Daniel A. Stephen

Gastropod evidence against the Early Triassic Lilliput effect

Size reduction in the aftermath of the Permian-Triassic mass extinction event has repeatedly been described for various marine organisms, including gastropods (the Lilliput effect). A Smithian gastropod assemblage from Utah, USA, reveals numerous large-sized specimens of different genera as high as 70 mm, the largest ever reported from the Early Triassic. Other gastropods reported from Serbia and Italy are also as large as 35 mm. Size frequency distributions of the studied assemblages indicate that they were not unusually small when compared with later Mesozoic and modern faunas. The occurrence of large-sized gastropods less than 2 Ma after the Permian-Triassic mass extinction refutes the Lilliput hypothesis in this clade, at least for the last ~75% of the Early Triassic.

Smithian ammonoid faunas from Utah: implications for Early Triassic biostratigraphy, correlation and basinal paleogeography

Intensive sampling of the lower portion of the Thaynes and Moenkopi Groups (Lower Triassic) at separate localities within the Confusion Range, Pahvant Range, Mineral Mountains, Star Range, Kanarraville, Cedar City, Torrey and San Rafael Swell areas (mainly central and southern Utah, USA) leads to the recognition of a new key regional Smithian ammonoid succession. The new biostratigraphical sequence, which is more precise than the long-recognized Meekocerasgracilitatis and Anasibiriteskingianus Zones, comprises twelve subdivisions, thus resulting in a sequence with much higher resolution that can be correlated not only with other western USA sites, but also with major worldwide localities as well. Middle and late Smithian faunas contain many taxa with wide geographic distribution, thus enabling long-distance correlation with faunal successions from other regions (e.g., British Columbia, Canadian Arctic, South China, Spiti and Oman). New assemblages from the lowermost beds are the least diversified and poorest preserved; they represent the earliest early/middle Smithian ammonoid faunas reported from the western North American basin. They highlight (a) the sudden Smithian advancement of the marine transgression within this epicontinental sea, (b) that this event is diachronous, and (c) that the paleotopography of the basin most likely was highly irregular. The newly obtained ammonoid succession also allows us to date and follow the transgression from the northern and central part of the basin to the southwesternmost and southeasternmost parts, which were reached during the late Smithian (Anasibiriteskingianus beds). In addition, we briefly discuss the now-limited previous regional biozonation in the light of these new results. One new genus (Minersvillites) and nine new species (Kashmirites utahensis, Kashmirites confusionensis, Kashmirites stepheni, ?Xiaoqiaoceras americanum, Minersvillites farai, Inyoites beaverensis, Meekoceras olivieri, Meekoceras millardense, Vercherites undulatus) are also described.

Unexpected Early Triassic marine ecosystem and the rise of the Modern evolutionary fauna.

A new marine fossil assemblage from the Early Triassic shows unexpected phyletic diversity and functional complexity. In the wake of the end-Permian mass extinction, the Early Triassic (~251.9 to 247 million years ago) is portrayed as an environmentally unstable interval characterized by several biotic crises and heavily depauperate marine benthic ecosystems. We describe a new fossil assemblage—the Paris Biota—from the earliest Spathian (middle Olenekian, ~250.6 million years ago) of the Bear Lake area, southeastern Idaho, USA. This highly diversified assemblage documents a remarkably complex marine ecosystem including at least seven phyla and 20 distinct metazoan orders, along with algae. Most unexpectedly, it combines early Paleozoic and middle Mesozoic taxa previously unknown from the Triassic strata, among which are primitive Cambrian-Ordovician leptomitid sponges (a 200–million year Lazarus taxon) and gladius-bearing coleoid cephalopods, a poorly documented group before the Jurassic (~50 million years after the Early Triassic). Additionally, the crinoid and ophiuroid specimens show derived anatomical characters that were thought to have evolved much later. Unlike previous works that suggested a sluggish postcrisis recovery and a low diversity for the Early Triassic benthic organisms, the unexpected composition of this exceptional assemblage points toward an early and rapid post-Permian diversification for these clades. Overall, it illustrates a phylogenetically diverse, functionally complex, and trophically multileveled marine ecosystem, from primary producers up to top predators and potential scavengers. Hence, the Paris Biota highlights the key evolutionary position of Early Triassic fossil ecosystems in the transition from the Paleozoic to the Modern marine evolutionary fauna at the dawn of the Mesozoic era.

