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Discussion
Ongoing phylogenetic research
next sectionWe recently obtained some limited material for both Potiicoara brasiliensis and Spelaeogriphus lepidops, as well as specimens of the soon-to-be-described species from Australia, with the intentions of conducting the first SEM based comparative analyses of these species. Such studies should reveal further details about the anatomy of these species that will hopefully lead to greater taxonomic refinement for these taxa with some added boost for understanding their phylogenetic relationships. This study may uncover some information relevant to possibly establishing either a distinct spelaeogriphacean clade, or confirming the paraphyletic status of the Spelaeogriphacea suggested herein. This kind of research, as well as the anticipated description of new fossil and Recent taxa, will go a great distance in helping to resolve the taxonomy of these and other closely related peracaridan orders.
Unfortunately, several obstacles continue to stand in the way of the determination of a definitive phylogeny for this group. First, some of the available information is not particularly informative for an analysis of the Peracarida. As can be seen in the data matrix used for our analysis here (see Table 2), or by examining representative taxa from each of the respective orders and suborders discussed here (see Schram 1986), many of the "obvious" morphological characters used to define these taxa are shared with most if not all of the other peracaridan orders. For example, most members of these 5 orders possess elongate, sub-cylindrical bodies with a reduced carapace that covers only the anterior-most region of the thorax. Is this character "facies" merely plesiomorphic, or is it a convergent set of features that appears in connection with a particular set of habitats? At this point we do not know.
Second, many of the characteristic features of each species we can employ actually appear at this time to be autapomorphies and thus uninformative in a phylogenetic analysis, e.g., the bi-lobed pleopodal exopod of L. quadripartitus, or the papillated distal margin on the ocular lobe of S. lepidops. It is conceivable, however, that as more spelaeogriphaceans are discovered, some of these features may be shared with these yet to be found taxa. Nevertheless, their inclusion here for the time being remains problematic.
Third, several features commonly used in previous discussions of peracaridan phylogenetic relationships often cannot be determined from fossil material, e.g., optic anatomy, or information concerning blood and excretory systems (see Watling 1983). To what extent this kind of information future analyses of phylogeny can incorporate without compromising any phylogenetic signal with high levels of uncertainty remains unclear.
Finally, many of the known peracaridan taxa live in habitats that are not often preserved in the fossil record, such as the ground-water habitats discussed here, and the deep-water habitats in which many peracaridan taxa live, e.g., the bathynellaceans Bathynella baicalensis and B. magna, which live in Lake Baikal at depths of up to 1440 m., or the many hemicarideans known from the deep sea (e.g., see Schram 1986). As a result, the information available from the fossil record will always be incomplete, resulting in ambiguities in any data matrix constructed.
Due to factors such as these, problems will continue to occur with any attempt to resolve the phylogenetic relationships among all peracaridan taxa. Whether these problems will remain insurmountable remains to be seen.
Biogeography
The number of species of Spelaeogriphacea is unfortunately sparse. Despite this, their distribution suggests a potentially interesting set of biogeographical scenarios. The known recent forms are restricted to the southern hemisphere, suggesting their geographic pattern is a remnant of a Gondwanan distribution: P. brasiliensis found in Brazil, S. lepidops in South Africa, and the as-yet-unnamed spelaeogriphacean from Australia. However, each of the known fossil taxa is located in the northern hemisphere, with A. novascotica found in eastern Canada, L. quadripartitus in China, and the as-yet-unnamed Cretaceous form in Spain (Figure 6). Schram (1977, 1982) , with only one fossil species in the Carboniferous, suggested an early Laurentian origin with later Gondwanan ‘diversification.‘ This pattern of Laurentian origin with Gondwanan dispersal (most likely occurring at the time of the formation of the Pangean supercontinent in the Permian) seems reasonable and finds reflection perhaps in other malacostracan taxa. The new species from Australia would seem to support the prediction of Schram (1974) that with further research, living spelaeogriphaceans would be found "in the Gondwana areas."
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Fig. 6. World map showing distributions of Recent (indicated by ‘o’) and fossil (indicated by ‘x’) spelaeogriphacean taxa. |
Nevertheless, the new Cretaceous form from Spain, in combination with our Late Jurassic species from China might suggest an alternative scenario for the evolution of Spelaeogriphacea. Might we look to the bathynellacean syncarids for another model? The distributional pattern of all spelaeogriphaceans with their relatively late persistence in northern continents, could be one of world wide ubiquity. Under such a scenario we would be tempted to predict that future discoveries of spelaeogriphaceans will not be confined to Gondwana localities, but will conform to a distribution more akin to that shown by the bathynellaceans. These crustaceans are currently found in ground-water habitats world-wide, and are believed to have originated in the Laurentian from a primitive syncarid eumalacostracan in the late Palaeozoic. The bathynellaceans later dispersed throughout northern and southern regions with the formation of Pangaea in the Permian (Schram 1986 ). We believe that such a distribution pattern may eventually become evident for the Spelaeogriphacea, with expanded exploration of ground-water systems world-wide.
Paleoecology
The few spelaeogriphacean taxa now known provide some interesting information as to some possible paleoenvironmental trends. Both of the Recent forms, P. brasiliensis and S. lepidops, are found in cavernicolous freshwater systems in the southern hemisphere. A. novascotica, on the other hand, is interpreted as having lived in a near shore marine habitat on the east coast of what is now North America (Schram 1974). Whether this is exactly true or not is not clear. Certainly the Carboniferous deposits containing Acadiocaris are marine. However, these deposits are black shales, and this has been suggested by others to reflect relatively deep-water habitats (e.g., see Heckel & Batemann 1975, O‘Neil & Schram 1975). Nevertheless, a distinct transition in environmental preferences has taken place at some point during the history of this group. Sometime between the Carboniferous and the Late Jurassic, certainly by the time of L. quadripartitus, a shift from marine to freshwater lacustrine habitats appears to have occurred. This shift might have been concurrent with invasion of cavernicolouos and/or ground water habitats.