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Tang et al. (2019) described new specimens of carbonaceous compression fossils from the early Cambrian Hetang Formation in South China, for which they established the new taxon Cambrowania ovata Tang and Xiao in Tang et al., 2019. Tang et al. (2019) interpreted these fossils as the remains of metazoans, representing either the carapaces of bivalve arthropods, or early life-cycle stages of sponges. We contest the animal affinity of these fossils; instead, we propose that the specimens described as Cambrowania ovata are actually large Leiosphaeridia—in other words, collapsed hollow organic spheroidal acritarchs. The features described by Tang et al. (2019) all fall into the morphology of carbonaceous compressions of Leiosphaeridia with pyritized/baritized folds and compaction wrinkles. Such Leiosphaeridia are a common component of Cambrian (and older) siliciclastic deposits, and frequently exhibit such a pattern of pyritization, baritization, and encrustation with other diagenetic minerals.
Gastropods often show signs of unsuccessful attacks by durophagous predators in the form of healed scars in their shells. As such, fossil gastropods can be taken as providing a record of predation through geological time. However, interpreting the number of such scars has proved to be problematic—Would a low number of scars mean a low rate of attack or a high rate of success, for example? Here we develop a model of population dynamics among individuals exposed to predation, including both lethal and nonlethal attacks. Using this model, we calculate the equilibrium distributions of ages and healed scars in the population and among fossilized specimens, based on the assumption that predation is independent of age or scar number. Based on these results, we formally show that the rates of attack and success cannot be disambiguated without further information about population structure. Nevertheless, by making the assumptions that the non-durophagous predatory death rate is both constant and low, we show that it is possible to use relatively small assemblages of gastropods to produce accurate estimates of both attack and success rates, if the overall death rate can be estimated. We consider likely violations of the assumptions in our model and what sort of information would be required to solve this problem in these more general cases. However, it is not easy to extract the relevant information from the fossil record: a variety of important biases are likely to intervene to obscure the data that gastropod assemblages may yield. Nonetheless, the model provides a theoretical framework for interpreting summary data, including for comparison between different assemblages.
The Upper Famennian (Upper Devonian) Strud locality has yielded very abundant and diversified flora as well as vertebrate and arthropod faunas. The arthropod fauna, mostly recovered from fine shales deposited in a calm, confined floodplain habitat including temporary pools, has delivered a putative insect and various crustaceans including eumalacostracans and notostracan, spinicaudatan and anostracan branchiopods. Here we present the Strud eurypterids, consisting of semi-articulated juvenile specimens assigned to Hardieopteridae recovered from the pool and floodplain deposits, as well as larger isolated fragments of potential adults recovered from stratigraphically lower, coarser dark sandy layers indicative of a higher-energy fluvial environment. The Strud fossils strongly suggest that, as proposed for some Carboniferous eurypterids, juvenile freshwater eurypterids inhabited sheltered nursery pools and migrated to higher-energy river systems as they matured.
We report the occurrence of organically preserved microfossils from the subsurface Ediacaran strata overlying the East European Platform in Poland, in the form of sclerites and cuticle fragments of larger organisms. They are morphologically similar to those known from Cambrian strata and associated with various metazoan fossils of recognized phyla. The Ediacaran age of the microfossils is evident from the stratigraphic position below the base of the Cambrian System and above the isotopically dated tuff layers at c. 551±4Ma. Within this strata interval, other characteristic Ediacaran microorganisms co-occur such as cyanobacteria, vendotaenids, microalgae, Ceratophyton, Valkyria and macroscopic annelidan Sabellidites. The recent contributions of organic sclerites in revealing the scope of the Cambrian explosion are therefore also potentially extendable back to the Ediacaran Period when animals first appear in the fossil record.
Amniotes first invaded saline lagoons and coastal seaways towards the end of the Palaeozoic (Early Permian, ~ 280 Ma: Piñeiro et al. 2012), but by the dawn of the Mesozoic (Early–Middle Triassic, ~ 250–235 Ma: Rieppel, 2002; McGowan & Motani, 2003) they had achieved a diversity of specialized body-forms requisite for an obligate oceanic lifestyle. Such an explosive ecomorphological radiation paved the way for amniote dominance of large-bodied aquatic carnivore/omnivore niches over the next 185 Ma, with some lineages (e.g. dyrosaurid crocodylomorphs and bothremydid turtles: Gaffney, Tong & Meylan, 2006; Barbosa, Kellner & Sales Viana, 2008) even persisting on into the Palaeogene (until ~ 50 Ma), and diversifying (i.e. chelonioid sea turtles: Hirayama, 1997) alongside emergent marine mammals through the Neogene (from ~ 23 Ma) and up until today.
