Allison C. Daley

Molecular Timetrees Reveal a Cambrian Colonization of Land and a New Scenario for Ecdysozoan Evolution

Ecdysozoans have been key components of ecosystems since the early Cambrian, when trilobites and soft-bodied Burgess Shale-type ecdysozoans dominated marine animal communities. Even today, the most abundant animals on Earth are either nematode worms or plankton-forming crustaceans, whereas the most diverse are the insects. Throughout geological time, several ecdysozoan lineages independently colonized land, shaping both marine and terrestrial ecosystems and providing an adequate environment for successive animal terrestrialization. The timing of these events is largely uncertain and has been investigated only partially using molecular data. Here we present a timescale of ecdysozoan evolution based on multiple molecular data sets, the most complete set of fossil calibrations to date, and a thorough series of validation analyses. Results converge on an Ediacaran origin of all major ecdysozoan lineages (∼587-543 million years ago [mya]), followed by a fast Cambrian radiation of the pancrustaceans (∼539-511 mya), a Cambro-Ordovician colonization of land of different arthropod lineages (∼510-471 mya), and a relatively recent radiation of extant nematodes, onychophorans, and tardigrades (∼442 mya). Arthropods colonized land nearly synchronously with land plants. Further diversification within flying insects, nematodes and onychophorans might be related to the evolution of vascular plants and forests.

The Burgess Shale Anomalocaridid Hurdia and Its Significance for Early Euarthropod Evolution

As the largest predators of the Cambrian seas, the anomalocaridids had an important impact in structuring the first complex marine animal communities, but many aspects of anomalocaridid morphology, diversity, ecology, and affinity remain unclear owing to a paucity of specimens. Here we describe the anomalocaridid Hurdia, based on several hundred specimens from the Burgess Shale in Canada. Hurdia possesses a general body architecture similar to those of Anomalocaris and Laggania, including the presence of exceptionally well-preserved gills, but differs from those anomalocaridids by possessing a prominent anterior carapace structure. These features amplify and clarify the diversity of known anomalocaridid morphology and provide insight into the origins of important arthropod features, such as the head shield and respiratory exites.

A new Burgess Shale–type assemblage from the “thin” Stephen Formation of the southern Canadian Rockies

A new Burgess Shale–type assemblage, from the Stephen Formation of the southern Canadian Rocky Mountains, is described herein. It occurs near Stanley Glacier in Kootenay National Park, 40 km southeast of the type area near Field, British Columbia. While at least a dozen Burgess Shale localities are known from the “thick” Stephen Formation, the Stanley Glacier locality represents the first discovery of Burgess Shale–type fossils from the “thin” Stephen Formation. The Cathedral Escarpment, an important regional paleotopographic feature, has been considered important to the paleoecologic set- ting and the preservation of the Burgess Shale biota. However, the Stanley Glacier assemblage was preserved in a distal ramp setting in a region where no evidence of an escarpment is present. The low- diversity assemblage contains eight new soft-bodied taxa, including the anomalocaridid Stanleycaris hirpex n. gen., n. sp. (new genus, new species). Pelagic or nektobenthic predators represent the most diverse group, whereas in relative abundance, the assemblage is dominated by typical Cambrian shelly benthic taxa. The low diversity of both the benthic taxa and the ichnofauna, which includes diminutive trace fossils associated with carapaces of soft-bodied arthropods, suggests a paleoenvironment with restrictive conditions. The Stanley Glacier assemblage expands the temporal and geographic range of the Burgess Shale biota in the southern Canadian Rockies, and suggests that Burgess Shale–type assemblages may be common in the “thin” Ste- phen Formation, which is regionally widespread.

