The degree of perceived taxonomic change in various lineages may be directly related to their general morphologic complexity: more complex forms appear to change more rapidly. “Rates of evolution” as customarily reported by paleontologists may therefore be a poor indication of evolutionary changes in the underlying genome. Two approaches were used to examine this problem. (1) We have estimated the degree of morphologic complexity by using the number of descriptive terms per genus, and per family, for 12 major groups of animals. Three general levels of complexity occur: (i) gastropods, bivalves and ectoprocts have relatively few terms; (ii) echinoids, foraminiferans, ostracodes, nautiloids, corals, trilobites, and brachiopods have an intermediate number of terms; (iii) mammals and ammonoids appear to have a relatively large number of terms. These 3 levels of complexity also increase in rate of taxonomic turnover; i.e., an increasing rate of evolution. (2) Using a cluster analysis based on morphologic similarity, we grouped 200 lineages of a computer-generated phylogenetic sequence according to 4 phenetic bases: 3, 5, 10 and 20 morphologic traits. Groups based on a few characters are longer lived and are commonly polyphyletic in comparison with groups based on many characters. In both the real world and the computer simulation, the bias of differential morphologic complexity may account for the observation that “only complicated animals evolve.” Most paleontologic studies of the “rate of evolution” may tell us more about morphologic complexity than about evolutionary rates of genomes.

Analysis of several thousand mammalian fossils from late Pleistocene sediments in California provide data on sampling in mammalian paleoecology. Recovery of bones and teeth from the screenwashed bulk sediment sample residue is considered nearly total. Neither surface collecting alone, nor small bulk samples provide satisfactory quantitative data on original community structure or postmortem alterations in community organization. Minimum sample size for the analysis of diversity is discussed. Diversity and size-trophic ratios of the total identifiable mammalian component of this fauna (N = 1222) are similar to those expected in living communities, and therefore suggest adequate sampling, and minimally biased samples.

The cranial structure of anteosaurid and many tapinocephalid dinocephalians became modified in a manner consistent with Geist's hypothesis that they used their heads for pushing and ramming during intraspecific combat. These modifications are most pronounced in certain tapinocephalids by the evolution of a strong dorsal head shield supported by a massive arch network suitable for receiving and supporting blows delivered to the dorsal surface of the head. The position of the occipital condyle reduced the torque created by such blows at the craniocervical joint. Evidence also indicates that the head was reoriented into a position suitable for butting. The cranial architecture displayed by dinocephalians suspected of headbutting differs from that of living mammalian rammers. The differences can be directly attributed to the modification, in the former, of a reptilian skull with its relatively unexpanded braincase into a ramming instrument.

Magadina cumingi (Davidson) lives in an environment of high current energy and mobile sediments using the pedicle as an elevating device. This previously undescribed substrate relationship is reflected in pedicle form, the muscular system, distribution of secondary thickening and characters of the beak. The cardinalia provide the evidence of key differences in the muscular system from an attached species Magellania australis (Quoy and Gaimard) and a free lying species with atrophied pedicle Neothyris lenticularis (Deshayes). The possession of an open foramen in articulate brachiopods is not synonymous with permanent attachment.

Amecystis Ulrich and Kirk is a rhombless rhombiferan cystoid exhibiting adaptation to an active mode of life in the bilaterally symmetrical, dorso-ventrally compressed theca and to an active mode of respiration suggested by the lack of pore rhombs and greatly enlarged periproct. Like other members of the Pleurocystitidae, Amecystis probably wriggled along the bottom, but may have adopted a temporarily sessile habit by pushing the distal stele into the substrate or by grasping. Evolution of the genus began in the Middle Ordovician, probably as a development from Pleurocystites. General trends in the stratigraphic succession of recognized morphospecies: A. raymondi Parsley, A. woodi n.sp., and A. laevis (Raymond) show a reduction in angularity of thecal outline and obsolescence of surface features on the thecal plates. The use of the periproct as a pumping organ in respiration is regarded as an advanced trend that was probably utilized by many rhomb-bearing cystoids as an accessory respiratory mode; alone, it proved to be an unsuccessful adaptation in Amecystis.

The extinction of endemic brachiopods in North America at the end of the Ordovician and recolonization by European species has been related to glacio-eustatic lowering of sea level which disrupted conditions in epicontinental seas. North American species may have been narrowly adapted to relatively stable conditions of broad, tropical shallow seas. European invaders may have been less specialized because they were adapted to conditions in both the open ocean and in narrow European epicontinental seas. Being less narrowly adapted, European species probably were better able to cope with changing environmental conditions than were North American species.

During the Lower and Middle Llandovery, shallow water, low diversity communities of Pentamerus Community depth were unstable and characterized by repeated extinctions and invasions. Following the crisis at the Ordovician-Silurian boundary 3 to 5 million years were needed to reestablish communities that were persistent in geologic time.

