Through an examination of the contents of the Journal of Paleontology (JP), this paper traces the growing interest in biological problems in mid twentieth-century invertebrate paleontology. While noting the continued dominance of descriptive morphology and systematics, the paper tracks the increasing attention paid to paleoecology, evolution and geographic distribution, and quantitative methods. An analysis of the debate over the relative importance of biology and geology for paleontology, and J. Marvin Weller's evolving views on the subject, further illustrate the main point. Neither Weller nor JP initiated the interest in biological questions, but both played an important role in bringing new developments to the attention of the paleontology community.

Archaeocyaths are calcareous, conical, Cambrian fossils with a long history of phylogenetic uncertainty and changing interpretations. The history of phylogenetic interpretation of archaeocyaths reveals five distinct schools of thought: the coelenterate school, the sponge school, the algae school, the Phylum Archaeocyatha school, and the Kingdom Archaeata school. Late nineteenth century and early twentieth century paleontologists worked within a paradigm of inexorably increasing diversity through time, and they did not believe in the concept of extinct phyla. Consequently, prior to about 1950, archaeocyaths were bounced around from coelenterates to sponges, to algae. By the 1930s, after considerable study, all workers agreed that archaeocyaths were sponges of one type or another. In the mid-twentieth century a significant paradigm shift occurred in paleontology, allowing the viability of the concept of a phylum with no extant species. Correspondingly, two new schools of thought emerged regarding archaeocyathan taxonomy. The Phylum Archaeocyatha school placed them in their own phylum, which was inferred to be closely related to Phylum Porifera within Subkingdom Parazoa. A second new school removed archaeocyaths and some other Paleozoic problematica from the animal kingdom and placed them in Kingdom Archaeata (later Kingdom Inferibionta). The Phylum Archaeocyatha school was the mainstream interpretation from the 1950s through the 1980s. However, the widespread use of SCUBA beginning in the 1960s ultimately led to the rejection of the interpretation that archaeocyaths belong in a separate phylum. SCUBA allowed biologists to study deep fore-reef and submarine cave environments, leading to the discovery of living calcareous sponges, including one aspiculate species that is morphologically similar to archaeocyaths. These discoveries in the 1960s and 1970s stimulated a re-examination of sponge phylogeny generally, and comparisons between archaeocyaths and sponges in particular. The result was the abandonment of the Phylum Archaeocyatha school in the 1990s. Present consensus is that archaeocyaths represent both a clade and a grade—Class Archaeocyatha and the archaeocyathan morphological grade—within Phylum Porifera.

The history of research on the “true” stromatoporoids, a presumably monophyletic group of sponges that occurred from the Ordovician through the Devonian, is examined in detail. Stromatoporoid published research is summarized in five categories: quantity of publication; biological affinities; systematics; skeletal microstructure; and paleoecology. Quantity of publication is measured from each of the 75 years. Moderate levels of publication in the late 1920s and 1930s declined in the early 1940s, and were reduced to zero for four years due to the impact of World War II. Levels similar to that of the 1930s returned in the 1950s, after which there was an overall increase until the mid-1980s, when levels began a decrease that persists today. The proportion of research on paleoecology has increased as research on systematics decreased through time. Post-Devonian forms assigned to the stromatoporoids are a polyphyletic grouping of several apparently unrelated taxa, possibly representing both Porifera and Cnidaria. Publications on the post-Devonian “stromatoporoids” amount to less than one-third that on the true stromatoporoids during the same 75 years.

The focus of this paper is to provide an overview of historical and modern accounts of scleractinian evolutionary relationships and classification. Scleractinian evolutionary relationships proposed in the 19th and the beginning of the 20th centuries were based mainly on skeletal data. More in-depth observations of the coral skeleton showed that the gross-morphology could be highly confusing. Profound differences in microstructural and microarchitectural characters of e.g., Mesozoic microsolenine, pachythecaliine, stylophylline, stylinine, and rhipidogyrine corals compared with nominotypic representatives of higher-rank units in which they were classified suggest their separate (?subordinal) taxonomic status. Recent application of molecular techniques resulted in hypotheses of evolutionary relationships that differed from traditional ones. The emergence of new and promising research methods such as high-resolution morphometrics, analysis of biochemical skeletal data, and refined microstructural observations may still increase resolution of the “skeletal” approach. Achieving a more reliable and comprehensive scheme of evolutionary relationships and classification framework for the Scleractinia will require close cooperation between coral biologists, ecologists, geologists, geochemists, and paleontologists.

