Stephen Q. Dornbos

A new ‘great‐appendage’ arthropod from the Lower Cambrian of China and homology of chelicerate chelicerae and raptorial antero‐ventral appendages

The uniramous ‘great appendages’ of several arthropods from the Early to Middle Cambrian are a characteristic pair of pre-oral limbs, which served for prey capture. It has been assumed that the morphological differences between the ‘great-appendage’ arthropods indicate that raptorial antero-ventral and anteriorly pointing appendages evolved more than once in arthropod phylogeny. One set of Cambrian ‘great-appendage’ arthropods has, however, very similar short antero-ventral appendages with a peduncle of two segments angled against each other (elbowed) and with stout distally or medio-distally directed spines or long flexible flagellate spines on each of the four distal segments. Moreover, the head appendages of all these forms comprise the ‘great appendages’ and three pairs of biramous limbs. To this set of taxa we can add a new form from the Lower Cambrian Maotianshan Shale of southern China, Haikoucaris ercaiensis n. gen. and n. sp. It is known from three specimens, possibly being little abundant in the faunal community. It can be distinguished from all other taxa by the prominence of the proximal claw segment of its ‘great appendages’ and by only three distal spines (one on each of the distal segments). The similarity of the short, spiky ‘great appendages’ of Haikoucaris with the chelicera of the Chelicerata leads us to hypothesize that this particular type of ‘great appendages’ was the actual precursor of the chelicera. Homeobox gene and developmental data recently demonstrated the homology between the antenna of ateloceratans and the antennula of crustaceans on one side and the chelicera of chelicerates on the other. To this we add palaeontological evidence for the homology between the chelicerae of chelicerates and the ‘short great appendages’ of certain Cambrian arthropods, which leads us to hypothesize that the evolutionary path went from the ‘short great appendages’, by progressive compaction, toward the chelicera with only a two-spined chela. The new form from China is regarded as the possible latest offshoot, whereas the other ‘great appendages’ arthropods with similar short grasping limbs were derivatives of the stem lineage of the crown-group Chelicerata. Consequently, the chelicera with a chela with one fixed and one mobile finger is an autapomorphy of the crown group of Chelicerata, whereas a raptorial, but more limb-like antenna, with more distal spine-bearing segments, characterized the ground pattern of Chelicerata. Further taxa having ‘great appendages’, including the large Anomalocarididae, are also discussed in the light of their possible affinities to the Chelicerata and possible monophyly of all of these arthropods with raptorial anterior appendages.

ASSESSING THE ECOLOGICAL DOMINANCE OF PHANEROZOIC MARINE INVERTEBRATES

Abstract Ecological studies have revealed that the functional roles of dominant species in modern communities are often more important than overall diversity in governing community composition and functioning. Despite this recognition that abundance and diversity data are both required for a complete understanding of ecological processes, many paleoecological studies focus on presence-absence data, possibly because of concerns regarding the taphonomic fidelity of time-averaged fossil accumulations. However, the abundance of organisms in shell beds has been shown to provide a fairly accurate record of the living community, suggesting that the benefits of relative-abundance data should be reconsidered. Recognition of ecologically dominant species in local fossil assemblages should be based on counts of relative abundance and assessment of ecological role. Ecological dominance at larger spatial or temporal scales can be quantified using the mean rank order of a clade and the proportion of assemblages where the clade is present, providing unbiased, quantitative values for measuring the ecological importance of a clade. Their utility has been tested with three case studies encompassing a range of geographic and taxonomic scales, using a database of 1221 Ordovician–Paleogene quantitative fossil collections. The dominance metrics for rhynchonelliform brachiopods, bivalves, and gastropods broadly parallel anecdotal trends, even including some more detailed patterns documented by regional studies. An examination of substrate preferences for brachiopod and bivalve orders confirms the abundance of infaunal bivalves in siliciclastics and epifaunal bivalves in carbonates, but it also reveals intriguing patterns regarding substrate preferences among rhynchonelliform brachiopod orders. The final case study analyzed changes in dominance between early Mesozoic fossil assemblages from Tethys and Panthalassa, documenting significant geographic differences in the ecological importance of rhynchonelliform brachiopods and bivalves. These large-scale dominance patterns often approximately matched those inferred from diversity trends; however, there are also times when dominance was decoupled from diversity, indicating that further investigation of ecological dominance will provide additional insights into ecological influences on the Phanerozoic history of life. “Are most species simply passengers in ecosystems that are run basically by a few dominants?” (Worm and Duffy, 2003, p. 631)