The records of columnar shell structures of linguloid brachiopods (Class Lingulata, Order Lingulida, Superfamily Linguloidea) are reviewed in the light of the discovery of two new taxa from the Middle Cambrian Forsemölla Limestone Bed of southern Sweden. The linguloid taxa, described here as Eoobolus? sp. aff. E. priscus (Poulsen) and Canalilatus? simplex sp. nov., are both characterised by a columnar shell structure, a structural type that is representative for acrotretoid brachiopods and that has previously only rarely been reported from the linguloids. Though the two taxa are superficially similar to known genera, i.e., Eoobolus and Canalilatus, their shell structure challenges such affiliations, as the shell structure of the type species of these genera is previously unknown. Linguloid families whose morphological characteristics agree the most with those of the new taxa, i.e., the Zhanatellidae and the Eoobolidae, and from which columnar shell structures have been reported, i.e., the Lingulellotretidae and the Kyrshabaktellidae, are reviewed briefly. Many taxa assigned to these families completely lack shell structure data and are in need of restudy in order to elucidate their systematic position. Knowledge of the representative type of shell structure of the various suprageneric taxa within the Linguloidea is considered crucial, in order to unravel their currently poorly resolved phylogenetic relationships.
Although the mathematical principles underpinning population-level evolution are now well studied, the origin and evolution of morphological novelties has received far less attention. Here, a broad but general theory for how this sort of change takes place is outlined, relying on functional continuity, least-constrained components of morphology, redundancy and preadaptation. At least four distinct sorts of redundancy are identified: (i) redundancy arising through duplication (amplification); (ii) redundancy arising through regionalisation (parcellation); (iii) redundancy arising through functional convergence; and (iv) redundancy arising from shared function (functional degeneracy). Although organisms are here recognised to be functionally constrained (“burdened”, in Riedl's terminology), these constraints can be overcome through the combination of the four principles given above. Contrary to its common treatment, functional constraint is neither an ever-increasing restriction on the scope of evolution, nor does it require drastic events to overcome or “break” it. Rather, it is an evolutionary quantity, subject to selection at some level. The rules that govern morphological evolution are the primary controls on what is allowed to happen in the evolution of the overall genotype-phenotype system, suggesting strong controls on the sorts of developmental changes that might be associated with macroevolution. This model, then, sees organism functionality as the primary control on evolvability, with exact genetic make-up being of secondary importance. It should prove possible to recast traditional notions of body-plan evolution into the formulations of complex system analysis, which in the future may prove a unifying discipline for fields as disparate as palaeontology and gene regulatory networks. In particular, understanding how morphology can evolve may provide the critical link between the ecological and morphological networks that are currently the intense focus of evolutionary investigations.
It has long been assumed that the extant bilaterian phyla generally have their origin in the Cambrian
explosion, when they appear in an essentially modern form. Both these assumptions are questionable. A strict
application of stem- and crown-group concepts to phyla shows that although the branching points of many
clades may have occurred in the Early Cambrian or before, the appearance of the modern body plans was
in most cases later: very few bilaterian phyla sensu stricto have demonstrable representatives in the earliest
Cambrian. Given that the early branching points of major clades is an inevitable result of the geometry of
clade diversification, the alleged phenomenon of phyla appearing early and remaining morphologically
static is seen not to require particular explanation. Confusion in the definition of a phylum has thus led to
attempts to explain (especially from a developmental perspective) a feature that is partly inevitable, partly
illusory. We critically discuss models for Proterozoic diversification based on small body size, limited
developmental capacity and poor preservation and cryptic habits, and show that the prospect of lineage
diversification occurring early in the Proterozoic can be seen to be unlikely on grounds of both parsimony
and functional morphology. Indeed, the combination of the body and trace fossil record demonstrates a
progressive diversification through the end of the Proterozoic well into the Cambrian and beyond, a picture
consistent with body plans being assembled during this time. Body-plan characters are likely to have been
acquired monophyletically in the history of the bilaterians, and a model explaining the diversity in just one
of them, the coelom, is presented. This analysis points to the requirement for a careful application of
systematic methodology before explanations are sought for alleged patterns of constraint and flexibility.
