The first major international trilobite conference (which also dealt with related arthropod groups) was convened by David Bruton in Oslo in 1973. That meeting resulted in a 467 page proceedings volume (Martinsson, 1975) that has become an indispensible reference on arthropod paleobiology. With its abundance of high-quality empirical work and influential ideas, the ongoing utility of “Fossils and Strata 4” is assured, yet it is also a fascinating microcosm of the themes and concerns of paleontology as a whole a quarter of a century ago. Paleoecology blossomed in the late 1960s and early 1970s. Autecology, and particularly functional morphology, dominates the volume, accounting for about one-third of the contributions. Plate tectonics was a relatively recent development in 1973 and, following the lead of Whittington and Hughes (1972), several papers tackled trilobite paleobiogeography. They vary from a bold attempt to determine oceanic circulation through the Ordovician by Ross to more conventional studies of biogeographic distribution that were aimed at providing further support for concepts of continental drift. There was a burgeoning interest in community paleoecology, and Richard Fortey's (1975) now-classic paper on the Early Ordovician faunas of Spitsbergen was the first to define trilobite “communities.” The volume was rounded out by diverse papers on trilobite anatomy based on material from the Burgess Shale and Beecher's Trilobite Bed, the microstructure of the trilobite cuticle, trilobites eyes, high level classification of trilobites, and various evolutionary themes (character displacement in agnostoids; adaptive radiation in Cambrian trilobites).

Based on range data and generic composition, four stages of evolution are recognized for late Paleozoic trilobites of the contiguous United States. Stage 1 occurs in the Lower Mississippian (Kinderhookian-Osagean) and is characterized by a generically diverse association of short-ranging, stenotopic species that are strongly provincial. Stage 2 species are present in the Upper Mississippian and consist of a single, eurytopic, pandemic genus, Paladin. Species of Stage 2 are much longer-ranging than those of Stage 1, and some species may have persisted for as long as 12 m.y. Stage 3 is present within Pennsylvanian and Lower Permian strata and consists initially of the eurytopic, endemic genera Sevillia and Ameura as well as the pandemic genus Ditomopyge. During the middle Pennsylvanian the very long-ranging species Ameura missouriensis and Ditomopyge scitula survived for more than 20 m.y. During the late Pennsylvanian and early Permian, a number of pandemic genera appear to have immigrated into what is now North America. Stage 4 is restricted to the Upper Permian (late Leonardian-Guadalupian) strata and is characterized by short-ranging, stenotopic, provincial genera.

The main causal factor controlling the four-stage evolution of late Paleozoic trilobites of the United States is interpreted to be eustacy. Whereas Stage 1 represents an adaptive radiation developed during the Lower Mississippian inundation of North America by the Kaskaskia Sequence, Stage 2 is present in strata deposited during the regression of the Kaskaskia sea. Stage 3 was formed during the transgression and stillstand of the Absaroka Sequence and, although initially endemic, Stage 3 faunas are strongly pandemic in the end when oceanic circulation patterns were at a maximum. A mid-Leonardian sea-level drop caused the extinction of Stage 3 fauna. Sea-level rise near the end of the Leonardian and into the Guadalupian created an adaptive radiation of stentopic species of Stage 4 that quickly became extinct with the latest Permian regression.

Since the publication of Darwin (1859), the biological meaning of the Cambrian radiation has been debated. Most commentators agree, however, that the Cambrian radiation is fundamentally a time of major metazoan cladogenesis. In and of itself this does not necessarily mean that unique evolutionary processes operated during the Cambrian radiation. Phylogenetic analysis has been used to study the tempo of speciation during the radiation, and thus far there is no need to invoke special rules relating to the tempo of evolution. Instead, what seems unique about the Cambrian radiation is its place as an important episode in the history of life—that is, as the first major radiation of the Metazoa. Although the tempo of evolution during the Cambrian radiation may not have been uniquely high, there were largely unique tectonic events that transpired during the late Neoproterozoic and Early Cambrian, such as extensive cratonic fragmentation. Biogeographic analysis of Early Cambrian olenelloid trilobites reveals that these tectonic events powerfully influenced evolutionary and distributional patterns in this diverse and abundant trilobite group. This emphasizes the importance of physical earth history in generating evolutionary patterns. In the general study of macroevolutionary patterns and processes, earth history phenomena emerge as powerful forces influencing the history of life and provide insights into evolution that can best be inferred by paleontological data.

