Loren E. Babcock

The Chengjiang Biota: Record of the Early Cambrian Diversification of Life and Clues to Exceptional Preservation of Fossils

The Chengjiang Biota, from Yunnan, China, is the most diverse assemblage of Early Cambrian marine fossils known. Just like the celebrated Burgess Shale (Middle Cambrian) of British Columbia, Canada, Chengjiang preserves not only fossils having hard skeletal parts (which is typical of most sedimentary deposits), but it also preserves in exquisite detail nonmineralized skeletal parts and internal soft parts of organisms (which is much more unusual in sedimentary deposits). The Chengjiang deposit, and the somewhat younger Burgess Shale, both provide important guides to diversity and evolutionary rates during the early Phanerozoic diversification event known as the Cambrian “explosion.” The Chengjiang Biota bridges a critical time between decline of the Late Neoproterozoic (latest Precambrian) Ediacaran biota and the terminal Early Cambrian extinction, and provides further evidence that the Cambrian explosion is part of an evolutionary transition that began in the Late Neoproterozoic. As we seek to understand the circumstances surrounding exceptional preservation generally, not just during the Cambrian, Chengjiang provides an important perspective on depositional conditions. Interpretation of the preservation of Burgess Shale–type organisms has been long dominated by the Burgess Shale model, in which organisms were washed from an oxic environment, where they were living, into an anoxic environment, where they were quickly buried. Anoxia inhibited the destructive activity of biodegraders (scavengers, bacteria, and burrowers) and probably played a role in early diagenesis. Other deposits of exceptional preservation indicate that biodegraders were limited long enough for the early stages of fossilization to occur under at least two other circumstances. First, immobile benthic creatures could be smothered in place by rapidly deposited mud, and then preserved through early diagenetic activity mediated by anoxic conditions developed within the sediment. Second, in Chengjiang muds, exceptional preservation is inferred to have occurred in a restricted-shelf, shallow sea. Here, as in some Carboniferous deposits, factors related to tidally influenced shelf conditions limited the activity of biodegraders, and high sedimentation rates provided for quick burial. With the addition of new models for exceptional preservation in the Cambrian, this phenomenon should be viewed less as a result of extraordinary, one-time-only, depositional conditions, and more the result of minor or short-lived perturbations in depositional circumstances common to epeiric seas. Similar perturbations led to exceptional preservation in similar environments, but at different times, during the Phanerozoic. INTRODUCTION The Neoproterozoic-Cambrian transition was a time of fundamental change in the history of life. Between the Late Neoproterozoic (ca. 570 Ma) and the late Early Cambrian (ca. 510 Ma), representatives of most important, multicellular marine animals and plants had evolved sufficiently to leave a fossil record, and had undergone early experimentation with body plans and habitats. During the transition interval, predation had emerged as a significant factor in evolution (McMenamin, 1986; Babcock, 1993; Bengtson, 1994), and probably had a causal link, along with geochemical factors, to the appearance and later development of mineralized skeletons. By the Early Cambrian, a transition from a microbial matdominated sediment-water interface to a more blurry, burrowed interface in shallowmarine settings was well under way (Seilacher and Pfluger, 1994; Bottjer et al., 2000). This time of rapid diversification among marine animals, experimentation with new body plans, and shifting ecological setting, is referred to as the Cambrian “explosion.” In terms of macroscopic organisms, this rather protracted “event” is represented in the rock record by a transition (Bengtson, 1994; Grotzinger et al., 1995) from Neoproterozoic strata yielding nonmineralized Ediacaran-type organisms (Seilacher, 1989; Fedonkin, 1994; Narbonne, 1998), a few small, hard part–secreting organisms (Grant, 1990; Bengtson, 1994; Gehling and Rigby, 1996; Grotzinger et al., 2000), and a limited number of trace fossils (e.g., Corsetti and Hagadorn, 2000) to Lower Cambrian strata having an increasing array of fossils (Figs. 1, 2). Lowermost Cambrian strata (Nemakit-Daldynian to Tommotian stages) yield small, isolated plates (small shelly fossils, which are disarticulated multielement skeletons) and few trace fossils, but overlying strata of the Atdabanian and Botomian stages have more abundant and diverse fossils comprising shelly skeletons of invertebrates (mollusks, brachiopods, echinoderms, hyoliths, and reef-forming archaeocyathid sponges) and an increasing number of trace fossils (Fig. 2). In this interval, the rich fossil record of the Phanerozoic begins. By about 518 Ma, and in the midst of this dramatic biological change, siliciclastic muds in present-day Chengjiang County and surrounding areas of Yunnan Province, China (Figs. 3, 4), buried and preserved in great anatomical detail the remains of animals, plants, and macroscopic bacterial colonies that comprise the Chengjiang Biota (Figs. 1, 5, 6; Table 1). Biological changes that occurred during the Neoproterozoic-Cambrian transition closely followed major physical and chemical changes of global scale. The Late Neoproterozoic witnessed the breakup of Rodinia, and collisional events that resulted in partial assembly of Gondwana (Hoffman, 1992; Unrug, 1997; Karlstrom et al., 1999). The Chengjiang Biota:

