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.
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