Large‐scale evolutionary trends of Acrochordiceratidae Arthaber, 1911 (Ammonoidea, Middle Triassic) and Cope’s rule

Abstract

Directed evolution of life through millions of years, such as increasing adult body size, is one of the most intriguing patterns displayed by fossil lineages. Processes and causes of such evolutionary trends are still poorly understood. Ammonoids (externally shelled marine cephalopods) are well known to have experienced repetitive morphological evolutionary trends of their adult size, shell geometry and ornamentation. This study analyses the evolutionary trends of the family Acrochordiceratidae Arthaber, 1911 from the Early to Middle Triassic (251-228 Ma). Exceptionally large and bedrock-controlled collections of this ammonoid family were obtained from strata of Anisian age (Middle Triassic) in north-west Nevada and north-east British Columbia. They enable quantitative and statistical analyses of its morphological evolutionary trends. This study demonstrates that the monophyletic clade Acrochordiceratidae underwent the classical evolute to involute evolutionary trend (i.e. increasing coiling of the shell), an increase in its shell adult size (conch diameter) and an increase in the indentation of its shell suture shape. These evolutionary trends are statistically robust and seem more or less gradual. Furthermore, they are nonrandom with the sustained shift in the mean, the minimum and the maximum of studied shell characters. These results can be classically interpreted as being constrained by the persistence and common selection pressure on this mostly anagenetic lineage characterized by relatively moderate evolutionary rates. Increasing involution of ammonites is traditionally interpreted by increasing adaptation mostly in terms of improved hydrodynamics. However, this trend in ammonoid geometry can also be explained as a case of Cope's rule (increasing adult body size) instead of functional explanation of coiling, because both shell diameter and shell involution are two possible paths for ammonoids to accommodate size increase.

Globacrochordicerasgen. nov. (Acrochordiceratidae, late Early Triassic) and its significance for stress-induced evolutionary jumps in ammonoid lineages (cephalopods)

“…an increase of shell involution), and (3) the clade furthermore experienced an increase of indentation of its suture line. Multiple ex- amples of similar morphological trends in ammonoid shells have already been illustrated and discussed (e.g., Haas 1942;Guex 1973Guex , 1992Guex , 2006Thierry 1982;Bayer & McGhee 1984;Kennedy & Wright 1985;Dommergues et al 1989;Dommergues 1990;Klug et al 2005;Monnet et al 2011Monnet et al , 2012De Baets et al 2012). The increasing body size of the studied lineage conforms with Cope's rule, known as the widespread tendency of animal groups to evolve toward larger body size (Newell 1949;Rensch 1960;Gould 1966).…”

“…The protracted changes in shell morphology of Acrochordiceratidae are robust and non-random (Monnet et al 2012) and can be interpreted at first sight as being constrained by the persistent, common selection pressure on this mostly anagenetic lineage with relatively moderate evolutionary rates during an ecologically stable period. As discussed by Monnet et al (2011), such trends in ammonoid shell (which acts as a buoyancy apparatus) could reflect that their morphology is mainly controlled by adaptive and constructional constraints.…”

“…Size increase is commonly interpreted in terms of adaptive advantages such as increased defence against predation, increased food competition, increased success in mating, increased individual longevity and/or better energy use, among others (Stanley 1973;Brown & Maurer 1986;Hone & Benton 2005). For ammonoids, several authors illustrated and discussed examples of increasing ammonoid shell size (Stanley 1973;Hallam 1975;Thierry 1982;Guex 2003;Dommergues et al 2002;Monnet et al 2011Monnet et al , 2012De Baets et al 2012).…”

“…Acrochordiceras covers a large morphological spectrum because of its long-term evolutionary trends during the middle Anisian (see Monnet et al 2010Monnet et al , 2012. It is a medium-to large-sized shell with moderately evolute to involute coiling, rather subquadrate, compressed to depressed whorl section, ceratitic to subammonitic suture line, and the presence of one row of tubercles.…”

“…Based on exceptionally large collections from North America, Monnet et al (2012) highlighted that the family evolved by displaying significant long-term mor-phological evolutionary trends during the Anisian (see discussion). Here we report on the oldest acrochordiceratids recently discovered from the Spathian of Nevada (USA) and of Guangxi (South China).…”