Information landscapes and sensory ecology of the Cambrian Radiation

Abstract Organisms emit, detect, and respond to a huge array of environmental signals. The distribution of a given signal is dependent, first of all, upon the original spatial distribution of signal sources, the source landscape. The signal sources can be fixed or moving and their output can be stable or ephemeral. Different sources can also occupy the same general spatial location, such as insects living on a host plant. The emitted signals are modified by relevant transport processes, which are often strongly scale and environment dependent. Chemical signals, for example, are propagated by diffusion and turbulence. The resulting complex, three-dimensional, and dynamic distribution of signals in the environment is the signal landscape; it is the environment of potentially available information in which sensory systems function and have evolved. Organisms also differ widely in what signals they can actually detect; the distribution of signals that an organism can potentially respond to is its information landscape. Although increasing the kinds and specificity of signals that can be detected and processed can lead to improved decision making, it almost always comes at an increased cost. The greater the spatial and temporal complexity of the environment, the greater are the costs of incomplete information and the more advantageous is the development of improved information-gathering capabilities. Studies with simulation models suggest how variability in the spatial structure of source and signal landscapes may control patterns of animal movement that could be represented in the trace fossil record. Information landscapes and the corresponding sensory systems should have evolved in concert with major transitions in the history of life. The Ediacaran to Cambrian interval is one of the most intensively studied periods in the history of life, characterized by the profound environmental and biological changes associated with the bilaterian radiation. These include the advent of macroscopic predation, an increase in the size and energy content of organisms, and the transition in seafloors from laminated matgrounds to mixgrounds produced by the development of macroscopic infaunal bioturbation. The overall effect of these transitions was to markedly increase the spatial complexity of the marine environment. We suggest that this increased spatial complexity, in turn, drove the evolution of macroscopic sense organs in mobile bilaterians, leading to their first appearance during the Cambrian. The morphology and distribution of these sense organs should reflect the life habits of the animals that possessed them. Our overall hypothesis was that there was a “Cambrian Information Revolution,” a coevolutionary increase in the information content of the marine environment and in the ability of and necessity for organisms to obtain and process this information. A preliminary analysis of the Maotianshan Shale (Chengjiang) biota indicates that the distribution of eyes and antennae in these animals is consistent with predictions based on their life habit.

Phosphatization Through the Phanerozoic

Phosphatization of soft tissues and skeletal remains has varied temporally and taxonomically through the Phanerozoic. During the Cambrian through early Ordovician, microscopic arthropods and animal embryos were preferentially preserved. Phosphatization was uncommon during the rest of the Paleozoic, as recalcitrant tissues of a few taxa were preserved in hospitable microenvironments. The Cretaceous through Eocene saw another expansion of phosphatization, with a strong bias toward fish remains already enriched in apatite. Throughout its Phanerozoic history, phosphatization exhibited a taphonomic bias toward taxa with recalcitrant tissues that could resist the early stages of organic decay, taxa with organic structures already enriched in calcium phosphate, and, in many cases, taxa with small body sizes. The pulse of phosphatization during the Cambrian through Early Ordovician may have been facilitated by the generally lower levels of mixed layer development in the upper few centimeters of seafloor sediments during that time period, whereas the Cretaceous through Eocene increase in phosphatization was possibly related to the enlargement of euxinic epicontinental seaways.

A new Burgess Shale-type deposit from the Ediacaran of western Mongolia

Preservation of soft-bodied organisms is exceedingly rare in the fossil record. One way that such fossils are preserved is as carbonaceous compressions in fined-grained marine sedimentary rocks. These deposits of exceptional preservation are known as Burgess Shale-type (BST) deposits. During the Cambrian Period, BST deposits are more common and provide a crucial view of early animal evolution. The earliest definitive fossil evidence for macroscopic animal-grade organisms is found in the preceding Ediacaran Period. BST deposits from the Ediacaran are rarer and lack conclusive evidence for animals. Here we report the discovery of a new Ediacaran BST deposit with exceptional preservation of non-mineralizing macro-organisms in thinly bedded black shale from Zavkhan Province, western Mongolia. This fossil assemblage, here named the Zuun-Arts biota, currently consists of two new species of probable macroscopic multicellular benthic algae. One species, Chinggiskhaania bifurcata n. gen., n. sp., dominates the biota. The other species, Zuunartsphyton delicatum n. gen., n. sp., is known from three specimens. SEM-EDS analysis shows that the fossils are composed of aluminosilicate clay minerals and some carbon, a composition comparable to fossils from the Cambrian Burgess Shale biota. This discovery opens a new window through which to view late Precambrian life.

Substrate adaptations of sessile benthic metazoans during the Cambrian radiation

Abstract. Many marine benthic metazoans must stabilize themselves upon the seafloor for survival, and as a result their morphologies are controlled in part by local substrate conditions. The Agronomic Revolution (AR), spurred by increasing vertical bioturbation during the Ediacaran—Cambrian transition, permanently altered the nature of shallow marine substrate conditions and led to a major shift in adaptive strategies among benthic metazoans. These ecological and evolutionary changes, known as the Cambrian Substrate Revolution (CSR), are generally understood from observations of benthic metazoan fossils across the Ediacaran/Cambrian boundary, but the timing and geographic extent of this transition are less well known. This analysis attempts to constrain the temporal and spatial pattern of the AR and CSR by performing a global-scale paleoecological analysis of the adaptive strategies of benthic fauna living during the Cambrian. This analysis focused on Burgess Shale-type (BST) faunas because of their exceptional preservation, and was conducted through direct observation of fossil specimens, analysis of data compiled from the Paleobiology Database, and literature review. From these analyses, faunal groups are assigned a metric, the Substrate Adaptability Index (SAI), that relates the overall affinity the fauna demonstrates toward either Proterozoic-style (SAI=0) or Phanerozoic-style (SAI= 1) substrate conditions. The results of this analysis demonstrate that most early and middle Cambrian faunas were mixtures of Phanerozoic- and Proterozoic-style adaptive strategists, suggesting that Proterozoic-style substrates were still influential in controlling adaptive strategies in marine environments until at least that time. This is further supported by ichnofabric analysis of many of these localities, where overall bioturbation levels are exceedingly low, indicating a lack of mixed-layer development and the prevalence of firm Proterozoic-style substrates well into the Cambrian.

