Download Cambrian Cannibals: Agnostid Trilobite Ethology and the Earliest

Cambrian Cannibals: Agnostid
Trilobite Ethology and the Earliest
Known Case of Arthropod
Cannibalism
Mark A. S. McMenamin
Department of Geology and Geography
Mount Holyoke College
2010
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
16%
84%
N=44; blue=juxtaposition; red=no juxtaposition
Proportion of multi-agnostid samples
showing evidence of cannibalism
42%
58%
red=no evidence; blue=evidence of cannibalism
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 with
data 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.
You are what you eat!
Photo Credit: Steve Jurvetson
Triops and agnostid compared
Photo Credit: USGS
Agnostus pisiformis: “possibly
raptorial antennae”—C.O.R.E.
Arthropod Cannibalism
• DiscoveryNews discussion of cannibal
agnostids; A. Horning comment.
• http://news.discovery.com/animals/earlyanimals-cannibals.html
Flip over all Burgess Shale agnostid
specimens in your teaching collection
MHC Sample 3020
Labelled 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.