The Geological Society of America Geologic Time Scale

The Geological Society of America has sponsored versions of the geologic time scale since 1983. Over the past 30 years, the Geological Society of America Geologic Time Scale has undergone substantial modifications, commensurate with major advances in our understanding of chronostratigraphy, geochronology, astrochronology, chemostratigraphy, and the geomagnetic polarity time scale. Today, many parts of the time scale can be calibrated with precisions approaching less than 0.05%. Some notable time intervals for which collaborative, multifaceted efforts have led to dramatic improvements in our understanding of the character and temporal resolution of key evolutionary events include the Triassic-Jurassic, Permian-Triassic, and Neoproterozoic-Phanerozoic boundaries (or transitions). In developing the current Geological Society of America Time Scale, we have strived to maintain a consistency with efforts by the International Commission on Stratigraphy to develop an international geologic time scale. Although current geologic time scales are vastly improved over the first geologic time scale, published by Arthur Holmes in 1913, we note that Holmes, using eight numerical ages to calibrate the Phanerozoic time scale, estimated the beginning of the Cambrian Period to within a few percent of the currently accepted value. Over the past 100 years, the confluence of process-based geological thought with observed and approximated geologic rates has led to coherent and quantitatively robust estimates of geologic time scales, reducing many uncertainties to the 0.1% level. (Less)

Global Standard Stratotype-section and Point (GSSP) of the Furongian Series and Paibian Stage (Cambrian)

The Global Standard Stratotype-section and Point (GSSP) of the Furongian Series (uppermost series of the Cambrian System) and the Paibian Stage (lowermost stage of the Furongian Series), has been recently defined and ratified by the International Union of Geological Sciences (IUGS). The boundary stratotype is 369 metres above the base of the Huaqiao Formation in the Paibi section, northwestern Hunan Province, China. This point coincides with the first appearance of the cosmopolitan agnostoid trilobite Glyptagnostus reticulatus, and occurs near the base of a large positive carbon isotopic excursion (SPICE excursion).

Reevaluation of the timing and extent of late Paleozoic glaciation in Gondwana: Role of the Transantarctic Mountains

Evidence from Antarctica indicates that a 2000-km-long section of the Transantarctic Mountains—including Victoria Land, the Darwin Glacier region, and the central Transantarctic Mountains—was not located near the center of an enormous Car- boniferous to Early Permian ice sheet, as depicted in many paleo- geographic reconstructions. Weathering profiles and soft-sediment deformation immediately below the preglacial (pre-Permian) un- conformity suggest an absence of ice cover during the Carbonif- erous; otherwise, multiple glacial cycles would have destroyed these features. The occurrence of glaciotectonite, massive and strat- ified diamictite, thrust sheets, sandstones containing dewatering structures, and lonestone-bearing shales in southern Victoria Land and the Darwin Glacier region indicate that Permian sedimenta- tion occurred in ice-marginal, periglacial, and/or glaciomarine set- tings. No evidence was found that indicates the Transantarctic Mountains were near a glacial spreading center during the late Paleozoic. Although these findings do not negate Carboniferous glaciation in Antarctica, they do indicate that Gondwanan glacia- tion was less widespread, and, therefore, that glacially driven changes to other Earth systems (i.e., glacioeustatic fluctuations, cli- mate) were much smaller than previously hypothesized.