Smithian shoreline migrations and depositional settings in Timpoweap Canyon (Early Triassic, Utah, USA)

In Timpoweap Canyon near Hurricane (Utah, USA), spectacular outcrop conditions of Early Triassic rocks document the geometric relationships between a massive Smithian fenestral-microbial unit and underlying, lateral and overlying sedimentary units. This allows us to reconstruct the evolution of depositional environments and high-frequency relative sea-level fluctuations in the studied area. Depositional environments evolved from a coastal plain with continental deposits to peritidal settings with fenestral-microbial limestones, which are overlain by intertidal to shallow subtidal marine bioclastic limestones. This transgressive trend of a large-scale depositional sequencemarks a long-term sea-level rise that is identified worldwide after the Permian-Triassic boundary. The fenestral-microbial sediments were deposited at the transition between continental settings (with terrigenous deposits) and shallow subtidal marine environments (with bioturbated and bioclastic limestones). Such a lateral zonation questions the interpretation of microbial deposits as anachronistic and disaster facies in the western USA basin. The depositional setting may have triggered the distribution of microbial deposits and contemporaneous marine biota. The fenestral-microbial unit is truncated by an erosional surface reflecting a drop in relative sea level at the scale of a medium depositional sequence. The local inherited topography allowed the recording of small-scale sequences characterized by clinoforms and short-distance lateral facies changes. Stratal stacking pattern and surface geometries allow the reconstruction of relative sea-level fluctuations and tracking of shoreline migrations. The stacking pattern of these small-scale sequences and the amplitude of corresponding high-frequency sea-level fluctuations are consistent with climatic control. Large-andmedium-scale sequences suggest a regional tectonic control.

Pathologic Gigantism in Middle Carboniferous Cephalopods, Southern Midcontinent, United States

Giant genera and species occur in virtually every phylum of the Kingdom Animalia. For the most part, their existence has been viewed as an illustration of Cope’s Rule: the tendency toward phylogenetic size increase among groups of organisms. Giant Mesozoic ammonites are frequently cited examples of this trend, and the giants are typically treated as discrete taxonomic entities. In contrast, pathologic gigantism is a rare condition that results in abnormal growth of an individual beyond the normal size limits of its species, and does not reflect Cope’s Rule. That condition is little known among invertebrates, although pathologic gigantism has been reported in prosobranch and pulmonate gastropods, where it is associated with infestation by larval trematodes that caused parasitic castration.

Possible Cephalopod Reproductive Mass Mortality Reflected by Middle Carboniferous Assemblages, Arkansas, Southern United States

Middle Carboniferous cephalopod assemblages across northern Arkansas are dominated by a small number of ammonoid species that occur in unusual abundances. Individuals of a particular species exhibit strikingly similar size, yet both small and large species are represented. Assemblages occur in either high energy deposits of conglomeratic sandstone or oolite, or low energy concretionary, dark shales, but in neither case do they seem to have undergone post-mortem sorting. Ammonitellas and protoconchs are notably absent in the matrix preserving the ammonoids in both settings. These assemblages may represent sexually mature adults that experienced mass mortality following reproduction (semelparity) in a manner similar to that found in some modern squids.

ONTOGENY AND HETEROCHRONY IN THE MIDDLE CARBONIFEROUS AMMONOID ARKANITES RELICTUS (QUINN, McCALEB, AND WEBB) FROM NORTHERN ARKANSAS