Specimens of Kerygmachela kierkegaardi Budd are described, from the Lower Cambrian Sirius Passet fauna of N Greenland. The cephalic region is characterised by a pair of stout unsegmented appendages each bearing long spinose processes, and an anterior mouth. The trunk shows alternating rows of tubercles and transverse annulations along the axis, to which are attached 11 pairs of gill-bearing lateral lobes and lobopodous limbs. The caudal region is small, and bears two long tail spines. There is some evidence for circular musculature arranged around the trunk and a dorsal, longitudinal sinus, and several details of the muscular pharynx have been preserved.
The combination of characters found in Kerygmachela allows it to be allied with the lobopods, represented in the extant fauna by the onychophorans, tardigrades, and possibly the pentastomids, and in the Cambrian fossil record by a morphologically diverse set of taxa, some of which are not assignable to the extant groupings. It also shares important characters with the previously problematic Burgess Shale forms Opabinia regalis Walcott and AnomalocarisWhiteaves, and the Sirius Passet form Pambdelurion Budd. These taxa together form a paraphyletic group at the base of the clade of biramous arthropods. The position of the so-called ‘Uniramia’ remains unclear. It can be demonstrated from the reconstruction of the arthropod stem-group that full arthropod segmentation has a different derivation from that of the annelids. In line with other recent analyses, this suggests that the ‘Articulata’ of Cuvier should be dismantled, and the arthropods considered to be a group of protostomes which are phylogenetically distinct from the classic spiralians. Arthropod affinities may rather lie with the other moulting animals, in the so-called ‘Ecdysozoa’.
The Lower Cambrian Sirius Passet fauna from Peary Land, North Greenland, is a rich repository of soft-bodied and poorly-sclerotised fossils. A new arthropod from the fauna, Kleptothule rasmusseni, is described. The animal is broadly trilobite-like, possessing a trilobed exoskeleton which is divided into distinct cephalic, thoracic and caudal regions. However, it is unusual in that it possesses a large number of segments, and demonstrates pronounced cephalic segmentation, and a very narrow cephalon and thorax. There is some evidence that the exoskeleton was lightly mineralised.
Kleptothule is compared to some of the olenellimd trilobites, especially those taxa that possess a many-segmented ‘opisthothorax’. Its morphology raises some issues discussed by Lauterbach (1983) in his assignment of some olenellids to the stem-group of the chelicerates. However, it is not considered herein that such a model can be supported. A complete analysis of basal trilobites and the stem-group leading to them must await a fuller description of key taxa from China and Greenland.
'Sirius Passet’ fauna arthropods from the Lower Cambrian Buen Formation of North Greenland form the major component of this exceptionally preserved biota. Early mineralisation of body cavities led to internal anatomy being preserved more readily than external structures such as limbs. Musculature and gut are known in three dimensional form. Many of the taxa present are more easily compared to extant arthropods than to flattened fossil material such as from the Burgess Shale.
The most common arthropod is represented by some 1600 specimens. Although widely differing in preservational style, these specimens may be reconciled to provide a coherent model of the three-dimensional anatomy of the arthropod. The affinities of this arthropod lie with the ‘Cheliceromorph’ rather than the Crustacean biramous-limbed arthropods, and may represent a fairly advanced lineage within the clade.
Recent discussions of the patterns produced by the early metazoan radiations have concentrated on data available from the Burgess Shale. The continuing discovery and description of other Cambrian lagerstätten such as from North Greenland and Chengjiang has highlighted the degree to which the Burgess Shale fauna should be considered to be an aliquot taken from the foment of the ‘Cambrian Explosion’. The discovery of more taxa is tending to flesh out the bare bones of the Burgess fauna. Conclusions about phylogentic patterns drawn from the Burgess Shale alone may thus be premature.
Body patterning in the arthropods, and the validity of the ‘Bauplan’ concept may be investigated by consideration of the actual mechanisms available for profound morphological change. One promising route is provided by the ‘homeotic’ and other hierarchically arranged developmental genes. When the mode of action of these genes is considered in conjunction with phylogenetic methods, it may prove possible to assess evolutionary pathways in terms of the feasibility of the morphological changes required by them rather than relying on what seems inherently reasonable or on marginal advantages in parsimony. Exceptionally preserved biotas also contain the evidence for the evolution of the developmental mechanisms themselves.
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