Trilobite malformations and the fossil record of behavioral asymmetry

Malformations of trilobites are classified as healed injuries, teratological conditions, and pathological conditions. An improved method of recognizing such malformations combines information about the conditions under which cell injury can occur, the processes by which animal tissues react to injury, and trilobite morphology. Study of healed injuries of polymeroid trilobites shows that injuries attributed to sublethal predation tend to be most commonly preserved on the pleural lobes, the posterior half of the body, and the right side. Statistically significant differences in the number of predation scars between the right and left sides is interpreted as evidence of right-left behavioral asymmetry in some predators of trilobites or the trilobites themselves. Asymmetrical, or lateralized, behavior in present-day animals is one manifestation of handedness, and is usually related to a functional lateralization of the nervous system. Evidence of behavioral lateralization in some Paleozoic predators or prey suggests that those organisms also possessed lateralized nervous systems. Right-left differences in preserved predation scars on trilobites date from the Early Cambrian ( Olenellus Zone), and are the oldest known evidence of behavioral asymmetry in the fossil record. Other examples of structural or behavioral asymmetry from the fossil record of animals are cited. Lateralization is recognized in representatives of the Arthropoda, Annelida, Bryozoa, Echinodermata, Cnidaria, Mollusca, Chordata, and Conodonta, and in trace fossils.

Tetrapod and Large Burrows of Uncertain Origin in Triassic High Paleolatitude Floodplain Deposits, Antarctica

Abstract Two types of large diameter burrows, recognized by non-overlapping size distributions, occur in high paleolatitude floodplain deposits of the Lower Triassic Fremouw Formation, Shackleton Glacier area, Antarctica. Type G (giant) burrows are gently dipping tunnels 8 to 19 cm in diameter. Type L (large) burrows are 2 to 6.5 cm in diameter, curved or subhorizontal tunnels that rarely branch; scratch markings on both burrow types generally are parallel or tangential to the long axis of the burrows. Type G burrows are interpreted as produced by tetrapods based on similarity in size, architecture, and surface markings to Permian burrows from South Africa that contain complete skeletons of therapsids. These are the first tetrapod burrows described from Antarctica. Type L burrows have characteristics of both fossil tetrapod and crayfish burrows, precluding identification of an unique producer. Triassic tetrapods, including therapsids, that lived in high southern latitudes probably burrowed to dampen the effects of seasonal environmental fluctuations, just as do many of their mammalian counterparts living today in high latitudes. The paleolatitudinal and paleooclimatic distributions of burrowing therapsids and their mammalian descendents can be assessed by focusing search efforts on very large burrows, and by identifying producers using criteria delineated herein; this will clarify the extent to which the burrowing habit originated and persisted in high latitudes.

Trilobites in Paleozoic Predator-Prey Systems, and Their Role in Reorganization of Early Paleozoic Ecosystems