It has been claimed that articulate brachiopods are excluded from turbulent environments by the weakness of their pedicle attachment. The evidence does not support this conclusion. Experiments on the force required to remove brachiopods from their substrate show that removal force often equals that of bivalve mussels from turbulent environments having the same valve area. Intertidal brachiopods probably occupy cryptic habitats to avoid desiccation, not to avoid waves. Subtidal brachiopods are abundant in tidal currents of 4 knots (2 m/sec) where they are usually attached in exposed positions.

Although paleoecologists often assume that the size of the pedicle foramen is directly proportional to attachment strength, the relationship is not precise. Experimental detachment of brachiopods resulted from failure of the substrate, pedicle-substrate attachment, pedicle, or pedicle adjustor muscles, in approximately equal frequencies. At the same attachment strength, the foramen in Terebratalia is four times the size of the foramen in Hemithiris.

Although subapical foramina are supposedly unable to enlarge as the brachiopod grows, the subapical foramen of Hemithiris appears to do so.

The phenomenon of selective biotic extinction is commonly explained as the result of the extinction of specialized organisms and the survival of generalized organisms during a time of environmental stress. Using 48 orders of aquatic free-living arthropods, we test the hypothesis that generalized organisms tend to have longer geologic ranges than specialized organisms. The degree of specialization, as expressed through morphological complexity, is measured by the diversity of limb pairs on the arthropod body. The resulting “tagmosis value” reflects the functional differentiation of limb pairs to perform specific tasks and thus may estimate ecological specialization. No statistically significant correlation exists between an order's typical degree of morphological specialization and its geologic range. Degree of specialization is not an important determinant of evolutionary persistence at this taxonomic level. Evolutionary longevity may depend on the adaptive type (e.g., trophic role, relationship to substrate, etc.) represented by the taxon and on the assortment of these adaptive types.

As Van Valen has demonstrated, the taxonomic survivorship curve is a valuable means of investigating extinction rates in the fossil record. He suggested that within an adaptive zone, related taxa display stochastically constant and equal extinction rates. Such a condition is evidenced by straight survivorship curves for species and higher taxa. Van Valen's methods of survivorship analysis can be improved upon and several suggestions are presented. With proper manipulation of data, it is possible to pool the information from extinct and living taxa to produce a single survivorship curve and therefore a single estimate of extinction rate. If extinction rate is constant at the species level (producing a straight survivorship curve), higher taxa in the same group should be expected to have convex survivorship curves. The constancy of extinction rates (here termed Van Valen's Law) can and should be tested rigorously. Several methods are available, of which the Total Life method of Epstein is particularly effective.

Lysorophus, a serpentiform, nearly limbless, aquatic Paleozoic amphibian was described in detail by Sollas in 1920. Although Sollas did not discuss cranial kinesis, his original reconstructions suggest that Lysorophus had an unusually kinetic skull for an amphibian. We have reexamined the skull of Lysorophus and find that kinesis, if present, was slight. The maxillae, premaxillae, and vomers might have moved in limited protraction and retraction, with the premaxillae rotating about their contacts with the nasals. There is a possibility of small medio-lateral movements of the lacrimal and maxilla as a unit at the lacrimal-prefrontal joint. The skull of Lysorophus shows many features convergent with the skulls of burrowing reptiles, such as amphisbaenians. Modifications in the skull of Lysorophus from a more primitive tetrapod condition seem associated with burrowng in a soft substratum rather than with kinesis per se.

Two communities dominated by deposit-feeding bivalves are recognized in Quisset Harbor, Massachusetts. Interference with life position and sediment preconditioning restrict the distribution of some species. The Eel Grass Community, dominated by Nucula proxima, comprises primarily non-siphonate forms which live in fecal-pellet rich, high-water-content, oxygenated sediment. The sediment texture results from active burrowing. This soft, mobile sediment readily clogs the siphons of siphonate bivalves. The Channel Community, dominated by Yoldia limatula, comprises primarily siphonate forms which live in partly compacted, low-water-content muds. Each community is characterized by abundant surface, shallow, and deep feeding species. Competition for food has generated this distinct stratification in feeding depth of the dominant species with each community.

Similar competitive relationships, affecting both species occurrence and trophic stratification, are apparent in three Silurian age deposit-feeding bivalve communities from the McAdam Brook Formation of Nova Scotia. A community dominated by four non-siphonate species inhabited soupy, high-water-content muds. A community dominated by three siphonate species lived in cohesive muds. A mixed community, dominated by four species, two non-siphonate and two siphonate species (the most abundant of which are present, but not as the most abundant forms, in the other communities), characterized the mid-way point of the ecotone between the other two communities.

These deposit-feeding bivalve dominated communities have similar structural patterns although separated by 400 million years and despite environmental differences and evolutionary changes. Both response to physical factors, such as sediment character, and interspecific competition impose and maintain this type of community structure.