A stroll in the forest of the fucoids: Status of Melatercichnus burkei Miller, 1991, the doctrine of ichnotaxonomic conservatism and the behavioral ecology of trace fossil variation

The ichnogenus Melatercichnus was described in 1991 from Cretaceous deep-ocean turbidte beds at Trinidad Bay, northern California. The type (and only) ichnospecies, Melatercichnus burkei, features all the primary characters (ichnotaxobases) of Phymatoderma, and is here reassigned to this ichnogenus. In so doing, Phymatoderma now consists of two palmate forms (P. penicillum and P. alcicorne), two digitate forms (P. melvillensis and P. granulata), and the reassigned ichnospecies (P. burkei) representing a kind of intermediate form. Reassignment follows the doctrine of ichnotaxonomic conservatism, by employing a limited number of primary characters and abandoning a superfluous ichnotaxon. All of the ichnospecies now included within Phymatoderma–as seems to be true of many of the so-called fucoids and certain other groups of trace fossils–are extremely variable in terms of morphology, although basic structure (in this case bunches of outward branching, unlined tunnels stuffed with fecal pellets) is consistent. This aspect of trace fossils needs more attention, both from an ichnotaxonomic and a biologic point of view. At the Trinidad Bay collecting site, the ‘perfect’ examples of P. burkei featuring all the characters of the ichnospecies grade into specimens that would be identified with P. granulata or possibly Alcyonidiopsis isp. if preserved in isolation or if the ‘perfect’ specimens had never been observed. These variants are difficult to accommodate in traditional ichnotaxonomy, pointing to a need for special categories in open nomenclature (e.g., identification of some of the ‘imperfect’ specimens as ‘Phymatoderma forma variabilis’ or possibly ‘Phymatoderma f. v. P. burkei–P. granulata’—to reflect the kind and degree of variation). In terms of behavioral ecology, P. burkei appears to be a fancy cesspit: the producer either fed at a different level and simultaneously deposited fecal pellets at depth (polychaete, echiuran?), or fed at or near the seafloor then dove into the turbidite sand blanket to discharge the pellets (sipunculan?). Variations in the structure could have resulted from the producer encountering different conditions in the substrate at different times and places (e.g., owing to dewatering), interruptions in the typical pattern caused by encounters with other organisms, limited time available to construct the usual pellet-filled structure, avoidance of other conspecific burrowers, or simply the inability of some individuals to execute precisely the construction plan.