PALEOECOLOGY OF THE MIDDLE CAMBRIAN EDRIOASTEROID ECHINODERM TOTIGLOBUS: IMPLICATIONS FOR UNUSUAL CAMBRIAN MORPHOLOGIES

Abstract The spheroidal edrioasteroid echinoderm Totiglobus, preserved in the middle Cambrian Chisholm Shale of eastern Nevada, was one of many interesting morphologies to develop during the period of rapid evolutionary diversification in the early and middle Cambrian. To understand more about how the unusual morphology of Totiglobus evolved, a detailed examination of fossil specimens and the strata in which they are preserved was performed. A total of 263 specimens were examined and placed into taphonomic categories based on their preserved orientation relative to the seafloor. Seventy-two well-preserved specimens were used for statistical analysis. Of these, 49% were preserved with the aboral surface down. The preference for preservation in this category, with the aboral surface (which contains a suctorial attachment disc) oriented directly on top of the sediment, was statistically significant (p < 0.025). These data support the hypothesis that Totiglobus lived attached to the seafloor through suction. Rock samples were also collected from seven localities in the Chisholm Shale and the bioturbation levels were determined using the ichnofabric index (ii) method. A total of 48 samples, comprising 4.09 m of strata, were collected and x-radiographed. Ichnofabric index (ii) data recorded from these rocks revealed extremely low bioturbation levels (∼ii  =  1) with no mixed layer development (1.01 average ii). These results likewise support the sediment attachment hypothesis for Totiglobus. This study indicates that some unusual Cambrian morphologies, including that of Totiglobus, evolved partly in response to the presence of nonactualistic seafloor conditions and are therefore not simply early evolutionary experiments.

PUNCTUATED GROWTH OF MICROBIAL CONES WITHIN EARLY CAMBRIAN ONCOIDS, BAYAN GOL FORMATION, WESTERN MONGOLIA

Abstract Oxygen bubbles produced during photosynthesis internally deform filamentous cyanobacterial mats, producing distinctive fenestral patterns. Similar textures preserved in ancient microbialites are useful biosignatures when filaments are no longer preserved, but have typically been observed within stromatolites. This study describes bubble-associated fenestrae within oncoids from the early Cambrian Bayan Gol Formation of Mongolia. Fenestrae appear in mm-scale micritic laminae which contain dense accumulations of large (10 × 300 μm) filamentous Girvanella microfossils. Many laminae are not spherical, often occurring with one flat side opposite a conical peak. Up to six generations of conical geometry are present, with each cone rotated with respect to the previous peak. We hypothesize that the oncoids experienced intermittent disturbances followed by periods of stasis and vertical growth. During resting periods, we hypothesize that flat areas formed the oncoid resting base and peaked areas the top. The presence of bubble laminae within peaks implies formation in part via entrapment of microbially produced gases. Examples of resting oncoids growing into stromatolites are well known, as well as irregularly laminated oncoids with no cones; the Bayan Gol Formation samples are intermediate between typical spherical oncoids and stromatolites. The preservation of cones also provides evidence for relatively rapid mineralization in the Cambrian ocean, as antecedent microbial tufts would likely have collapsed if disturbed before calcification.

Penetrative trace fossils from the late Ediacaran of Mongolia: early onset of the agronomic revolution

The Cambrian radiation of complex animals includes a dramatic increase in the depth and intensity of bioturbation in seafloor sediment known as the ‘agronomic revolution’. This bioturbation transition was coupled with a shift in dominant trace fossil style from horizontal surficial traces in the late Precambrian to vertically penetrative trace fossils in the Cambrian. Here we show the existence of the first vertically penetrative trace fossils from the latest Ediacaran: dense occurrences of the U-shaped trace fossil Arenicolites from late Precambrian marine carbonates of Western Mongolia. Their Ediacaran age is established through stable carbon isotope chemostratigraphy and their occurrence stratigraphically below the first appearance of the trace fossil Treptichnus pedum. These Arenicolites are large in diameter, penetrate down to at least 4 cm into the sediment, and were presumably formed by the activity of bilaterian animals. They are preserved commonly as paired circular openings on bedding planes with maximum diameters ranging up to almost 1 cm, and as U- and J-shaped tubes in vertical sections of beds. Discovery of these complex penetrative trace fossils demonstrates that the agronomic revolution started earlier than previously considered.