Mark A. S. McMenamin Department of Geology and Geography Mount Holyoke College 2010

1. 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

2. 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”

3. Peronopsis interstricta • Middle Cambrian • Wheeler Formation • Millard County, Utah

4. Possible Evidence for Cannibalism

5. Bite marks to pygidial margin Source: L. E. Babcock, 2003, in Kelley et al., ed., Predator-Prey Interactions in the Fossil Record

6. Agnostid damage: shredded thorax N.B.: Trilobites are on the same bedding plane.

7. Captured small agnostid Photo credit: Marian Rice

8. Damaged remains of smaller agnostid Photo credit: Marian Rice

9. Seafloor “Snapshots”

10. Proportion of samples showing large and small agnostids juxtaposed 16% 84% N=44; blue=juxtaposition; red=no juxtaposition

11. Proportion of multi-agnostid samples showing evidence of cannibalism 42% 58% red=no evidence; blue=evidence of cannibalism

12. 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.”

13. Chemotaxis and a possible spiralling approach pattern

14. Low-Res Movie Simulation

15. 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.

16. Burgess Shale Stem-Group Priapulid Ottoia prolifica Photo credit: Mark A. Wilson

17. 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.

18. 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.

19. Triops longicaudatus • Jessica McMenamin and our home school Triops experience. You are what you eat! Photo Credit: Steve Jurvetson

20. Triops and agnostid compared Agnostus pisiformis: “possibly raptorial antennae”—C.O.R.E. Photo Credit: USGS

21. Arthropod Cannibalism • DiscoveryNews discussion of cannibal agnostids; A. Horning comment. • http://news.discovery.com/animals/early-animals-cannibals.html

22. 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!

23. 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.