Long Live the Anomalocaridids!

Describing Anomalocaris to someone who is unfamiliar with the fossil species is a difficult task. No living creature is quite like it. Sometimes this 505 million year old invertebrate has been called a ?killer shrimp?, but even that is not quite right. With a segmented body, eyes on stalks, and a pair of formidable, grasping appendages set on either side of a mouth that looked like a camera-shutter-of-doom, Anomalocaris was one of the weirdest and most wonderful creatures of the Cambrian seas.

But this image of Anomalocaris has only been around since 1985. Prior to that, various pieces of this invertebrate oddball were taken to be parts of sea cucumbers, arthropods, and jellyfish. Once a relatively complete image of the animal was assembled, though, paleontologists realized that, despite its name, Anomalocaris was not actually an anomaly. The creature was just one of a widespread and long-lived group of marine predators now grouped under the name anomalocaridids, and a discovery reported today in Nature indicates that they remained major players in the seas for about 30 million years longer than previously thought.

A large specimen of the Ordovician anomalocaridid preserved in a concretion. The animal is estimated to have been over three feet long in life. Photo credit: Peter Van Roy.

Along with other Cambrian weirdos, the anomalocaridids were thought to have disappeared by about 488 million years ago. That is the boundary line between the Cambrian and the next geologic period, the Ordovician, and it appeared to be marked by a mass extinction that swept away many of the lineages that cropped up during the Cambrian. But anomalocaridids and other creatures similar to those dug out of Canada?s famous Burgess Shale have now been found in the Early Ordovician rock of Morocco. The surprising persistence of the Cambrian types was reported one year ago in Nature, and the today?s paper adds a meter-long anomalocaridid into the mix.

I wrote a brief summary of the new find for the AAAS website ScienceNOW ? which can be seen here ? and, in preparing it, I contacted two paleontologists for their insights about the discovery. Derek Briggs, a Yale University scientist and one of the paper?s authors, and paleontologist Allison Daley of London?s Natural History Museum very kindly answered my questions about the as-yet-unnamed fossil, and their replies were so good that I could not just let them sit in my inbox. (Only a small fraction of what they wrote was included in the ScienceNOW summary.) I am posting their complete answers below in a sort of ?supplemental material? section to today?s article. I have lightly edited the responses to take care of a few typos, but otherwise I have left the responses intact. Some of the discussion gets a little technical, but, in my view, that?s all the more reason to share the comments.

Questions sent to Derek Briggs (Yale University and Peabody Museum of Natural History)

Brian Switek: What is the size range for these anomalocaridids?

Derek Briggs: Our specimens were complete when buried but the concretions do not capture the entire animal.? Concretions form around a buried carcass and often the periphery is not incorporated ? so here the front of the head and the grasping appendages are missing.? The two more complete concretions are about 90 cm and 30 cm long so we estimate that the full animal reached lengths of at least one meter.? We have fragments of smaller individuals in the shale that is more typical of the Fezouta deposits, which indicate examples less than 10 cm in length.

BS: The anatomy of anomalocaridid great appendages vary from species to species. How do the great appendages of the Ordovician animals compare to those of Anomalocaris?

DB: The appendages are most similar to those of Laggania and Hurdia from the Burgess Shale, and unlike those of Anomalocaris.

BS: Does the new species have a name yet? I didn?t see one in the paper but I wanted to make sure that I didn?t miss it.

DB: No we did not name the species.? We were keen to announce the discovery in terms of size and occurrence in much younger rocks.? We are confident that we will find more specimens that will allow a full description of the animals (one or more) in due course.

BS: Last fall there was a report that Anomalocaris would not have been capable of feeding on trilobites and was not a hard-object feeder. Is there any indication of what this new species would have been eating?

DB: We don?t have direct evidence of the preferred prey.? The report you mention is based on assumptions about structural strength and biomechanics and concludes that anomalocaridids could not have bitten the hard dorsal carapace of trilobites.? We have always argued that their preferred prey was soft-bodied? arthropods with unmineralized cuticle, worms and so forth. Why try to consume armored food if you have a choice?

BS: And, finally, what does the existence of this form in the Ordovician mean for the big picture of evolution during a time when life was still confined to the sea?

DB: The discovery of anomalocaridids in the Ordovician shows that they did not become extinct around 510 million years ago but persisted at least a further 30 million years.? More exploration in the right kind of settings to preserve soft-bodied fossils might well reveal more examples.? But clearly anomalocaridids persisted as important predators at later times. They played a role as marine communities became more complex and the number of animal genera in the seas increased nearly four times during the Great Ordovician Biodiversification Event

Questions sent to Allison Daley (Natural History Museum)

Brian Switek: Compared to other known anomalocaridids, how unusual is the giant Moroccan species?

