Cambrian Cannibals: Agnostid Trilobite Ethology and the Earliest

Known Case of Arthropod Cannibalism

Mark A. S. McMenaminDepartment of Geology and Geography

Mount Holyoke College2010

The Puzzling Agnostids

• Due to their small size, agnostid trilobites have defied attempts to properly interpret their:

• Affinities• Environmental preferences• Ethology• Feeding strategies or “eat-ology”

Peronopsis interstricta

• Middle Cambrian• Wheeler Formation• Millard County, Utah

Possible Evidence for Cannibalism

Bite marks to pygidial margin

Source: L. E. Babcock, 2003, in Kelley et al., ed., Predator-Prey Interactions in the Fossil Record

Agnostid damage: shredded thorax

N.B.: Trilobites are on the same bedding plane.

Captured small agnostid

Photo credit: Marian Rice

Damaged remains of smaller agnostid

Photo credit: Marian Rice

Seafloor “Snapshots”

Proportion of samples showing large and small agnostids juxtaposed

N=44; blue=juxtaposition; red=no juxtaposition

16%

84%

Proportion of multi-agnostid samples showing evidence of cannibalism

red=no evidence; blue=evidence of cannibalism

58%42%

How might a “seek and destroy” . . .

• cannibal predator be blind?• Hypothesis: agnostids used an alternate

sensory modality, such as chemotaxis (or, say, response to electrosensory stimuli), to locate their prey.

• Is there any way to test this?• Let’s take a second look at the “snapshots.”

Chemotaxis and a possible spiralling approach pattern

Low-Res Movie Simulation

The origins of cannibalism

Modified from J. Keith Rigby, 1978, Jour. Paleo. 52:1327 withdata from: D. Collins et al., 1983, Science 222:166, fig. 2.

Burgess Shale Stem-Group Priapulid Ottoia prolifica

Photo credit: Mark A. Wilson

Ottoia cannibalism

• Ottoia—Earliest known case of cannibalism, 505 my.

• The case for cannibalism here is fairly certain (as opposed to the alternative of scavenging dead priapulids by swallowing them whole), as cannibalism is common in modern priapulids.

• The Burgess Shale is slightly older than the Wheeler Shale; both are Middle Cambrian.

• No direct evidence yet to my knowledge for cannibalism in the Early Cambrian.

Early History of Cannibalism• Early cannibals are not necessarily associated

with vision-directed predation.• Ottoia and Peronopsis were both presumably

blind animals.• The earliest Cambrian ichnofossil Treptichnus

pedum may have been formed by a stem-group priapulid.

• The behavioral tools associated with macropredation may have been refined within a single species before being unleashed on the rest of the biosphere.

Triops longicaudatus

• Jessica McMenamin and our home school Triops experience.

Photo Credit: Steve Jurvetson

You are what you eat!

Triops and agnostid compared

Agnostus pisiformis: “possiblyraptorial antennae”—C.O.R.E.

Photo Credit: USGS

Arthropod Cannibalism

• DiscoveryNews discussion of cannibal agnostids; A. Horning comment.

• http://news.discovery.com/animals/early-animals-cannibals.html

Flip over all Burgess Shale agnostid specimens in your teaching collection

MHC Sample 3020Labelled Pagetia bootes (Walcott)

Unlabelled, new specimens of Ottoia on the reverse side!

Acknowledgments

• Thanks to: • Lee Bouse, Douglas Fleury, Jerry Marchand,

Jessica McMenamin, Steve Dunn, Marian Rice and Jacqueline Boisvert for assistance with various aspects of this research.