Did shell-crushing predators drive the evolution of ammonoid septal shape?

Abstract

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.

A new index for quantifying the ornamentational complexity of animals with shells

“…Buckman's law of covariation, which discusses the relationship of ribbing and shell geometry of ammonites (Guex et al, 2003; Hammer & Bucher, 2005a; Monnet et al, 2015; Moulton et al, 2015; Westermann, 1966; Yacobucci, 2004), could also be quantified through OI. The link between the complexity of suture lines and ornaments was inferred by the Buckman's law (Johnson et al, 2021; Lemanis et al, 2016), could be quantitatively assessed by the OI.…”

Osteology and reassessment of Dineobellator notohesperus, a southern eudromaeosaur (Theropoda: Dromaeosauridae: Eudromaeosauria) from the latest Cretaceous of New Mexico

“…Additionally, some of this variation is particularly hard to definitively identify in the fossil record, especially sexual dimorphism (e.g., Mallon, 2017). Many studies have been conducted on morphological variation, particularly within fossil taxa (e.g., Arbour et al, 2016; Bell, 2011; Burns et al, 2015; Carter et al, 2021; Currie, 2003a, 2003b; Dalman et al, 2017, 2021; Dalman, Jasinski, & Lucas, 2022; Dalman, Lucas, et al, 2022; Delcourt & Iori, 2018; Dodson, 1976; Evans et al, 2013, 2014; Fabrezi et al, 2017; Gee & Jasinski, 2021; Grillo & Delcourt, 2017; Jasinski, 2011, 2013, 2015b, 2018; Jasinski et al, 2018, 2022; Jasinski & Moscato, 2014, 2017; Jasinski & Wallace, 2014, 2015; Ji et al, 2011; Johnson, 2020; Johnson et al, 2021; Lacovara et al, 2014; Lehman, 1987; 2001; Longrich, 2014; Lucas et al, 2011, 2016; Machado et al, 2013; Moscato & Jasinski, 2016; Osborn, 1923; Rivera‐Sylva et al, 2012; Rowe, Colbert, & Nations, 1981; Sampson et al, 2010; Sullivan et al, 2013; Sullivan & Jasinski, 2012; Sullivan, Jasinski, Guenther, & Lucas, 2011; Sullivan, Lucas, & Jasinski, 2011c, 2011d; Vamberger et al, 2020; Voris et al, 2019). While the only definitive specimen of Dineobellator notohesperus is the holotype (SMP VP‐2430), other specimens from the Naashoibito Member argue for the presence of more than one dromaeosaurid taxon in this stratigraphic unit.…”

“…Additionally, some of this variation is particularly hard to definitively identify in the fossil record, especially sexual dimorphism (e.g., Mallon, 2017). Many studies have been conducted on morphological variation, particularly within fossil taxa (e.g., Arbour et al, 2016;Bell, 2011;Burns et al, 2015;Carter et al, 2021;Currie, 2003aCurrie, , 2003bDalman et al, 2017Dalman et al, , 2021Delcourt & Iori, 2018;Dodson, 1976;Evans et al, 2013Evans et al, , 2014Fabrezi et al, 2017;Gee & Jasinski, 2021;Grillo & Delcourt, 2017;Jasinski, 2011Jasinski, , 2013Jasinski, , 2015bJasinski, , 2018Jasinski et al, 2018Jasinski et al, , 2022Jasinski & Moscato, 2014Jasinski & Wallace, 2014Ji et al, 2011;Johnson, 2020;Johnson et al, 2021;Lacovara et al, 2014;Lehman, 1987;2001;Longrich, 2014;Lucas et al, 2011Lucas et al, , 2016Machado et al, 2013;Moscato & Jasinski, 2016;Osborn, 1923;Rivera-Sylva et al, 2012;Rowe, Colbert, & Nations, 1981;Sampson et al, 2010;Sullivan et al, 2013;…”

Updating studies of past life and ancient ecologies using defossilized organismal proxies

“…Recent advances in the fabrication of physical models, most notably 3D printing, have created direct conversions of virtual models to physical ones (Long et al, 2006;Mcinroe et al, 2016;Talori et al, 2018;Johnson and Carter, 2019;Nyakatura et al, 2019;Talori et al, 2019;Ibrahim et al, 2020;Johnson et al, 2021). Now, in addition to performing experiments on casts or physical approximations of specimens (Chamberlain, 1976;Jacobs, 1992;Schulp, 2005;Whitenack and Herbert, 2015), more intricate morphologies can be created at high levels of detailed combinations for uses spanning robotic gait reconstruction (Nyakatura et al, 2019;Ibrahim et al, 2020), compression experiments (Johnson, 2020;Johnson et al, 2021), hydrostatic balancing (Peterman et al, 2020d) and dynamic fluid flow experiments . These experiments may not always reflect absolute physical properties or behaviors; however, they provide valuable insight from relative comparisons of results between different morphologies using materials with similar bulk properties (Johnson et al, 2021).…”

“…Now, in addition to performing experiments on casts or physical approximations of specimens (Chamberlain, 1976;Jacobs, 1992;Schulp, 2005;Whitenack and Herbert, 2015), more intricate morphologies can be created at high levels of detailed combinations for uses spanning robotic gait reconstruction (Nyakatura et al, 2019;Ibrahim et al, 2020), compression experiments (Johnson, 2020;Johnson et al, 2021), hydrostatic balancing (Peterman et al, 2020d) and dynamic fluid flow experiments . These experiments may not always reflect absolute physical properties or behaviors; however, they provide valuable insight from relative comparisons of results between different morphologies using materials with similar bulk properties (Johnson et al, 2021). Even the most fundamental syn vivo capabilities have not yet been determined for many extinct organisms, especially those lacking clear modern analogs.…”