From near extinction to recovery: Late Triassic to Middle Jurassic ammonoid shell geometry

Abstract

. 2014: From near extinction to recovery: Late Triassic to Middle Jurassic ammonoid shell geometry. Lethaia, Vol. 47, pp. 337-351.The Triassic-Jurassic extinction resulted in the near demise of the ammonoids. Based on a survey of ammonoid expansion rates, coiling geometry and whorl shape, we use the Raup accretionary growth model to outline a universal morphospace for planispiral shell geometry. We explore the occupation of that planispiral morphospace in terms of both breadth and density of occupation in addition to separately reviewing the occurrence of heteromorphs. Four intervals are recognized: pre-extinction (Carnian to Rhaetian); aftermath (Hettangian); post-extinction (Sinemurian to Aalenian) and recovery (Bajocian to Callovian). The pre-extinction and recovery intervals show maximum disparity. The aftermath is marked by the disappearance of heteromorphs and a dramatic reduction in the range of planispiral morphologies to a core area of the morphospace. It is also characterized by an expansion into an evolute, slowly expanding part of the morphospace that was not occupied prior to the extinction and is soon abandoned during the post-extinction interval. Aftermath and post-extinction ammonoid data show a persistent negative correlation whereby rapid expansion rates are associated with narrow umbilical widths and often compressed whorls. The permanently occupied core area of planispiral morphospace represents generalist demersals whose shells were probably optimizing both hydrodynamic efficiency and shell stability. All other parts of the planispiral morphospace, and the pelagic modes of life the shells probably exploited, were gradually reoccupied during the post-extinction interval. Planispiral adaptation was by diffusion away from the morphospace core rather than by radical jumps. Recovery of disparity was not achieved until some 30 Myr after the extinction event.

Ichthyosaurs from the French Rhaetian indicate a severe turnover across the Triassic–Jurassic boundary

“…Hallam and Wignall 1997;Wignall and Bond 2008;Smith et al 2014). But other groups such as plesiosaurs and neoichthyosaurians radiated immediately after (Benson et al 2012;Fischer et al 2013), implying a Rhaetian or older divergence of lineages.…”

Ammonoids at the Triassic-Jurassic Transition: Pulling Back from the Edge of Extinction

“…During the Late Triassic, which, according to Gradstein et al (2012) had a duration of 35.7 million years, there existed in excess of 225 ammonoid genera whereas during the Early Jurassic, with a duration of 27.2 million years, there existed in excess of 325 genera. During the Middle Jurassic with a supposed duration of 10.6 million years there existed in excess of 325 genera (Smith et al 2014).…”

“…The advantage of this approach is that it is somewhat more objective than a taxonomic approach because it is not based on taxa definition and the interpretation of phylogenetic relationships. A recent study by Smith et al (2014) employed Raup's accretionary growth model (Raup 1966(Raup , 1967 to develop a simple morphospace concept describing planispiral shell geometry and whorl shape using three variables to define a universal space. The occupation of this space before and after the event offers insights into the dynamics of the near extinction and the evolutionary, functional and ecological constraints that might have influenced recovery.…”

“…c Schematic of ammonoid shell morphology in the W-U morphospace (modified from Raup 1967). Figure 1 and 2b of Smith et al (2014); Figure was originally modified from Raup (1966), following the terminology of Smith (1986) and Liang and Smith (1997) are plotted on the W-U graph and then contoured based on frequency of occurrence per unit area. Figure 18.3c provides illustrations of hypothetical shell geometries for several different W-U coordinates.…”

The Rhaetian vertebrates of Chipping Sodbury, South Gloucestershire, UK, a comparative study

“…This species is known from Triassic deposits in many European countries including Belgium (Duffin and Delsate, 1993;Duffin et al, 1983), France (Cuny et al, 2000) and Luxembourg (Godefroit et al, 1998) as well as the UK (Norden et al, 2015). Due to the curved, knife-shaped structure of the teeth, it is reasonable to suggest that R. minor may have fed on smaller osteichthyans and invertebrates known to have been found in the Rhaetian (Smith et al, 2014).…”