Three historical phases can be distinguished in the study of brachiopod systematics over the past 75 years. Prior to 1956, systematic neontologists and paleontologists struggled to reconcile differences in perceived evolutionary patterns (and thus classifications) based largely on static morphological differences observed separately among living brachiopods and among fossil brachiopods. Following 1956, patterns of morphological distribution began to be interpreted relative to the processes by which they were formed, and a more dynamic view of brachiopod phylogeny and classification resulted. Over the past decade, newer methodologies (phylogenetic systematics) have allowed older phylogenetic hypotheses to be tested and evaluated. The major challenges that brachiopod systematists now face are not unique to brachiopods; they concern improving the methods of phylogeny (and classification) reconstruction so that all the sources of data available to paleontologists can be utilized more effectively. In the future, I predict that more intensified, global fossil collecting, together with further investigation of the embryology and development of brachiopods, and molecular systematic research, will play an increasingly larger role in revising the classification currently in use.

Over the past 75 years, the higher-level taxonomy of bivalves has received less attention than that of their fellow molluscs, gastropods. The publication of the bivalve volumes of the Treatise on Invertebrate Paleontology in 1969 was not followed by an explosion of study into the evolution of bivalves; rather, with only one or two exceptions, bivalve workers were noticeably absent from the cladistic and molecular revolutions that were taking place during the 1970s and 1980s, even as gastropods received considerable attention. Over the past ten years, cladistics and molecular systematics have begun to be applied to solve problems of bivalve evolutionary biology. These studies, most of which have been undertaken by paleontologists, have halted the stagnation in bivalve systematics. Bivalve systematics looks to have an exciting future, as the excellent fossil record of the Bivalvia will be used in conjunction with cladistics and molecular systematics to solve problems in not just bivalve evolution but evolutionary biology in general.

Twentieth century fossil gastropod systematics relied extensively on neontological paradigms. However, recent appreciation of the extant gastropod diversity suggests that those early paradigms provided very unsound models. This likely is a greater problem for Paleozoic taxa than for Meso-Cenozoic gastropods because Meso-Cenozoic taxa frequently have easily recognized extant relatives whereas Paleozoic taxa frequently do not. Also, many of the taxa that apparently diverged in the Paleozoic now are limpets and retain little information about the morphologies of their coiled ancestors.

Snails could be a model taxon for investigating macroevolutionary patterns because of the clade's dense fossil record. However, paleontologists usually study only adult shells (teleoconchs), and many malacologists maintain that teleoconch characters reflect phylogeny poorly if at all. This is important because many macroevolutionary hypotheses make their most specific predictions given phylogeny. Studies evaluating species- or genus-level relationships typically use more shell characters and states than do studies evaluating suprageneric relationships, as expected if shells evolve rapidly. Monte Carlo tests reject a null hypothesis that rates of homoplasy are equal among shell and soft-anatomy characters for two neogastropod clades, but suggest that these rates differ by less than an order of magnitude. Finally, teleoconch characters fail to unite bellerophontiform species with gastropod muscle scars but successfully unites clusters bellerophontiform species with tergomyan muscle scars. These results corroborate the conventional wisdom that teleoconch character distributions reflect abundant homoplasy, but the results also suggest that these distributions reflect phylogeny, too.

If we can control the effects of homoplasy, then gastropods are an excellent “model” group for testing macroevolutionary hypotheses such as changing rates of evolution. Two obvious candidates are rates of morphologic evolution among basal neogastropods, and rates of molecular evolution within clades radiating after the K/T mass extinction.

The progress achieved in trilobite systematics over the last 75 years is briefly reviewed. Different approaches to phylogenetics have influenced the way trilobites have been classified. Classical evolutionary taxonomy, the stratigraphical approach, and cladistics have all contributed in different ways to the current classification, which has evolved piecemeal, and is still unsatisfactory is some ways. Nonetheless, progress towards a phylogenetic classification has been made, especially as the result of information from ontogenies provided by well-preserved silificified material. Trilobites are a well-defined clade within a larger arachnomorph group. Agnostida have been excluded from Trilobita, but are perhaps best considered as specialised trilobites, at least until limbs of eodiscids are described. The outstanding problems in classification of each trilobite order are reviewed. Most are concerned with the recognition of the appropriate Cambrian sister taxa, and the discovery of the relevant ontogenies. It is very likely that post-Cambrian clades “root” deeply into the Cambrian. The coherence, or otherwise, of Proetida, Asaphida, Corynexochida and the lichid/odontopleurid groups will be resolved by such studies. The problems of paraphyly in Ptychopariida and Redlichiina may prove more obdurate. The temporal brevity of certain Cambrian family ranges may be partly a taxonomic artefact. The possibility of a late Cambrian gap in the record on some clades should be considered.

Significant investigators and aspects in the past century of insect paleontology are briefly reviewed. Despite the pervasive influence of the paleoentomologist Willi Hennig in systematic biology, the study of fossil insects remains more descriptive than most other paleontological areas. Hypotheses are reviewed on relationships and chronologies of early divergences in insects (Paleozoic, Lower Mesozoic), particularly living and extinct orders of the lower pterygotes and putative monophyly of the Paleoptera (Odonata Ephemeroptera). The Dictyoptera (Mantodea, Isoptera, Blattaria) illustrate relationships and discrepencies between stratigraphic record and phylogenetic relationships. Future directions in the field are suggested.

