Richard Cowen

Algal Symbiosis and Its Recognition in the Fossil Record

Symbiosis describes an association between two organisms that is of benefit to both. The association is generally long-term rather than transitory, and one or both partners have structural and/or behavioral adaptations that foster the association. In extreme cases of symbiosis, the reproductive systems of the partners are linked, so that continuation of the symbiosis is almost automatic; in these cases there are strong analogies with the meiotic sexual cycle (Margulis, 1980; Knoll, this volume).

Stereoscopic vision in one eye; paleophysiology of the schizochroal eye of trilobites

The schizochroal eye of trilobites bears lenses of high optical quality. Cooperation between adjacent lenses, which must have had retinas rather than rhabdomeres, might have allowed the trilobite stereoscopic vision throughout its visual field. We propose a simple neurophysiological model for the schizochroal eye in which neural connections between lenses on vertical “visual strips” could have provided the integration necessary for stereoscopic vision. Simple neural signals could have coded information on the position, size, speed and nature of an object in the field of view. The model of stereoscopic vision is consistent with data on the shape of the eye, the arrangement of lenses on its surface, and with current views on the origin and early evolution of the schizochroal eye. It implies that the lens arrangement, neural network and shape of the eye surface were a coadapted morphological complex, and it is consistent with accepted ideas of the Phacopina as living in benthic habitats in the photic zone.

Analogies between the recent bivalve Tridacna and the fossil brachiopods lyttoniacea and richthofeniacea

Abstract The modern bivalve Tridacna has enormous size and very rapid growth; it has an unusual feeding mechanism and an unusual diet. It is confined to well-lit warm waters, i.e., shallow clear tropical seas. All these features are associated directly or indirectly with the presence of symbiotic Zooxanthellae within exposed tissues. The Upper Paleozoic brachiopods Lyttoniacea and Richthofeniacea probably had large areas of mantle tissue exposed to the external environment. They were fairly large, and can be interpreted as having had fairly rapid growth, abnormal feeding mechanisms and abnormal diets. They appear to be confined to environments which can be interpreted as shallow-water carbonate substrates. It is suggested that they also had symbiotic Zooxanthellae. The suggestion is testable because it imposes narrow limits on the environments in which these brachiopods should be found.

The Distribution of Punctae on the Brachiopod Shell

The model recently proposed by Kemežys for the distribution of punctae on the brachiopod shell is examined and found to be insufficient. An alternative model is proposed: punctae appear to conform to a hexagonal close-packing pattern on the shell surface. A hypothesis to account for this is introduced, and further implications are discussed.

RESPIRATION IN METAZOAN EVOLUTION

The delineation of almost all metazoan phyla took place in pre-Cambrian times, and there is practically no direct fossil evidence of the morphological trends or the evolutionary pathways involved in this major episode in the history of Earths biota. Models for rational sequences of events which could have produced the diverse array of metazoans now present on Earth can be tested only by assessing their compatibility with meager geological and paleontological data from late pre-Cambrian and early Cambrian rocks, and their compatibility with features of living inetazoan groups which are now removed at least 600 million years from their first divergence. It is hardly surprising that there are several competing models for early metazoan evolution, and it is obvious that there is a need for much more careful analysis in this area. Many current models of metazoan evolution derive the major invertebrate phyla from flatworms through annelid-like worms by the acquisition of a coelom. Clark has thoroughly discussed this question, and constructs the following adaptive pathway by which the process might have occurred. He argues (1964:206-214) that the first metazoans were probably small swimming or creeping organisms employing ciliary action for movement. An increasing exploitation of detritus as a food resource would have led to the differentiation of a mouth and gut; ciliary creeping on a soft substrate would have necessitated mucous secretion, and dorso-ventral flattening might have accompanied size increase among surface creepers. Essentially, a plexus of benthic flat-worms would have resulted from these selective pressures. Clark envisages that an adaptive radiation of flat-worms on the substrate would have generated in turn a selective advantage for any faunal member which could burrow into the substrate, primarily for escape from predators. Body-wall musculature alone would have allowed a rather inefficient burrowing mechanism, but the development of any internal fluid-filled cavity as a hydrostatic skeleton would have conferred a greatly improved capacity for burrowing by peristaltic movement. Clark (1964:214-219) stresses that there have been several different ways of developing such an internal fluid cavity, and that the coelom is mostly likely polyphyletic. Early coelomate worms would have been circular in shape, rather than flat, and they would have most likely engaged in intermittent burrowing near the surface. Clark suggests that they were probably relatively sedentary, living rather like echiuroids or sipunculids, with a variety of food-gathering devices. In association with the development of peristaltic body waves for locomotion, Clark points out that respiratory currents could have been generated by a minor modification of the same mechanism, generating water flow instead of body movement through the sediment. Considerable complexity and organization of muscular and nervous systems is implied in either of these activities. There have been at least two main lines of evolution from these early coelomate worms. The oligomerous phyla have remained sedentary, and are capable of weak burrowing, if they burrow at all. The metameric body architecture developed from an adaptive strategy favoring active burrowing, in which segmentation and the development of paired external appendages

History of Life

The origin of life Earths earliest life Sex and nuclei: eukaryotes The evolution of animals Life in a changing world Paleoclimate and evolution The early vertebrates Leaving the water Amphibians and reptiles Reptiles and thermoregulation The Triassic takeover Dinosaurs The evolution of flight The origin of mammals Marine reptiles Why flowers are beautiful The end of the dinosaurs Cenozoic guilds and trends Geography and evolution Primates Evolving toward humans Humans and the Ice Age