Predation is a fundamental ecological process that has profound effects on the morphology, distribution, abundance, and evolution of metazoans. The earliest verified records of predation date to the Neoproterozoic-Cambrian transition interval (e.g., Conway Morris and Jenkins, 1985; Babcock, 1993a; Bengtson and Yue, 1992; Bengtson, 1994; Conway Morris and Bengtson, 1994; Nedin, 1999; Jago and Haines, in press), but the impact of predation almost certainly has a much deeper evolutionary history. Among the earliest and most widespread lines of evidence for the importance of predation in the early Paleozoic comes from the record of trilobites. As biomineralized animals, trilobites have left an excellent fossil record that extends from the latter part of the Early Cambrian (e.g., Zhang, 1987; Geyer, 1996, Geyer, 1998; Geyer and Palmer, 1995; Luo and Jiang, 1996; Hollingsworth, 1999; Geyer and Shergold, 2000; Peng and Babcock, 2000; Peng and Babcock, 2001), c. 520 Ma, to the end of the Permian (e.g., Brezinski, 1992), c. 248 Ma. Predation on and by trilobites evidently exerted influence on the morphological development of metazoans, as well as on ecosystem development, through the Paleozoic.

Paleolimulus, an early limuline (Xiphosurida), from Pennsylvanian‐Permian Lagerst tten of Kansas and taphonomic comparison with modern Limulus

The Pennsylvanian-Permian horseshoe crab Paleolimulus signatus (Beecher), incorporating as a junior synonym P. avitus Dunbar, is one of the earliest species of the Limulina (Xiphosurida). Some specimens from Kansas, USA, are exceptionally well preserved, retaining intact book gills and appendages. Indistinct, bilobed burrowing traces of variable width occur in association with some examples of P. signatus and may have been produced by that animal. Based on actualistic taphonomic experiments on Limulus polyphemus, ancient horseshoe crabs and other arthropods having non-mineralized exoskeletons are inferred to have become pliable soon after death or moulting, and to have disarticulated slowly prior to burial. Extreme compression, wrinkling, and loose folding of sclerites are attributed to burial of a pliable exoskeleton. Slow preburial disarticulation partly accounts for the exceptional preservation of Paleolimulus remains. Also relevant for the exceptional preservation of these arthropods was burial in estuarine, tidal flat, or lacustrine environments. Because of fluctuating salinity and possibly dessicating conditions, these settings were limiting to scavengers, burrowers, and some microbes that could potentially disarticulate or decompose xiphosurid remains.

A Parvancorina-like arthropod from the Cambrian of South China

Constraining the origin of animal groups is allowed, to some extent, by discoveries of Cambrian Lagerstätten that preserve both mineralizing and nonmineralizing organisms. A new species is reported here of the Cambrian arthropod Skania, which bears an exoskeleton that shares homologies with the Neoproterozoic (Ediacaran) organism Parvancorina and firmly establishes a Precambrian root for arthropods. A new monophyletic group, Parvancorinomorpha, is proposed as the first clade within the arthropod crown group demonstrably ranging across the Neoproterozoic–Paleozoic transition. The Parvancorinomorpha is interpreted to be the sister group of the Arachnomorpha. Incipient cephalization in Skania and related genera represents a step in the progression toward division of a cephalon from a large posterior trunk as shown in Cambrian arachnomorphs such as naraoiids and the addition of a pygidium and thoracic tergites as shown in the arachnomorph clade basal to trilobites. This evidence can serve as a new calibration point for estimating the divergence time for the last common ancestor of arthropods and priapulids based on molecular clock methods.

Morphology-Based Phylogenetic Analysis of the Clawed Lobsters (Family Nephropidae and the New Family Chilenophoberidae)

ABSTRACT The phylogeny of extinct and extant clawed lobsters is interpreted using a morphology-based phylogenetic analysis. Twenty-one genera, representing two clades, are grouped into either the redefined family Nephropidae or the new family Chilenophoberidae. Chilenophoberids, the primitive sister group of nephropids, share a close common ancestry with members of the Erymidae, and arose by the Middle Jurassic. Nephropids arose by the Early Cretaceous. Among clawed lobsters, no one morphological character is generally more reliable than any other as a guide to phylogeny. Homoplasy in aspects of groove pattern, ornamentation, and the appendages is widespread. The ubiquity of homoplasy in lobster evolution argues against the practice of erecting suprageneric taxonomic groups based on a few, intuitively selected characters. Cretaceous lobsters strongly resemble Recent ones, and no long-term, directional evolutionary trends were detected.