10.5061/DRYAD.PVMCVDNK6
Johnson, Erynn
0000-0002-1913-0492
University of Pennsylvania
DiMarco, Briana
Drexel University
Peterman, David
University of Utah
Carter, Aja
0000-0003-2285-8668
University of Pennsylvania
Allmon, Warren
Cornell University
Did shell-crushing predators drive the evolution of ammonoid septal shape?
Dryad
dataset
2021
ammonoid
Paleontology
Septa
Compression
National Science Foundation
https://ror.org/021nxhr62
DGE-1845298
2021-01-15T00:00:00Z
2021-01-15T00:00:00Z
en
3732782 bytes
2
CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
For centuries, paleontologists have sought functional explanations for the
uniquely complex internal walls (septa) of ammonoids, extinct shelled
cephalopods. Ammonoid septa developed increasingly complex fractal
margins, unlike any modern shell morphologies, throughout more than 300
million years of evolution. Some have suggested these morphologies
provided increased resistance to shell-crushing predators. We perform the
first physical compression experiments on model ammonoid septa using
controlled, theoretical morphologies generated by computer-aided design
and 3D printing. These biomechanical experiments reveal that increasing
complexity of septal margins does not increase compression resistance. Our
results raise the question of whether the evolution of septal shape may be
tied closely to the placement of the siphuncle foramen (anatomic septal
hole). Our tests demonstrate weakness in the centers of uniformly thick
septa, supporting work suggesting reinforcement by shell-thickening at the
center of septa. These experiments highlight the importance of 3D
reconstruction using idealized theoretical morphologies that permit the
testing of long-held hypotheses of functional evolutionary drivers by
recreating extinct morphologies once rendered physically untestable by the
fossil record.
Figure S1. Materials test of rigid resin. Load versus displacement curved
for models cured at 80°C for (A) 15 minutes, (B) 30 minutes, (C) 45
minutes, (D) 60 minutes, (E) 120 minutes, (F) 180 minutes. Figure S2.
Generalized depiction of the box method of fractal analysis modified from
Peterman and Barton 2019. For methodological details and original figure
see Peterman and Barton 2019. Figure S3. Septal shape and shell strength
with genera grouped by shape. Peak load sustained by each genus with (A)
ammonitic septa, (B) ceratitic septa, (C) goniatitic septa, (D) nautiloid
septa. Red triangles represent average peak load for each genus. Red
dotted line represents average peak load for septal shape type.