REDUCTIO AD ABSURDUM: TESTING THE EVOLUTIONARY RELATIONSHIPS OF EDIACARAN AND PALEOZOIC PROBLEMATIC FOSSILS USING MOLECULAR DIVERGENCE DATES

Journal of Paleontology, Jan 2004 by Waggoner, Ben, Collins, Allen G

ABSTRACT-Many of the late Neoproterozoic "Ediacaran fossils" have been referred to the Cnidaria, often on the basis of vague or poorly known features. However, representatives of the living Chondrophorina (=Porpitidae, Hydrozoa), Pennatulacea (Anthozoa), and Coronatae and/or Stauromedusae (Scyphozoa) have all been identified in Ediacaran biotas, based on specific morphological features preserved in a number of specimens. These three cnidarian groups have plausible Paleozoic representatives as well, but many of their Paleozoic fossils are also somewhat problematic. We test these systematic hypotheses by using them to calibrate divergence dates across the Cnidaria, based on an extensive molecular phylogeny of extant cnidarians. In this reductio ad absurdum approach, if a calibration based on one interpretation of a problematic fossil yields a glaringly inconsistent age for a better-known clade, that interpretation is likely to be mistaken. We find that assuming the existence of Pennatulacea and Scyphozoa in the "Ediacara biota" places the root of the Cnidaria between 800 and 1,000 Ma, a figure which is, at least, not out of line with other molecular clock estimates. However, assuming the existence of the Chondrophorina in the Neoproterozoic, or anywhere in the Paleozoic, pushes the root of the Cnidaria back to between 1,500 and 2,000 Ga, which is considerably older than the oldest previous estimates for the origin of the Cnidaria. We suggest that the likeliest explanation is that chondrophorines were not present in the late Precambrian or Paleozoic. The Ediacaran and Paleozoic fossils previously interpreted as chondrophorines probably represent other taxa.

INTRODUCTION

THE LATE Precambrian and earliest Cambrian soft-bodied fossils of the "Ediacara biota" are well-known paleontological enigmas. Consisting of a variety of structural types ranging from disc-shaped "medusoids" to leaflike "fronds," they have been interpreted as everything from invertebrate animals in or near extant phyla (e.g., Glaessner, 1984; Fedonkin, 1990) to members of other kingdoms, such as protists or lichens (e.g., Zhuravlev, 1993; Retallack, 1994) to members of independently multicellular taxa, not directly related to anything alive (e.g., Seilacher, 1992, 1994). Probably no single explanation will fit all the fossils. While some Ediacaran forms are quite unlike any known animal, others are still most plausibly interpreted as animals in or near extant phyla. These include representatives of the Porifera (Gehling and Rigby, 1996), Mollusca (Fedonkin and Waggoner, 1997), and possibly Arthropoda (Waggoner, 1999) and Annelida or Pogonophora (Sokolov, 1968).

Most Ediacaran fossils have historically been interpreted as cnidarians. The "medusoids," which are the most abundant fossils at most Ediacaran sites, were generally described as jellyfish (e.g., Wade, 1972; Glaessner, 1984). A few "medusoids" show tetraradial symmetry, such as Conomedusites and Stauridinium (e.g., Fedonkin, 1990), which is typical for non-anthozoan cnidarians: the Medusozoa. However, many Ediacaran "medusoids," if not all, are now thought to be benthic (e.g., Bruton, 1991; Gehling, 1991; Jenkins, 1992; Gehling et al., 2000). Some "medusoids" could be cnidarian polyps, and others are probably holdfasts for "frondlike" organisms that could be colonial cnidarians, but working out just what each specific type of "medusoid" was is difficult at this time. Other "medusoids," such as Tribrachidium and Albumares, are triradially symmetrical fossils with relatively complex structure. These triradial fossils cannot easily be placed in any known metazoan group, although they may be of a cnidarian grade of complexity. However, most of the Ediacaran "medusoids" show no defined degree of symmetry. "Medusoid" taxonomy is currently highly problematic due to the extreme taphonomic variability and morphological simplicity of many of the fossils (Gehling el al., 2000).

However, there are three extant cnidarian groups which have been claimed, on well-defined anatomical grounds, to be represented in the Ediacara biotas. (Fig. 1) The first is the Chondrophorina or "sailors-by-the-wind," a subclade of the Hydrozoa, whose extant genera Porpita and Velella are usually classified together in the family Porpitidae. Chondrophorines are disc-shaped pelagic colonial organisms that secrete a gas-filled float composed of multiple concentric chambers. The Ediacaran genera Ovatoscutum, Chondroplon, Kaisalia, Kullingia, and Eoporpita have been referred to the Chondrophorina (Gehling, 1991; Jenkins, 1992). Some of these are poorly known, taxonomically suspect, and/or interpreted as non-chondrophorines (e.g., Hoffman, 1988; Jenkins, 1992; Jensen et al., 2001). However, the Ediacaran genus Ovatoscutum looks very similar to the float of the living chondrophorine Velella. The genus Eoporpita shows extensive tentacles radiating from a central boss, which in some specimens appears chambered.

The second extant clade that may be represented in Ediacaran assemblages is the Pennatulacea or the "sea pens," a clade of colonial anthozoans within the Octocorallia (Anthozoa). Pennatulaceans are composed of one large primary zooid which forms a central stalk or rachis, with multiple secondary feeding zooids branching from the primary zooid. In the most derived pennatulaceans, the secondary polyps are organized into leaflike structures branching from the rachis. Exemplified by living genera such as Ptilosarcus, Pteroeides, Virgularia and Pennatula, this arrangement produces the featherlike appearance that is "typical" of pennatulaceans; however, this shape actually is found in a minority of extant genera. In most pennatulaceans, the secondary polyps branch directly from the rachis, and the colony form may be club-shaped or fan-shaped (Williams, 1992, 1995). A number of frondlike Ediacaran fossils have been linked with the Pennatulacea, notably Charnia, Charniodiscus, Rangea, and Glaessneria; these have a leaflike shape with a stalk and round bulbous holdfast, and a few have structures interpreted as spicules, zooids, and polyp leaves (Ivantsov, 1996; Jenkins, 1985, 1996). These fossil taxa have been linked specifically with derived living pennatulaceans. Waggoner (1998) suggested a similarity between certain Ediacaran "fronds" and the extant pennatulacean Renilla, which has a broad frondlike expansion of the rachis with variable arrangements of the secondary polyps, but which lacks polyp leaves. The fossil Ausia from Namibia has been interpreted as similar to a more basal pennatulacean, resembling living pennatulaceans in the Veretillidae (Hahn and Pflug, 1985). Taken at face value, all these reports would suggest that the Pennatulacea was not only present in the Vendian, but had diversified into several extant subclades by the Vendian.