No trilobite species, and very few genera, pass from the Lower (Ibexian) into the Middle Ordovician (Whiterockian), which is a turning point in Laurentian trilobite history. Trilobites from three sections exposing the base of the Whiterock Series (basal Middle Ordovician) in Nevada are described and illustrated. They are attributable to different, and generally more open-shelf, biofacies from the Bathyurid biofacies trilobites described from the Ibex area, Utah, by Fortey and Droser (1996), but include species in common, which allow correlation into the Ibexian type section. At Little Rawhide Mountain the basal Middle Ordovician is developed in Olenid biofacies, described for the first time in western North America from rocks of this age. Correlation based on species-level similarity shows that the “spike” that has been used to define the type base of the Whiterockian (and hence the Middle Ordovician) at Whiterock Canyon is at a level younger than the base of the Whiterockian assumed in recent discussions of its international correlation. The type Whiterock base correlates with the Psephosthenaspis glabrior trilobite Subzone at Ibex, well above the major change in trilobite faunas at the base of the Psephosthenaspis Zone (P. microspinosa Subzone). In all study sections there is an abrupt change of facies after trilobite Zone J, possibly associated with regression. Four new species are described: Cloacaspis tesselata, Harpillaenus rossi, Acidiphorus? lineotuberculatus, and Benthamaspis serus.

The distributions of trilobite species were controlled by a combination of habitat preferences and paleogeographic constraints, which tend to limit their extent. The Lower Ordovician trilobite Carolinites genacinaca Ross, 1951, however, had a remarkable range unequaled among polymerid trilobites; it has been recognized on all Ordovician paleocontinents. Its distribution has been explained by an epipelagic mode of life, based on evidence from functional morphology, analogy with modern pelagic crustaceans, and geological occurrence. In such a case, morphological identity throughout its range might be anticipated, if all occurrences can be postulated to be members of a single pandemic population. Rotational superimposition has been used to compare variation within samples drawn from Alberta, Spitsbergen, and Australia with a benchmark population from the western United States. All are morphometrically similar. By any criterion, specimens identical to the benchmark population are found within the Alberta, Spitsbergen and Australia samples, which represent the extremes of the species' geographic range. A lone cranidium from France, previously referred to Carolinites vizcainoi, may be a juvenile of C. genacinaca or C. tasmaniensis; its differences are consistent with ontogeny. A small number of specimens from Siberia and central China show differences in cranidial proportions from the Utah specimens that may be the result of preservational factors and/or photographic technique, or may represent genuine morphological disparity; this could be clarified if more specimens were to become available. This study suggests that C. genacinaca was ubiquitous in the epipelagic environment in a belt that encircled the planet between paleolatitudes of approximately 30°N and 30°S.

The ontogenies of three new species of Telephinidae, Telephina calandria, Telephina chingolo, and Telephina problematica are described from Arenig-Caradoc strata in the Argentine Precordillera, and compared with the larval stages of some other Proetida, including other telephinids.

New findings reveal 1) a radical metamorphosis in the ontogenies of these Telephina species late in the meraspid period, not previously described among Trilobita; and 2) distinctive hypostomes of Telephinidae containing long, thin anterolaterally and dorsally splayed anterior wings. Early ontogenies of different species currently assigned to the genus Telephina fall into at least two distinct morphological and life history groups, and hypostomes (if correctly assigned in previous works) vary significantly. The three new species strengthen the hypothesis of a phylogenetic connection between Oopsites and Telephina.

Three morphological types of protaspid instars are described for proetide trilobites. Two are anaprotaspides, and the third is a metaprotaspis. They always occur in the same sequence in the ontogeny, but no cases are known of all three types in the same species. These larval types are important for understanding the systematics and life cycles of the Proetida. Benthic/pelagic transitions identify four life history patterns among the the Proetida. The best larval synapomorphy for the Proetida is the distinctive metaprotaspid larval type, which is absent in very few proetides (some Telephinidae), perhaps as a result of heterochronic displacement of this stage into the meraspid period.

Exceptionally rich and well-preserved silicified exuviae comprising numerous immature growth stages allow the tracing of detailed morphological and morphogenetical features of the reduced-eyed phacopid species Cryphops? ensae (Richter and Richter, 1926) from the latest Famennian of Thuringia (Central Germany). Ontogenetic features of this species are more similar to those of Phacops than to Trimerocephalus.