Morphology of Anomalocaris canadensis from the Burgess Shale

Abstract Recent description of the oral cone of Anomalocaris canadensis from the Burgess Shale (Cambrian Series 3, Stage 5) highlighted significant differences from published accounts of this iconic species, and prompts a new evaluation of its morphology as a whole. All known specimens of A. canadensis, including previously unpublished material, were examined with the aim of providing a cohesive morphological description of this stem lineage arthropod. In contrast to previous descriptions, the dorsal surface of the head is shown to be covered by a small, oval carapace in close association with paired stalked eyes, and the ventral surface bears only the triradial oral cone, with no evidence of a hypostome or an anterior sclerite. The frontal appendages reveal new details of the arthrodial membranes and a narrower cross-section in dorsal view than previously reconstructed. The posterior body region reveals a complex suite of digestive, respiratory, and locomotory characters that include a differentiated foregut and hindgut, a midgut with paired glands, gill-like setal blades, and evidence of muscle bundles and struts that presumably supported the swimming movement of the body flaps. The tail fan includes a central blade in addition to the previously described three pairs of lateral blades. Some of these structures have not been identified in other anomalocaridids, making Anomalocaris critical for understanding the functional morphology of the group as a whole and corroborating its arthropod affinities.

The oral cone of Anomalocaris is not a classic ‘‘peytoia’’

The Cambro-Ordovician anomalocaridids are large ecdysozoans commonly regarded as ancestors of the arthropods and apex predators. Predation is indicated partly by the presence of an unusual “peytoia”-type oral cone, which is a tetraradial outer ring of 32 plates, four of which are enlarged and in perpendicular arrangement. This oral cone morphology was considered a highly consistent and defining characteristic of well-known Burgess Shale taxa. It is here shown that Anomalocaris has a different oral cone, with only three large plates and a variable number of smaller and medium plates. Its functional morphology suggests that suction, rather than biting, was used for food ingestion, and that anomalocaridids in general employed a range of different scavenging and predatory feeding strategies. Removing anomalocaridids from the position of highly specialized trilobite predators forces a reconsideration of the ecological structure of the earliest marine animal communities in the Cambrian.

Morphology and systematics of the anomalocaridid arthropod Hurdia from the Middle Cambrian of British Columbia and Utah

In Cambrian fossil Lagerstätten like the Burgess Shale, exceptionally preserved arthropods constitute a large part of the taxonomic diversity, providing opportunities to study the early evolution of this phylum in detail. The anomalocaridids, large presumed pelagic predators, are particularly relevant owing to their unique combination of morphological characters and basal position in the arthropod stem lineage. Although isolated elements and fragmented specimens were first discovered over 100 years ago, subsequent findings of more complete bodies of Anomalocaris and Peytoia, especially in the 1980s, allowed for a better understanding of these enigmatic forms. Their evolutionary significance as stem group arthropods was further clarified by the recent discovery of a third anomalocaridid taxon, Hurdia. Here, examination of hundreds of Hurdia specimens from different stratigraphical layers within the Burgess Shale and Stephen Formation, combined with statistical analyses, provides a detailed description of the taphonomy, morphology and diversity of the genus and further elucidates anomalocaridid systematics. Hurdia is distinguished from other anomalocaridids in having mouthparts with extra rows of teeth, a large frontal carapace complex and diminutive swimming flaps with prominent setal structures. The two original species, H. victoria Walcott, 1912 and H. triangulata Walcott, 1912, are confirmed based on morphometric outline analyses of the frontal carapace components combined with stratigraphical evidence; a third species, Hurdia dentata Simonetta & Delle Cave, 1975, is synonymized with H. victoria. Morphology, preservation and stratigraphical distribution suggest that H. victoria and H. triangulata share the same type of frontal appendage; a second type of appendage, previously assigned to Hurdia (Morph A), belongs to Peytoia nathorsti. These and other morphological differences between the anomalocaridids may reflect different feeding strategies. Appendages and mouthparts of Hurdia indet. sp. are also identified from the Spence Shale Member of Utah, making Hurdia and Anomalocaris the most common and globally distributed anomalocaridid taxa.

