A new Anomalocaris mystery

by Matthew Cobb

[Originally posted at z-letter.com]

Anomalocaris – literally “unusual shrimp” – was first identified in 1892 by Joseph Frederick Whiteaves from mid-Cambrian deposits in British Columbia. It looked pretty much like this fossil, and was thought to look something like the drawing below.

One of the many things that was odd about this “shrimp” is that it never seemed to have a body or a head. All they ever found was the “tail”. There are plenty of these fossils about, and you can pick them up on eBay for a few hundred dollars (not recommended unless you are certain of provenance, that appropriate permission has been obtained, etc).

As is now well known, in 1985 Harry Whittington in Cambridge and Derek Briggs solved the mystery of the missing head of Anomalocaris, and at the same time also clarified the nature of Laggania, which was thought to be a non-descript sponge, and Peytoia, which was seen as a pineapple ring-like jellyfish thing:

They all turned out to be part of the same animal – a vicious predator which is now thought to be related to the arthropods. It still bears the nameAnomalocaris, but what was thought to be the body of the “strange shrimp” is in fact a predatorial claw, while the “legs” are thorny projections. Peytoia is the mouth of Anomalocaris, while Laggania turned out to be its body:

This was part of the Whittington/Briggs/Conway Morris redescription of the Burgess Shale animals which led Stephen Jay Gould to write Wonderful Life, and which is still the source of much debate today. At up to 60 centimetres long,Anomalocaris, and related members of the “great appendage” group are thought to have been the top predators in the Cambrian seas, spearing passing prey with their raptorial claws.

Here’s a nice model of Anomalocaris, from the Manchester Museum:

Here’s a close-up (annoyingly, I couldn’t get to see its mouth) – you can see a Burgess Shale fossil of Waptia below it:

Indeed, these predators are now known to have extended their domination of the seas into the Devonian, as shown by this recently discovered fossil ofSchinderhannes bartelsi from 407 MY ago (Kühl et al. 2009):

Many reconstructions of great appendage predators – and in particular ofAnomalocaris – show them munching away on trilobites. This video from Phleschbubble is particularly striking (Anomalocaris turns up at around 50 seconds. NB the file is pretty large so may take some time to download)

And this great drawing by Sam Gon III shows:

“two Anomalocaris canadensis converging on an Olenoides trilobite. This doesn’t necessarily imply that they engaged in cooperative hunting. The second Anomalocaris could have merely been attracted to the commotion caused by the activities of the other. It would be interesting to consider what kinds of agonistic behaviors occurred between individuals, and whether they engaged in any specialized territorial or courtship behaviors.”

This video (sorry about the music!) not only shows one eating what looks like a trilobite, it also has a pair engaging in either mating or intra-sexual conflict. This is cool but, of course, entirely gratuitous! NB the “streamers” seen on these reconstructions are typical of both Anomalocaris saron and a related anomalocaridid, Amplectobelua symbrachiata:

So, if you believe the videos (and you shouldn’t), the main diet of anomalocaridids would appear to be trilobites. But for the last couple of years Professor “Whitey” Hagadorn of Amherst College has been arguing that the evidence just isn’t there. In 2009 he presented a paper to “Walcott 2009”, a conference to mark the centenary of the discovery of the Burgess Shale deposits, and then three weeks ago he presented more data to the Geological Society of America’s annual meeting.

In an interview with Wired magazine, Hagadorn points out that there’s no direct evidence (e.g. trilobite traces in anomalocaridid guts, or clear anomalocaridid coprolites containing trilobite bits for example), and, as he explains in his 2009 abstract, while the teeth in the mouthparts look pretty sharp (see the picture above):

“Anomalocaridid mouth plates and their tips are never broken, nor are tips worn.  If plates were hard, and were used to manipulate, puncture, crush, or masticate biomineralized prey, they would be expected to show evidence of abrasion or breakage.  Absence of this evidence is striking given the frequency (0.01-1%) of healed malformations in extant marine arthropods, most of which are due to prey manipulation or feeding.  Moreover, anomalocaridid plates and their biting tips are commonly wrinkled, exhibit preburial shearing and tearing, and mantle or are deformed by biomineralized fossils such as brachiopods, trilobites, and Scenella.  Plates are preserved as organic carbon and exhibit fracture patterns typical of desiccating arthropod cuticle.  Thus anomalocaridid plates, including their tips, were unmineralized and pliable in life.”

Furthermore, in his latest presentation, Hagadorn has made a 3-D reconstruction of the mouth of Anomalocaris and found that not only was the mouth soft, it also couldn’t completely close. So – says Whitey – Anomalocaris and its fellows could do no more than suck nastily on stuff.

I’m neither a paleontologist nor do I do biomechanics, so will find it hard to judge when the data are eventually published (it may be in review, though there’s no trace of it on his website). On the basis of Hagadorn’s talks, opinion for the moment seems to be divided as to whether he is right. That may change when his work is published.

However, let’s assume he is right, and anomalocaridids didn’t eat trilobites. That simply begs the question – who did eat them? Because one thing is certain – those trilobites did get munched by something. There are apparent coprolites (= fossilised turds) that contain trilobite bits, as seen in this picture by Vannier & Chen (2005):

Furthermore, there are plenty of trilobite fossils that have chunks taken out of them, as seen here in this reference to the work of Babcock & Robinson (1989 – taken from my next Evolution of Invertebrates lecture):

The final answer will come, I suppose, when someone finds a clear association between a predator and a consumed trilobite. Until then, studies like that of Whitey Hagardorn are the best we can do.

h/t: Ray Moscow

References and links:

Babcock, L.E. and Robinson, R.A. 1989. Preferences of Palaeozoic predators.Nature, 337, 695-696.

Kühl, G., Briggs D E. G. & Rust J (2009) A great-appendage arthropod with a radial mouth from the Lower Devonian Hunsrück Slate, Germany. Science323:771-773

Vannier J. & Chen J. (2005) Early Cambrian food chain: New evidence from fossil aggregates in the Maotianshan Shale Biota, SW China. Palaios 20:3-26.

Anomalocaris homepage by Sam Gon III – this contains loads of anatomical and taxonomic information, as well as references, links and reconstructions – a goldmine! This is contained within an excellent website devoted to their apparent prey: trilobites.info