Etched slices of the cuticle of the Silurian trilobite Homalonotus rhinotropis Angelin, 1854, from two localities in Skåne, southern Sweden, have been examined with the scanning electron microscope. The two major subdivisions of the cuticle are a “prismatic” (originally laminated) outer layer, about 20-30 μm thick, and a principal layer, 200-300 μm thick, which may be foliated or roughly laminated. The original finely-laminated aspect of the outer layer may sometimes be seen as a continuous layer, but is more frequently detected as “shaped inclusions” set in a prismatic background. These “shaped inclusions” are considered to have been produced by accumulation of minerals around the apices of 3-6 μm perpendicular canals early in calcification and preferentially preserved because of subtle differences from the products of later stages of the process. It is possible that calcification started before ecdysis. The difficulties involved in ascertaining the original structure of the principal layer are outlined, and possible reasons for differences in its preservation from the outer layer are proposed.

Cladistic relationships of Trilobita, Naraoiidae (five ingroup taxa), Helmetiida (five ingroup taxa), Xandarellida, and the Cambrian arachnates Retifacies, Sinoburius, Emeraldella, and Sidneyia are investigated based on 29 characters. Documentation of appendage morphology and other ventral structures in Saperion from the Chengjiang fauna permits an appraisal of helmetiid relationships. A monophyletic Trilobita [=“Olenellida” (Emuellida + Eutrilobita)] is defined by numerous synapomorphies, including exoskeletal calcification and dorsal eyes with calcified lenses and circumocular sutures. Helmetiida is a robust clade, resolved as (Helmetiidae (Tegopeltidae (Saperiidae + Skioldiidae))). Naraoiid monophyly is well-supported, but neither a naraoiid-trilobite nor a naraoiid-Retifacies clade are parsimonious, the latter grouping (“Nectopleura”) being explicitly paraphyletic. A sister group relationship between Xandarellida and Sinoburius is endorsed, although character support is novel compared to previous groupings of these taxa. The fourth postantennal limb pair in trilobites, naraoiids, and apparently helmetiids is based beneath the cephalothoracic articulation. Reweighted characters favor Trilobita and Helmetiida as closest relatives, with Petalopleura and then Naraoiidae as sister groups.

The Walcott-Rust Quarry, at Trenton Falls, New York, has yielded a large number of well-preserved, fully articulated fossils from the lower third of the Middle Ordovician Rust Formation, Trenton Group. Along with three species of the more common trilobites from the Trenton Group, fourteen species of rare and uncommon trilobites are found within a thin package of micritic limestones and shales. The first trilobites with preserved appendages, Ceraurus pleurexanthemus Green, 1832 and Flexicalymene senaria (Conrad, 1842), were described from one layer from this quarry. Unique specimens of Isotelus walcotti UIrich in Walcott, 1918, and Sphaerocorphe robusta Walcott, 1875, were found in the next higher bed. Re-excavation of the quarry yielded information about the taphonomy of the trilobites and stratigraphy of the trilobite layers. Nearly half of the beds surveyed (n = 50) yield direct evidence of obrution (i.e., rapid post-mortem or live burial) of benthic organisms. Unusual anaerobic microenvironments in partially enrolled trilobites of the Ceraurus layer facilitated very early calcification of appendages and other soft parts.

A new, low-density trilobite body cluster from an Upper Ordovician Kope Formation mudstone in the Cincinnati region is characterized by well-preserved, articulated specimens. This cluster is polytaxic, size-segregated, and shows an unusual variety of flexure postures in the most common taxon, Flexicalymene aff. granulosa. Specimens show internal pyritization and associated pyritefilled tubular burrows, and were preserved by rapid deposition of mud from a sediment-laden flow. Size-segregation among species, but size-consistency within species, coupled with sedimentological data and apparently haphazard body orientation data, suggest that the trilobites represent a biological association, rather than the product of mechanical sorting. While the majority of specimens are inverted, many are dorsal-up, and some are inclined to bedding. Postures include prone (i.e., nonflexed), simple dorsal or ventral flexure, and complex stances that combine both dorsal and ventral flexure. Flexure pattern appears largely unrelated to sediment compaction. We interpret the cluster as an association of live trilobites that were entombed in situ. Specimens may have inverted prior to burial as a behavioral posture, or as a response to onset of burial. The animals were likely preserved in the process of escaping, as they tried to free themselves from being buried. This is in contrast to the tightly enrolled stance in some trilobites, especially other Flexicalymene, commonly associated with rapid burial events, and indicates a new behavioral aspect of trilobite paleobiology. The distribution of associated burrows suggests that the rotting carcasses attracted scavengers.

