Geographic Information Systems technology as a morphometric tool for quantifying morphological variation in an ammonoid clade

Abstract

Ammonoids are known for their intraspecific and interspecific morphological variation through ontogeny, particularly in shell shape and ornamentation. Many features covary and individual elements are difficult to homologize, which make qualitative descriptions and widely-used morphometric tools inappropriate for quantifying these complex morphologies. However, spatial analyses conducted in a geographic information systems (GIS) environment allow for quantification and visualization of global shell form. Here, we present a GIS-based methodology in which the variation of shell features is assessed to evaluate evolutionary patterns in an ammonoid clade from the Late Cretaceous Western Interior Seaway of North America. We investigate the ancestral and more variable scaphitid heteromorph ammonoid Hoploscaphites spedeni and its less variable descendant H. nebrascensis. We created three-dimensional (3D) digital models of the fossil shells' lateral surfaces using photogrammetric software and imported the reconstructions into a GIS environment. We used the number of discrete aspect patches and the surface-to-planimetric (3D to 2D) area ratios of the lateral surface as terrain roughness indices. These 3D spatial analyses demonstrate the overlapping morphological ranges of H. spedeni and H. nebrascensis in ornamentation despite differences in shell compression. Additionally, the target for evolutionary change in this clade resides in the macroconch body chamber, which becomes less variable through development from H. spedeni to subsequent H. nebrascensis in macroconchs; however, microconchs retain the ancestral variation through ontogeny. These geospatial analyses not only successfully quantified variation in complex morphologies, but also demonstrated the versatility of this method to address questions related to ontogeny and phylogeny.

Ammonoid Intraspecific Variability

“…Many authors have acknowledged high intraspecific variability in heteromorph ammonoids (e.g., Egojan 1969;Rawson 1975a, b;Dietl 1978;Ropolo 1995;Delanoy 1997;Wiedmann and Dieni 1968;Wiedmann 1969;Kennedy 1972;reviewed in Kakabadze 2004;De Baets et al 2013a, b;Bert 2013), maybe even more than in normally coiled planispiral ammonoids (Dietl 1978;Kakabadze 2004). However, besides studies on Scaphites (reviewed in Landman et al 2010;Knauss and Yacobucci 2014), which only uncoils at the end of ontogeny, intraspecific variation has been only rarely studied in numerically large samples and/or quantitatively in heteromorphs with openly coiled and/or trochospirally coiled whorls (Aguirre- Urreta and Riccardi 1988;Dietl 1978;Ropolo 1995;Delanoy 1997;De Baets et al 2013a;Bert 2013). This lack of research might be due to their fragmentary preservation (De Baets et al 2013a) and difficulties in quantifying some of their shell characters using traditional morphometrics and classical "Raupian" parameters (Tsujino et al 2003;Parent et al 2009Parent et al , 2011Bookstein and Ward 2013).…”

“…The few studies using a large amount of material and employing quantitative methods have particularly focused on Mesozoic coiled ammonoids showing extensive ranges of intraspecific variation (e.g., Reeside andCobban 1960, Dagys andWeitschat 1993b;Checa et al 1996;Morard and Guex 2003;Weitschat 2008;Monnet et al 2010), but a large range of intraspecific variation also seems to be present in at least some Paleozoic taxa (e.g., Klug and Korn 2007;De Baets et al 2013a) and Mesozoic heteromorph ammonoids (e.g., Bert 2013; Knauss and Yacobucci 2014). The presence of discrete morphologies within a species in the form of intraspecific sexual dimorphism and non-sexual polymorphism has been suggested based on polymodal distributions in shell shape, ornamentation and/or size in late and/or early ontogeny in contemporary specimens.…”

“…Various statistical methods are available and easily accessible through free software (PAST: Hammer et al 2001; The R Project for Statistical Computing: http://www.r-project.org/), which makes it possible to analyze interpopulation and intrapopulation variation within ammonoid species quantitatively. New approaches to quantifying morphological variation that are rooted in spatial statistics can also be applied to many ammonoid groups (Manship 2004(Manship , 2008Yacobucci and Manship 2011;Knauss and Yacobucci 2014). A better understanding of the types and drivers of intraspecific variation in ammonoids can only be achieved by quantitative analyses of numerically large samples from a wide variety of ages, paleoenvironments, geographic areas, taxa and shell morphologies.…”

Ammonoid Septa and Sutures

“…In recent years, a number of researchers have experimented with methods to quantitatively evaluate suture line morphology. They have employed GIS (Geographic Information System; Manship 2004;Yacobucci and Manship 2011;Knauss and Yacobucci 2014), which can help comparing sutures quantitatively. Eigenshape analysis was employed by Ubukata et al (2009) to quantitatively characterize ammonoid sutures.…”

Buckman’s Rules of Covariation

“…It has often been stated that heteromorph ammonoids have a higher degree of intraspecific variation than normally coiled ammonoids (e.g., Wiedmann 1969;Dietl 1978;Ropolo 1995;Kakabadze 2004 and references therein). The phenomenon has only rarely been quantified (Urreta and Riccardi 1988;Landman and Waage 1993b;Tsujino et al 2003;De Baets et al 2013;Knauss and Yacobucci 2014). This might be partially related to fragmentary preservation and problems of dealing with the unconventional shell morphology (e.g., classic Raup parameters cannot be used in some of these forms).…”