Distribution and dispersal history of Eurypterida (Chelicerata)

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Abstract

The distribution and dispersal histories of Eurypterida are examined in a phylogenetic and palaeogeographic context. It is suggested that all the eurypterid clades originated and had most of their evolutionary history on the palaeocontinents of Laurentia, Baltica, Avalonia, the Rheno–Hercynian Terrane and some presence in Siberia. The Siluro–Devonian Stylonurina and the eurypterine superfamilies Eurypteroidea and Waeringopteroidea are confined in their entirety to these continents. The Late Palaeozoic records of Adelophthalmoidea and Hibbertopteroidea in China, South America and Africa can be explained by ‘rafting’ on the continental plates that amalgamated into the supercontinent Pangaea. However, two clades have distributions that suggest abilities to cross open stretches of ocean; the Silurian and Devonian pterygotoids and some Devonian adelophthalmoids. However, one Silurian mixopteroid and the basal swimming form Onychopterella outside the ‘core-area’ suggest the dispersal patterns are more complicated. The pterygotoids might be the only clade whose members frequently undertook oceanic excursions. Since almost all eurypterid evolution took place in Laurentia, Baltica and Avalonia (and some in the Early Devonian of the Rheno–Hercynian Terrane) prior to the Carboniferous, it is unlikely that many taxonomically rich pre-Carboniferous eurypterid faunas will be encountered outside of these palaeocontinents.

Introduction

The eurypterids, commonly referred to as sea scorpions, were the most diverse chelicerate Order in the Palaeozoic, with around 200 valid species. There are around 235 species listed in Table 1, Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, but not all of these are based on diagnostic material (pers. obs.), and revisions are still ongoing. The eurypterids were primarily aquatic animals, but a potential dual respiratory system has been described that might have made it possible for them to undertake short terrestrial excursions (Selden, 1985, Manning and Dunlop, 1995, Braddy, 2001). Amongst the eurypterids, we find the largest known arthropods; the pterygotids from the Early Devonian of Germany (Poschmann pers. comm., 2002), with estimated total lengths (excluding their long chelicerae) of 2.5 m. However, even many of the smaller forms might have been formidable predators in their time, and sizes approaching 100 cm were common in most clades. The earliest known eurypterid is from the early Late Ordovician (Sandbian — ca 460 million years ago (Ma)) of Wales (Brachyopterus stubblefieldi Størmer, 1951) and the last record is from the Late Permian (250 Ma) of Russia (Hibbertopterus permianus Ponomarenko, 1985), giving a total range of approximately 210 Ma. They are most diverse between the Middle Silurian and Early Devonian and have their absolute peak of diversity in the latest Silurian (Přídolí — ca 418.7 to 416.0 Ma). The eurypterids can broadly be divided into two groups (Fig. 1), the basal Stylonurina (which is here interpreted as monophyletic, but might in fact be paraphyletic when a broader range of taxa are considered) and the monophyletic Eurypterina. These two groups are differentiated most easily on the morphology of the posteriormost prosomal appendage; in Stylonurina, this is a long slender walking leg, lacking a modified spine (termed podomere 7a). In Eurypterina, this leg is usually broadened into a swimming paddle, but always has a podomere 7a (Tetlie and Cuggy, in press). The swimming forms dominate and following the definition of Eurypterina possessing a podomere 7a, they represent around 75% of the known species, while the walking forms represent the remaining 25%. In terms of specimen numbers, the swimming forms are even more dominant, representing somewhere between 95 and 99% of known specimens. However, the morphology of walking forms appears almost as diverse as in swimming forms, and it is likely that the fossil record of the swimming forms is vastly more complete than for the walking forms (see also Fig. 1), possibly because of varying habitat preferences. Both the oldest and the youngest eurypterids are walking forms, so they existed for the entire 210 Ma interval, while the swimming forms existed for around 185 Ma (455 to 270 Ma). Almost all eurypterids have been found in near-shore marine, estuarine, fluvial or lacustrine environments. The vast majority of eurypterid occurrences are from eastern North America and Europe, and whether this pattern represents the true distribution of eurypterids, or a research and collecting bias has been questioned (Plotnick, 1983, Plotnick, 1999). Scotland and eastern North America have the most diverse assemblages, both in terms of species numbers and morphology, followed by England/Wales, the Baltic region, western Germany and Siberia. The rest of the world has a very poor record of eurypterids.

Herein the eurypterid fossil record is interpreted from both a palaeogeographic and phylogenetic perspective, providing some evidence for origination and dispersal of different eurypterid clades. The results suggest that the observed fossil record with a vast eurypterid diversity in Laurentia and Baltica, at least partly represents a true signal and not primarily a collecting or research bias. It is concluded that almost all eurypterids were inhibited from crossing vast expanses of ocean and were limited to dispersal along coastlines, or the slow movements of the continents they ‘rafted’ on. The only clear exception to this seems to have been the pterygotoids, that apparently could cross open oceans, and are found throughout the world in the short time span of their existence (∼ 40 Ma).

Section snippets

Previous work

It has long been known that there is a prevalence of eurypterid fossils in certain parts of the world (e.g. Clarke and Ruedemann, 1912, Størmer, 1955). Other parts of the world have proved more or less devoid of eurypterids, especially so in pre-Carboniferous strata. Every time a eurypterid occurrence from outside North America and Europe has been described, the authors have usually pointed out the rarity of these occurrences (e.g. Kjellesvig-Waering, 1964, Kjellesvig-Waering, 1973, Waterston

Material and methods

Information about the phylogenetic relationships of most complete eurypterids is now available (Tetlie, 2004, Tetlie, 2006a, Tetlie, 2006b, Ciurca and Tetlie, 2007, Tetlie and Cuggy, in press, Tetlie and Poschmann, in review). Even many eurypterids whose phylogenetic position has not been precisely determined can be confidently placed in one of the major clades and this assignment is shown in Table 1, Table 2, Table 3, Table 4, Table 5, Table 6, and the few that cannot currently be placed are

Results

The results are discussed cladewise, but more than one clade is plotted on each palaeomap. Fig. 2, Fig. 3, Fig. 4, Fig. 5 demonstrate that Laurussian occurrences dominate; however the dominance of eurypterids in the Laurussian area is actually much greater than indicated by the figures. For instance, Silurian stylonurids from Scotland are only indicated once, but there are a total of nine taxa present (Table 1). Similar simplifications were made for many of the eurypterid core areas of

Discussion

Eurypterids are non-mineralized arthropods and their fossil record is therefore relatively incomplete (Tetlie and Cuggy, in press, Tetlie and Poschmann, in review). However, recent advances in resolving the phylogeny of the group have resulted in this first attempt to put together a spliced tree containing all the major clades of swimming eurypterids and some walking forms (Fig. 1). The distribution of eurypterids is here interpreted from both a phylogenetic and palaeogeographical perspective.

Acknowledgements

The palaeomaps were provided by Professor R.C. Blakey (Northern Arizona University). T. Hegna (Yale University), M. Poschmann (Dept. for Protection of Cultural Monuments of Rhineland–Palatinate, Mainz), V.P. Tollerton (New York State Museum) and P. Van Roy (Ghent University) provided valuable discussions that greatly improved this contribution. The work was supported by the Norwegian Research Council grant 166647/V30 into eurypterid and fossil scorpion phylogeny and palaeobiology. Two anonymous

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