Silurian trilobites of the central United States belong to a series of temporally-successive associations which appeared abruptly, maintained taxonomic stasis for a time, and then disappeared abruptly. Their disappearance resulted from global perturbations of short-term duration and moderate magnitude, which caused substantial taxonomic replacement but no reorganization of major ecosystems. The most significant extinction and replacement in Silurian trilobite associations in the study area occurs near the Llandovery–Wenlock boundary. This turnover in trilobite associations appears to correspond to Jeppsson's Ireviken Event in his model of oceanic and climatic cyclicity. Major sea-level changes earlier in the Llandovery did not have a similar impact on trilobite associations.

Trilobites from thrombolitic reefs in the Ore Hill Member of the Upper Cambrian Gatesburg Formation are assigned to (in ascending order) the Pseudosaratogia magna, Cliffia lataegenae, and Irvingella major Subzones of the Elvinia Zone. These reef faunas contrast markedly with those from coeval off-reef grainstones (tempestites), representing separate biofacies rather than biostratinomic variants. Associated lithologies suggest that the fauna of the P. magna Subzone at the base of the member comprises a biofacies associated with shallower microbial reef environments than those occupied by faunas in the overlying C. lataegenae Subzone. Reefs in the C. lataegenae Subzone yield a fauna dominated by Buttsia drabensis, extending the documented range of catillicephalid abundance in microbial reef biofacies downward into the highest strata of the Steptoean Stage. This reef fauna also includes several new taxa, including Cheilocephalus quadratus new species (described herein), an atypical species of Cheilocephalus with catillicephalid-like cranidial features. The fauna in off-reef tempestites low in the C. lataegenae Subzone contains the genera that characterize the Xenocheilus–Kindbladia Biofacies, expanding the known environmental and geographic range of that biofacies. Sulcocephalus, typically a minor component of faunas in the I. major Subzone, dominates the fauna in reefs within this subzone such that a distinct Sulcocephalus Biofacies can be recognized immediately above the extinction horizon that defines the base of the Sunwaptan Stage.

Analysis of 164 collections from shelf facies of Laurentian North America indicates that three successive trilobite mass extinctions at Late Cambrian stage boundary intervals (“biomere” boundaries) are characterized by a common pattern of change in distributional paleoecology and species diversity. In all cases, the extinction intervals are marked by a shift to biofacies that have broader environmental distributions than those prior to the onset of extinctions, implying a reduction in between-habitat (beta) diversity. Significant declines in within-habitat (alpha) diversity also characterize each extinction and the compositions of shelf biofacies record extensive immigration of taxa from off-shelf and shelf-margin sites. The nature and extent of ecologic disruption of the shelf appears to be comparable to changes associated with major mass extinctions, such as those at the end of the Ordovician and Permian. Unlike major mass extinctions, the Cambrian events are followed by a complete recovery of diversity and biofacies structure within a few million years.

Morphometric analyses of silicified and nonsilicified (preserved in shale) specimens of the olenelloid trilobites Olenellus (Olenellus) gilberti Meek (in White, 1874) and Nephrolenellus geniculatus Palmer, 1998, from the Lower Cambrian C-Shale Member of the Pioche Formation show that even well-preserved specimens in shales have undergone significant changes in lateral as well as vertical dimensions as a result of compaction. Analyses of cephalic landmarks show that in both species compaction causes posteriordirected collapse of the anterior lobe of the glabella, adaxial deformation of the ocular lobes, and abaxial and anterior splaying of genal regions. These shape changes are explicable in terms of observed exoskeletal fracture patterns. Landmarks show an increase in scatter around their ontogenetic trajectories that is generally proportional to the degree of lateral shift each landmark has undergone. Interspecific differences in compactional response may depend on the relative convexity of the cephalon. Olenellus (Olenellus) gilberti is a low-convexity species and shows marked lateral shape change, particularly in the genal region. Nephrolenellus geniculatus is more convex and shows less severe lateral shape change. Landmarks of both species exhibit an average trebling of the degree of scatter around their average ontogenetic trajectories in compacted samples. Because even well-preserved specimens in shales differ in shape from their precompactional appearance, results of morphometric studies utilizing metric distances between landmarks in trilobites where compaction can be detected must be interpreted with caution.