Allison Daley: Although the giant size and younger age of the Moroccon anomalocaridid material is unusual, the morphology of the specimens is strikingly similar to anomalocaridids of the Cambrian. The partial body specimens are very similar to the body of Laggania, in that it has wide bands of blades covered the entire dorsal surface, with lateral lobes bearing strengthening rays extending laterally from the ventral surface. The scelerites found in excavation 2 are very similar to the H- and P-elements of Hurdia, although they exhibit an ornamentation of tubercles not observed in Hurdia specimens from the Cambrian. The raptorial appendages found at the slightly younger excavations looks very similar to the frontal appendages of Hurdia (especially figure S3cd) and possibly ?Laggania (especially figures 1I and S4f), although the terminal dorsal spines are a bit more robust than the Cambrian taxa. All in all, I would say the Moroccan material is not that unusual as compared to the Cambrian anomalocaridids, other than the large size.

BS: Other than the size of the new species, does anything else stand out about the giant form?

AD: As the authors state, the dorsal blades in the articulated specimens from excavation 1 are very prominent. These specimens add to a growing body of evidence (from comments made by Bergstr?m on Laggania and Opabinia in 1985 & 1986, and more recently from Hurdia and again from Opabinia my myself and co-authors) showing that the body trunks of most anomalocaridid taxa were covered in thin, elongated blades.?The clear dorsal-view specimens from Morocco confirm this morphology, which was first suggested by the Cambrian taxon Laggania. It is however different from the morphology?of?Hurdia, where the blades are divided along the dorsal mid-line axis.

Very few well-preserved dorsal-view specimens of Laggania have been found, so these Moroccan specimens in dorsal-orientation are interesting for showing more clearly the dorsal morphology of the anomalocaridids. Not only does it show the dorsal covering of the blades, as discussed above, but one of the specimens also preserves the posterior margin of the head. Based on what I can tell from the images, there seems to be a dorsal covering of carapace at least on the posterior region of the head. My co-authors and I suggested the presence of this head shield in Laggania (in the paper describing Hurdia), which is relevant because if a head shield was present in the anomalocaridids, it extends this euarthropod feature deep into the stem lineage of the arthropods. The Moroccan material seems to confirm the presence of this head shield.

The fragments of head shield carapace from Morocco (from excavation 2) are very similar to the H- and P-elements of Hurdia, but unusually they are covered in an ornamentation of tubercles, which is not seen in the Cambrian specimens. External tubercles are often thought to have a protective function, either as mechanical protection against attack or mechanical increase in the strength of carapaces (similar to reticulate patterns), or perhaps by allowing protective resemblance to the surroundings by providing an attachment point of foreign bodies for concealment and disguise (much like some modern crabs do today).

BS: How does the discovery of this giant form affect our understanding of what happened to the disparate array of creatures that first appeared during the Cambrian?

AD: In some ways, the discovery of these anomalocaridids in the Ordovician is not that surprising, considering the fact that we know that an animal with many anomalocaridid characteristics has been found in the Devonian [Schinderhannes bartelsi], so it is expected that some form of anomalocaridid existed in the Ordovician. That the specimens found in Morocco are so similar in morphology to Hurdia and Laggania from the Cambrian indicates that these taxa at least survived the end Cambrian extinction event that severely reduced numbers of trilobites, brachiopods and reefs, etc. and their giant size could in part be a response to the increasing ecological pressures imposed upon them by the Ordovician radiation, which saw the establishment of several large predators such as cephalopods and eurypterids. Such a large increase in body size is likely the result of complex ecological interactions that made being larger advantageous, perhaps for protection, dispersal, speed, ease in finding conspecifics, etc.

The discovery of the Orodovician Moroccan assemblage, containing a mix of Cambrian Burgess Shale type taxa and more typical Ordovician taxa, indicates that there was no sharp distinction between the Cambrian and Palaeozoic Evolutionary Faunas. The previous lack of soft-bodied taxa in the Ordovician reflects a closing of the taphonomic window that allowed their preservation in the Cambrian, as opposed to a real extinction event. Thus, the extinction event at the end of the Cambrian, which was thought to have wiped out all the Burgess Shale-type soft-bodies taxa, appears not to have had a dramatic effect on these marine communities, meaning that a re-assessment of the ecology of early Ordovician ecosystems in warranted. The large size of the anomalocaridids is just one example of the impact of the smearing of Cambrian taxa up into the Orodovician, and the interplay of Burgess Shale-type taxa existing concurrently with other more typical Palaeozoic taxa.

Top Image: A sculpted restoration of the Cambrian anomalocaridid Laggania, created by Esben Horn. Photo credit: Esben Horn.

Source: http://www.wired.com/wiredscience/2011/05/long-live-the-anomalocaridids/

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