A POSSIBLE ANOMALOCARIDID FROM THE CAMBRIAN SIRIUS PASSET LAGERSTATTE, NORTH GREENLAND

Abstract The Sirius Passet biota of North Greenland is one of the oldest Cambrian lagerstätten, and although it is dominated by non-mineralized arthropods and lobopods, anomalocaridids have never been identified. Based on a single specimen, we herein describe for the first time an appendage with possible anomalocaridid affinities as suggested by an overall gross morphology similar to that of the frontal appendage of Anomalocaris from other localitites. Tamisiocaris borealis n. gen. and n. sp. has an elongated appendage with paired spines along one margin, and differs from the frontal appendage of Anomalocaris in that segment boundaries are absent and ventral spines are relatively long and spineless. These differences may be taphonomic, but the entire surface of the appendage is covered in a fine fabric, making it unlikely that this appendage was originally segmented or sclerotized. The taxon is tentatively placed within Radiodonta, but this systematic placement cannot be confirmed while complete body specimens are lacking.

Morphology of the radiodontan Lyrarapax from the early Cambrian Chengjiang biota

Abstract. The recently described radiodontan Lyrarapax unguispinus Cong et al., 2014 from the Chengjiang biota (Cambrian Series 2, Stage 3) highlighted a new morphological type of frontal appendage and unique mouth structures, a functional combination reinforcing the diversification of feeding strategies of radiodontans during the early Cambrian. Here we describe Lyrarapax trilobus n. sp. from the same fossil Konservat-Lagerstätte. The new species differs from L. unguispinus in the morphology and distribution of endites on the frontal appendage and the strengthening structure of the body flaps. The two species resemble each other in body shape (pattern of flap size), neck segment number, cephalic plates, and most importantly a mouth characterized by concentric wrinkled furrows. The latter confirms that a soft mouth without sclerotized plates is a real feature of Lyrarapax and supports the idea that oral structures provide valid diagnostic characters within Radiodonta.

The fossil record of ecdysis, and trends in the moulting behaviour of trilobites.

Ecdysis, the process of moulting an exoskeleton, is one of the key characters uniting arthropods, nematodes and a number of smaller phyla into Ecdysozoa. The arthropod fossil record, particularly trilobites, eurypterids and decapod crustaceans, yields information on moulting, although the current focus is predominantly descriptive and lacks a broader evolutionary perspective. We here review literature on the fossil record of ecdysis, synthesising research on the behaviour, evolutionary trends, and phylogenetic significance of moulting throughout the Phanerozoic. Approaches vary widely between taxonomic groups, but an overall theme uniting these works suggests that identifying moults in the palaeontological record must take into account the morphology, taphonomy and depositional environment of fossils. We also quantitatively analyse trends in trilobite ecdysis based on a newly generated database of published incidences of moulting behaviour. This preliminary work reveals significant taxonomic and temporal signal in the trilobite moulting fossil record, with free cheek moulting being prevalent across all Orders and throughout the Phanerozoic, and peaks of cephalic moulting in Phacopida during the Ordovician and rostral plate moulting in Redlichiida during the Cambrian. This study and a review of the literature suggest that it is feasible to extract large-scale evolutionary information from the fossil record of moulting.

A morphological and taxonomic appraisal of the oldest anomalocaridid from the Lower Cambrian of Poland

Material previously referred to as Cassubia infercambriensis was re-examined and found to represent a composite fossil of a Peytoia -like anomalocaridid frontal appendage and an arthropod of uncertain affinities comparable to some bivalved arthropod taxa. The frontal appendage is referred to the genus Peytoia based on the presence of elongated ventral spines bearing a single row of auxiliary spines. As well as representing the oldest example of an anomalocaridid in the fossil record (Series 2, Stage 3), Peytoia infercambriensis is also the only record of this group from the East European Craton, therefore extending both the temporal and geographic range of the anomalocaridid family Hurdiidae.