver of the Cambrian diversification, since prey that can be seen must hide, armour itself or move faster
What it ate, and the argument about it
The obvious reading is a hunter of trilobites, and it was supported for years by fossil trilobites bearing distinctive W-shaped bites and by coprolites containing shattered trilobite shell. That interpretation has been substantially revised. The mouth ring, examined closely, does not close fully and its plates show no wear of the kind that crushing armour would leave, and a biomechanical study concluded that the jaws could not have processed a hard shell. The current view is that the great appendages were used on soft-bodied prey, gripping and passing it whole, and that the group diversified quickly into different feeding strategies, with some members filtering plankton with fine bristles on the same appendages that others used as spined graspers. Whatever it ate, it occupied the top of a food web for the first time in the history of animals, and the appearance of a large mobile predator is one of the defining events of the period.
Where it sits on the tree
Its relationships were argued over for decades, and the resolution is one of the better results in the study of early animals. It belongs to a group called the radiodonts, named for the circular tooth-bearing mouth, and the radiodonts sit near the base of the arthropods, the enormous group that contains insects, crustaceans, spiders and the extinct trilobites. They are not arthropods in the strict sense, lacking the hardened jointed legs, but they are on the line leading to them, and the flaps along the body may be an early version of the limb branch that became a gill in later forms. Fossils from China of related animals with both flaps and stubby legs have filled in the sequence, and the group now includes dozens of species from every continent, ranging in age across roughly forty million years of the Cambrian and into the Ordovician. What began as a shrimp abdomen, a jellyfish and a sponge has turned into one of the best-understood branches of the early animal tree, and a standing reminder of how much interpretation sits between a fossil and a reconstruction.
The takeaway
Three fossils catalogued separately as a headless shrimp, a jellyfish and a sponge were shown in 1985 to be the appendages, mouth and body of one animal, a swimming predator up to sixty centimetres long with stalked eyes carrying over sixteen thousand lenses and a pair of spined graspers. It was the largest hunter of its world, though later work suggests it took soft-bodied prey rather than crushing trilobites, and it belongs to the radiodonts, a group sitting just below the arthropods on the family tree.