Abstract The reticuloceratid ammonoid Arkanites relictus (Quinn, McCaleb, and Webb, 1962) is represented by hundreds to thousands of individuals from horizons isolated both stratigraphically and geographically in northern Arkansas. These assemblages appear to represent mass mortality events resulting from a semelparous reproductive strategy. Arkanites relictus occurs as a dimorphic pair (depressed, widely umbilicate, cadiconic conchs and compressed, narrowly umbilicate, pachyconic conchs) thought to reflect sexual dimorphism. Late stage ontogenetic modifications, such as septal crowding and change in aperture profile, are widely cited evidence of sexual maturity in ammonoids. Septal crowding begins at a predictable ontogenetic stage in the compressed forms of A. relictus, but specimens with cadiconic conchs do not have crowded septa even at the largest diameters available. Depending on the trait examined and the proxy for age of individuals, the dimorphism in Arkanites relictus (using the depressed form as the reference morph) is the result of acceleration, neoteny, or hypermorphosis plus neoteny. If size (diameter) is considered a proxy for age, the dimorphs were the same age at death, and the septa in the compressed variants developed via acceleration relative to the depressed variants. Regarding conch shape (width vs. diameter), the compressed morphs developed via neoteny relative to the depressed morphs. If septal count is considered a proxy for age, the dimorphs were not the same age at death, and the compressed forms were produced by a combination of hypermorphosis plus neoteny, i.e., they grew longer yet slower than the depressed forms. In A. relictus, the heterochronic processes of hypermorphosis and neoteny may have been operating simultaneously, which is an interesting possibility because it is an example of a combination of both peramorphic and paedomorphic processes.

Superstesaster promissor gen. et sp. nov., a new starfish (Echinodermata, Asteroidea) from the Early Triassic of Utah, USA, filling a major gap in the phylogeny of asteroids.

We describe Superstesaster promissor gen. et sp. nov., a starfish from the Smithian (Early Triassic) of Utah (USA) that fills a major gap in the fossil record of the Asteroidea. The post-Palaeozoic crown group Asteroidea are distinct from any of the diverse Palaeozoic forms. However, current understanding of the Palaeozoic–Mesozoic transition is blurred by a large gap in the fossil record between the Early Permian and the Middle Triassic. Building on the newly described taxon, a phylogenetic analysis investigates the relationships between Palaeozoic and Mesozoic Asteroidea. Including 30 species and 70 morphological characters, it is the most comprehensive phylogeny produced for fossil starfishes so far. Relationships among Palaeozoic forms remain poorly resolved, but their position in the tree is grossly consistent with stratigraphy. The tree topology implies the appearance of a wide range of morphologies during the Ordovician, a bottleneck during the end-Devonian events, and a second diversification during the Carboniferous, before a diversity decline at the end of the Permian. Superstesaster promissor nests above Palaeozoic taxa and appears as the sister group to the post-Palaeozoic Asteroidea. It represents the first record of a member of the stem group in the Mesozoic, and it likely reflects the ancestral morphology of the crown group. S. promissor shares with the crown group typical ambulacral and adambulacral plate shape and articulation. Phylogenetic relationships within the crown group remain poorly resolved, although three clades are consistent with molecular and morphological phylogenies available for extant forms: Valvatacea (Comptoniaster, Pentasteria, Advenaster, Noviaster), Forcipulatacea (Germanasterias, Argoviaster) and Velatida (Tropidaster, Protremaster). A Triassic and Jurassic radiation of the crown group is confirmed. Most Triassic and Jurassic forms do not share all synapomorphies with extant clades and usually represent separate clades or stem members of modern clades rather than true members of modern families. http://zoobank.org/urn:lsid:zoobank.org:pub:8CAC421A-DC0C-4EEB-B872-C7505FA8C2EA

Late Smithian microbial deposits and their lateral marine fossiliferous limestones (Early Triassic, Hurricane Cliffs, Utah, USA)

Recurrent microbialite proliferations during the Early Triassic are usually explained by ecological relaxation and abnormal oceanic conditions. Most Early Triassic microbialites are described as single or multiple lithological units without detailed ecological information about lateral and coeval fossiliferous deposits. Exposed rocks along Workman Wash in the Hurricane Cliffs (southwestern Utah, USA) provide an opportunity to reconstruct the spatial relationships of late Smithian microbialites with adjacent and contemporaneous fossiliferous sediments. Microbialites deposited in an intertidal to subtidal interior platform are intercalated between inner tidal flat dolosiltstones and subtidal bioturbated fossiliferous limestones. Facies variations along these fossiliferous deposits and microbialites can be traced laterally over a few hundreds of meters. Preserved organisms reflect a moderately diversified assemblage, contemporaneous to the microbialite formation. The presence of such a fauna, including some stenohaline organisms (echinoderms), indicates that the development of these late Smithian microbial deposits occurred in normal-marine waters as a simple facies belt subject to relative sea-level changes. Based on this case study, the proliferation of microbialites cannot be considered as direct evidence for presumed harsh environmental conditions.