Phylogenetic analysis of some basal early Cambrian trilobites, the biogeographic origins of the Eutrilobita, and the timing of the Cambrian radiation

Abstract

This paper presents a phylogenetic analysis of the “Fallotaspidoidea,” a determination of the biogeographic origins of the eutrilobites, and an evaluation of the timing of the Cambrian radiation based on biogeographic evidence. Phylogenetic analysis incorporated 29 exoskeletal characters and 16 ingroup taxa. In the single most parsimonious tree the genus Fallotaspidella Repina, 1961, is the sister taxon of the sutured members of the Redlichiina Richter, 1932. Phylogenetic analysis is also used to determine the evolutionary relationships of two new species of “fallotaspidoids” distributed in the White-Inyo Range of California that have been previously illustrated but not described. These species had been referred to Fallotaspis Hupé, 1953, and used to define the occurrence of the eponymous Fallotaspis Zone in southwestern Laurentia. However, these two new species need to be reassigned to Archaeaspis Repina in Khomentovskii and Repina, 1965. They are described as Archaeaspis nelsoni and A. macropleuron. Their phylogenetic status suggests that the Fallotaspis Zone in southwestern Laurentia is not exactly analogous to the Fallotaspis Zone in Morocco, where that division was originally defined. Thus, changes to the biostratigraphy of the Early Cambrian of southwestern Laurentia may be in order. Furthermore, specimens of a new species referable to Nevadia Walcott, 1910, are recognized in strata traditionally treated as within the Fallotaspis Zone, which is held to underlie the Nevadella Zone, suggesting further biostratigraphic complexity within the basal Lower Cambrian of southwestern Laurentia.Phylogenetic analyses of the Olenellina and Olenelloidea, along with the phylogenetic analysis presented here, are used to consider the biogeographic origins of the eutrilobites. The group appears to have originated in Siberia. Biogeographic patterns in trilobites, especially those relating to the split between the Olenellid and Redlichiid faunal provinces are important for determining the timing of the Cambrian radiation. Some authors have argued that there was a hidden radiation that significantly predated the Cambrian, whereas others have suggested that the radiation occurred right at the start of the Cambrian. The results from trilobite biogeography presented here support an early radiation. They are most compatible with the notion that there was a vicariance event relating to the origin of the redlichiinid trilobites, and thus the eponymous Redlichiid faunal province, from the “fallotaspidoids,” whose representatives were part of the Olenellid faunal province. This vicariance event, based on biogeographic patterns, is likely related to the breakup of Pannotia which occurred sometime between 600–550 Ma, suggesting that the initial episodes of trilobite cladogenesis occurred within that interval. As trilobites are relatively derived arthropods, this suggests that Númerous important episodes of metazoan cladogenesis precede both the earliest trilobitic part of the Early Cambrian, and indeed, even the Early Cambrian.

Paleobiogeography: The Relevance of Fossils to Biogeography

“…Although some (e.g., Upchurch et al 2002) have argued for the dominant role of vicariance in dinosaur paleobiogeography, there are significant concerns about the quality and quantity of the data they utilized (Sereno 1997). The recognition of congruent, temporally correlated range expansion in independent clades matches the phenomenon of geo-dispersal identified by Lieberman & Eldredge (1996) and Lieberman (1997Lieberman ( , 2000. Lieberman & Eldredge (1996) coined the term geo-dispersal to distinguish it from traditional dispersal [sensu (Humphries & Parenti 1986)] that involves individual species moving over geographic barriers.…”

“…However, because ecological biogeography is such a rich discipline (e.g., Brown & Maurer 1989, Rosenzweig 1995, Brown & Lomolino 1998, Maurer 1999, it is anticipated that ecological paleobiogeographic studies are likely to represent an area of future growth for the field (Lieberman 2000). Still, the contributions of phylogenetic paleobiogeographic studies have been substantial.…”

“…Understanding biogeographic patterns connecting extinct and modern lineages remains a major challenge (Lieberman 2000(Lieberman , 2002bMurphy et al 2001), yet important progress has been made. In particular, the analysis of ancient DNA and the refinement of approaches for dating divergence events using molecules enabled 58 LIEBERMAN several new approaches to paleobiogeography.…”

“…Not restricted to one research school or mode of thought, it is organized into two subdisciplines aimed at probing patterns in the genealogical and economic hierarchies: phylogenetic and ecological biogeography, respectively (Brooks & McLennan 1991, 2002Lieberman 2000Lieberman , 2003a. The analysis of biogeographic patterns in fossil taxa, paleobiogeography, is also an important area of research (Hallam 1994, Lieberman 2000, Brooks & McLennan 2002. There have been recent important gains and developments in paleobiogeography.…”

“…Using a similar approach, Lieberman (2002a;2003b,c) conducted a phylogenetic paleobiogeographic analysis of Early Cambrian trilobites. Again, as with the Middle Devonian, the vicariance tree was well resolved, suggesting an important earth history control of evolution, with the formation of geographic barriers owing to tectonic rifting and sea-level fall leading to speciation.…”