Development of a phylogenetic classification has been a primary pursuit of crinoid paleontologists during the 20th century. Wachsmuth and Springer and Bather vigorously debated crinoid classification during the waning years of the 19th century, and although tremendous progress has been made a comprehensive phylogenetic classification is still the primary objective for crinoid research during the early 21st century. Twentieth century crinoid studies are divisible into four periods. The direct influence of Frank Springer and Francis Bather continued until approximately 1925. Descriptive studies dominated the period of 1926–1943 and culminated in a comprehensive classification of Paleozoic crinoids that was a combination of the ideas of Wachsmuth and Springer and Bather. The end of the third period, 1944–1978, was marked by publication of the Treatise on Invertebrate Paleontology. The Treatise compilation brought together classification ideas for the entire class into a truly comprehensive classification, although problems remained with the phylogenetic underpinnings of the Treatise classification. During the third period, pioneering work on crinoid paleobiology laid the foundation for significant paleobiology advances for the fourth, 1979–1999, period. This last period also witnessed significant advances in the taxonomy of crinoid faunas at critical intervals, the taxonomy of crinoids from new geographic areas, and working toward the solution to the origin and early evolution of the Crinoidea.

Continued work on crinoids in the 21st century promises to provide significant advances both for understanding the evolutionary history of crinoids and for understanding the history of epifaunal benthic communities through time. Immediate challenges include completion of a comprehensive phylogenetic classification, which will open the door for evolutionary paleoecologic and paleobiologic studies; utilization of computerized morphometric techniques in the analysis of functional morphology; systematic studies of new faunas in critical intervals; discovery of faunas in new geographic areas to better constrain knowledge of crinoid biogeography; and modern systematic revision of classic North American and European faunas.

Conodonts were mostly small, elongate, eel-shaped marine animals that inhabited a variety of environments in Paleozoic and Triassic seas. Although long enigmatic, conodonts are now regarded as vertebrates and their closely controlled fossil record is not only the most extensive of all vertebrates, but it also makes conodonts the fossils of choice in upper Cambrian through Triassic biostratigraphy. Conodonts were soft-bodied except for a variety of phosphatic elements that formed a distinctive feeding apparatus. Post-mortal dissociation of the apparatus and subsequent jumbling of its elements on the sea floor led, from 1856 to about 1966, to development of an artificial, form-based taxonomy that was utilitarian, but clearly unsatisfactory as a vehicle for understanding the group in biologic terms. Natural assemblages of elements, discovered between 1879 and 1952, have been interpreted as undisturbed skeletal apparatuses, and in the mid-1960s it was determined that original composition of the apparatuses of many species could be reconstructed and statistically evaluated from collections of disjunct elements by various grouping procedures. These determinations led to an emphasis on multielement taxonomy by most (but not all) students of conodonts. Even so, only about a third of the approximately 550 valid conodont genera, have been established (or re-interpreted) in multielement terms and this makes any of the several extant schemes of suprageneric classification phylogenetically suspect. We comment on a recent scheme that recognizes 41 families assigned to some 7 orders, and suggest how it might be modified so as to square with principles of phylogenetic systematics.

The basic structure of archosaurian phylogeny is understood to include two primary crown-group lineages—one leading to living crocodiles and including a broad diversity of Triassic animals (e.g., phytosaurs, rauisuchians, aetosaurs), and another leading to dinosaurs (living and extinct). These lineages were established by the middle Triassic. A few extinct groups remain controversial, such as the pterosaurs, and debate persists over the phylogenetic relationships among extant bird lineages, which have proved difficult to resolve, and divergence timing estimates within Aves and Crocodylia remain the source of contention. A few analyses support a close relationship between archosaurs and turtles, or even a nesting of turtles within Archosauria. All sources of information used to resolve these issues have weaknesses, and these problems all involve highly derived lineages when they first appear in the fossil record.

The origin of tetrapods from sarcopterygian fish in the Late Devonian is one of the best known major transitions in the history of vertebrates. Unfortunately, extensive gaps in the fossil record of the Lower Carboniferous and Triassic make it very difficult to establish the nature of relationships among Paleozoic tetrapods, or their specific affinities with modern amphibians. The major lineages of Paleozoic labyrinthodonts and lepospondyls are not adequately known until after a 20–30 m.y. gap in the Early Carboniferous fossil record, by which time they were highly divergent in anatomy, ways of life, and patterns of development. An even wider temporal and morphological gap separates modern amphibians from any plausible Permo-Carboniferous ancestors. The oldest known caecilian shows numerous synapomorphies with the lepospondyl microsaur Rhynchonkos. Adult anatomy and patterns of development in frogs and salamanders support their origin from different families of dissorophoid labyrinthodonts. The ancestry of amniotes apparently lies among very early anthracosaurs.