Journal of Paleontology, 91(5), 2017, p. 919–932 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2017.60
Comparing phylogenetics and linear morphometrics to solve the generic assignment of Parabolinella? triarthroides Harrington (Trilobita, Olenidae)
Daniela S. Monti, and Viviana A. Confalonieri
Instituto de Ecología, Genética y Evolución de Buenos Aires, CONICET-UBA, Departamento de Ecología Genética y Evolución, Facultad de ciencias Exactas y Naturales, UBA, Intendente Güiraldes 2160, Ciudad Universitaria, Buenos Aires, C1428EGA, Argentina 〈
danielamonti@ege.fcen.uba.ar〉
Abstract.—The use of different methodological approaches together with an exhaustive qualitative study has helped to recognize important morphological traits to distinguish species in a systematic and phylogenetic framework. Parabolinella triarthroides Harrington, 1938 was described based on two cranidia from the Quebrada de Coquena, Purmamarca, Jujuy province. The generic assignment of P. triarthroides has been questioned by a phylogenetic ana- lysis, which resolves this species as the sister group of Bienvillia Clark, 1924. To explore the generic assignment of this species, a revision of the type material, plus a morphometric analysis including specimens of Parabolinella Brøgger, 1882 and Bienvillia were performed. In addition, the original matrix used in the published phylogeny was reviewed and enlarged, including more species of Bienvillia. Continuous characters were coded in different ways in order to compare how they could affect the ordering of specimens and their phylogenetic relationships. Finally, both methodologies were compared, especially in regard to the behavior of the quantitative characters included in the ana- lyses. From the combined analyses, it is shown that similarities between the cranidium of P. triarthroides and all other Parabolinella species are true homologies instead of a by-product of evolutionary convergence. Therefore, P. triarthroides should be considered a member of this genus. Finally, this study demonstrates that the best strategy for solving systematic problems in groups where the morphological variation is the only source of information (i.e., fossil taxa without living representatives) is the implementation of an integrative approach, combining different methodological techniques and a good description of specimens.
Introduction
Morphometrics and systematic biology share a common interest in the analysis of morphology and the nature of morphological variation (MacLeod, 2002), although both fields have a history of complex dialogue, which derives, at least partially, from the underlying differences in their theoretical bases (Rae, 2002; Jensen, 2003; Clouse et al., 2011). The use of ratios (i.e., quotients between morphometric measurements) has become thoroughly ingrained into scientific methodology (Atchley et al., 1976), in part given their capability to present, in concise and easily interpretable ways, quantities that are otherwise non- comparable (Leirmann et al., 2004). Consequently, their use is extensive throughout taxonomic and systematic literature (Reyment et al., 1984; Winston, 1999; Schuh, 2000). Among these fields, the purpose behind the use of ratios is either to control the body size component in the observed variation of an anatomical feature (Atchley et al., 1976) or to represent shapes or proportions (Baur and Leuenberger, 2011) that are useful to identify and describe living organisms. Morphometrics can be an important tool in systematic studies given that it incorporates strong elements of quantification and formal hypothesis testing (MacLeod, 2002), and linear morphometrics have proven to be valuable tools for the resolution of taxonomic problems (e.g., Hughes, 1994; Żylińska et al., 2013). On the other hand, the use
of ratios as cladistic characters has raised an intense debate. Most of the criticisms are related to several undesirable statistical properties that violate the fundamental principle of character independence of cladistics studies. Furthermore, Mongiardino Koch et al. (2015) stated out that the often- arbitrary choice between which measurement is used as numerator and which as denominator affects the phylogenetic outcome of the analysis, so the use of log-transformed ratios would be a suitable solution. Nonetheless, many authors con- tinue to use ratios as continuous characters without any sort of modification. In addition, they have proven to be useful in fossil taxa, in which the morphological variation is the only source of information (e.g., Prevosti, 2010; Escapa and Catalano, 2013; Mannion et al., 2013; Monti and Confalonieri, 2014). Parabolinella triarthroides Harrington, 1938 (p. 194, pl. 7,
figs. 10, 11) was first described based on two cranidia from the Quebrada Coquena, Purmamarca, Jujuy Province. In its original description, Harrington (1938) mentioned a simple preoccipital furrow (S1) in the type specimens of P. thriarthroides, which is the only character that distinguishes this species from the rest of the species of Parabolinella Brogger, 1882 (Harrington and Leanza, 1957). The generic assignment of P. triarthroides has been questioned by a phylogenetic analysis, which resolved this species as the sister group of Bienvillia Clark, 1924 and recov- ered a bifurcate preoccipital furrow as the only nonhomoplasic
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