Ancestral locomotor modes, placental mammals, and the origin of euprimates: Lessons from history

Abstract

The title of this chapter has a double entendre embedded in it. It is a truism that biological history, in addition to ongoing adaptive demands, is decisive in shaping properties of lineages. But it is also uncontestable that precedent notions, influential contributors, or specific papers, right or wrong, channel and continue to profoundly influence thinking on many issues in science. There is a difference, however, in these two processes of canalization. In the evolutionary dynamic, there is no right or wrong, and the inherited attributes are the initial and boundary conditions that define the avenues open for subsequent phylogenetic/adaptive change. These paths do not only constrain but facilitate as well. At any rate, whales are not fish, so history fundamentallymatters in all biological science. Genotype encoded factors which sum up history, beyond the maternal contribution in the egg, guide the change, adaptive or not, which is phylogeny. Such largely adaptive phylogeny is an ongoing probabilistic outcome of environmental demands, which determine the frequency of individuals that make it through the survival and reproductive bottlenecks of each generation. The necessity to consider this theoretical foundation should be, therefore, neither surprising nor burdensome for natural historians who study morphological attributes or behaviors. Sundry disciplines, in particular research from behavioral ecology, provide fundamentally important plausibility hypotheses for paleobiologists, who seek such questions as this conference set out to do. The task, however, to reconstruct adaptive phylogeny is within the realm of paleontologists and morphologists who must tie the fossil record, through a variety of procedures referred to as modeling (see Szalay and Sargis, 2001), to information and ideas from neontology. This is done by testing specific historical-narrative explanations (i.e., phylogeny or taxon hypotheses; see Figure 1 in Szalay, 2000) against various areas of information. Historical narratives of science are tested against evidence of all sorts (Bock, 1981) an activity not indulged in by Kipling. So contrary to Popperian thinking, much of science consists of historical-narrative explanations offered within the confines of law-like explanations, in juxtaposition to Cartmills (1990) opinion that only the law-like statements are scientific. Law-like statements must be part of the context within which the various topics of becoming are explained (Bock, 1981; Szalay and Bock, 1991). But nomological-deductive explanations (law-like statements) alone, obviously, do not suffice in any science where history played a role. It is all those specific and contextual historical "mistakes" in the law-like workings of chemistry that result in consequences for replication, transcription, and translation of nucleic acids where the science of evolutionary change begins and couples with the vicissitudes of the environment. The aim of this chapter is relatively straightforward, but because of space constraints, it is more of a review of some literature debates and an outline of some issues related to the origin of both the Plesiadapiformes and Euprimates (perhaps best considered as sister orders at present) rather than detailed documentation. To achieve these goals I will (1) examine, in a historical framework, selected examples of hypotheses in which conceptual methods, as well as empirical emphasis or de-emphasis, have had a significant role in the construction of these hypotheses, as well as their consequences, for analyzing the phylogeny of adaptations for plesiadapiforms and euprimates; (2) present my views on modeling in paleobiology and the testing of homology hypotheses; (3) remark on the evidence related to locomotor strategies of Cretaceous and some recent therians that are relevant to the assessment of the ancestral pattern in the Eutheria, and of a clade within that group, the Placentalia; and (4) reassert the importance of the "morphotype locomotor mode" concept as a critical connection between phylogeny estimation and adaptational (functional, in a broad sense) assessment. As an example, I point to some evidence from hard anatomy for the ancestral euprimate locomotor mode. The latter was first referred to as "grasp-leaping" in Szalay and Delson (1979) and subsequently more fully developed in Szalay and Dagosto (1980, 1988). R. H. Crompton (1995) appears to strongly second this view. The Placentalia, diagnosed elsewhere based on tarsal attributes and four premolars, is the taxon that stems from (back in the Cretaceous) the last common ancestor of the Cenozoic and surviving eutherians. This is not just the living crown group because it includes now extinct orders as well. The corresponding stem group of the Eutheria from which the Placentalia arose is the paraphyletic Eoeutheria that diverged from Metatheria at least 125 MYA. © Springer Science+Business Media, LLC 2007.