A test of whether rates of speciation were unusually high during the Cambrian radiation

The Cambrian radiation represents an interval when nearly 20 animal phyla appear in the fossil record in a short geological time span; however, whether this radiation also represents a period of extremely rapid speciation remains unclear. Here, a stochastic framework is used to test the null hypothesis that diversity changes in one of the dominant Early Cambrian groups, the olenelloid trilobites, could be produced by tempos of speciation known to have operated during later time periods. Two continuous-time models, the Yule model and the birth and death process model, and one discrete-time model, the Bienaymé-Galton-Watson branching process model, were used. No statistical evidence for uniquely high rates of speciation during the radiation in these trilobites was found when the continuous-time models were used with low or moderate extinction rates, the rates typically associated with the Cambrian radiation, although the p values are fairly low or, in one case, significant when high extinction rates were used. However, rates of speciation were higher than the average Phanerozoic rates of speciation. The discrete-time model produced equivocal results: either rates were unusually high or the model is inapplicable during the Cambrian radiation. This suggests that there was nothing unique about evolutionary processes relating to the tempo of speciation during the Cambrian radiation.     

The Cambrian evolutionary 'explosion': decoupling cladogenesis from morphological disparity

The origin and differentiation of major clades is often assumed to have occurred in tandem with the 'explosion' of fossil evidence of diverse morphologies ('disparity') at the base of the Cambrian. Evidence is presented that this was not the case. Biogeographical and morphological differentiation among the earliest trilobites reveals incompleteness in the known early Cambrian record; similar evidence can be accrued for other major groups. Phylogenetic analysis proves the likelihood of 'ghost' lineages extending into the Precambrian. The important events in the generation of clades were earlier than the Cambrian 'explosion', at which time the groups become manifest in the fossil record. It is likely that the important phylogenetic changes happened in animals of small size; sister taxa of major groups are shown to be small animals. Decoupling cladogenesis from the Cambrian 'explosion' removes the necessity of invoking unknown evolutionary mechanisms at the base of the Phanerozoic. Genes controlling development may have played a role in generating new morphologies, through heterochrony for example, in the early differentiation of metazoan body plans.     

Two middle Cambrian diceratocephalid trilobites, Cyclolorenzella convexa and Diceratocephalus cornutus, from Korea: development and functional morphology

Park, T.-Y. & Choi, D.K. 2010: Two middle Cambrian diceratocephalid trilobites, Cyclolorenzella convexa and Diceratocephalus cornutus , from Korea: development and functional morphology. Lethaia , Vol. 43, pp. 73–87.
Silicified sclerites of the latest middle Cambrian trilobites, Cyclolorenzella convexa and Diceratocephalus cornutus , have been recovered from the Sesong Formation, Korea. Their morphological similarity and stratigraphic occurrences suggest that D. cornutus is a descendant of C. convexa . The ontogenies of both trilobites demonstrate that a pair of long frontal horns in the cephalon of D. cornutus is an evolutionarily novel structure. It is inferred that redeployment of some pre-existing regulatory gene played a significant role in constructing the frontal horns of D. cornutus . The frontal horns may have been a defensive structure to deter predators. The facial suture of D. cornutus , which extends onto the frontal horns and splits them into the dorsal and ventral halves, was a solution to enable easier forward egression during ecdysis. □ Functional morphology, Korea, Middle Cambrian, ontogeny, trilobites .     

Taking the pulse of the cambrian radiation

The Cambrian radiation is that key episode in the history oflife when a large number of animal phyla appeared in the fossilrecord over a geologically short period of time. Over the last20 years, scientific understanding of this radiation has increasedsignificantly. Still, fundamental questions remain about thetiming of the radiation and also the tempo of evolution. Trilobitesare an excellent group to address these questions because oftheir rich abundance and diversity. Moreover, their complexmorphology makes them readily amenable to phylogenetic analysis,and deducing the nature of macroevolutionary processes duringthe Cambrian radiation requires an understanding of evolutionarypatterns. Phylogenetic biogeographic analysis of Early Cambrianolenellid trilobites, based on a modified version of BrooksParsimony Analysis, revealed the signature of the breakup ofPannotia, a tectonic event that most evidence suggests is constrainedto the interval 600 to 550 Ma. As trilobites are derived metazoans,this suggests the phylogenetic proliferation associated withthe Cambrian radiation was underway tens of millions of yearsbefore the Early Cambrian, although not hundreds of millionsof years as some have argued. Phylogenetic information from Early Cambrian olenellid trilobiteswas also used in a stochastic approach based on two continuoustime models to test the hypothesis that rates of speciationwere unusually high during the Cambrian radiation. No statisticalevidence was found to support this hypothesis. Instead, ratesof evolution during the Cambrian radiation, at least those pertainingto speciation, were comparable to those that have occurred duringother times of adaptive or taxic radiation throughout the historyof life.     

A tiny eye indicating a planktonic trilobite

Abstract: Cambrian trilobites mainly lived on the sea floor, and up till now few, if any, unequivocally planktonic trilobites have been reported from earlier than the Ordovician. The late Cambrian (Furongian) to late Ordovician olenids are a distinctive group of benthic (sea‐floor dwelling) or nekto‐benthic trilobites. Here we show, however, that one recently described, miniaturized and very spiny olenid species, Ctenopyge ceciliae must have been planktonic (passively drifting or feebly swimming in the upper waters of the sea). This interpretation is based not only upon body form but also on the analysis of its visual system and may be one of the earliest records of the planktonic realm being invaded by trilobites.     

The Geography of Evolution and the Evolution of Geography

Insights into the geography of life have played a fundamental role in motivating major developments in evolutionary biology. The focus here is on outlining some of these major developments, specifically in the context of paleontology, by emphasizing the significance of geographic isolation and allopatric speciation, punctuated equilibria, and the Turnover Pulse Hypothesis to evolutionary theory. One of the major debates in evolution concerns the relative contributions of abiotic and biotic factors to macroevolution, and each one of these developments increasingly suggested that it was climatic and geologic factors, rather than competition, that played the primary role in motivating macroevolution. New technical developments, including in the area of Geographic Information Systems, allow continued detailed testing of the relative roles that biotic as opposed to abiotic factors play in causing evolution, and some of the work in this area will also be described.     

Quantitative palaeobiogeography: GIS, phylogenetic biogeographical analysis, and conservation insights

Aim  The utility of GIS-based and phylogenetic biogeographical analysis in palaeobiogeography is reviewed with reference to its ability to elucidate patterns of interest for modern conservation biology, specifically the long-term effects of invasive species.
Location  Emphasis is on biogeographical patterns in the Appalachian basin and mid-continent of North America during the Devonian. Global palaeobiogeographical patterns of the Cambrian are also considered.
Methods  Palaeobiogeographical patterns are assessed within a GIS framework, including both direct range reconstruction and niche modelling methods, and within phylogenetic biogeographical analysis. Biogeographical patterns are considered within multiple clades of fossil invertebrates, including trilobites, crustaceans, brachiopods, and bivalves.
Results  GIS-based analysis (including niche modelling methods) of Devonian invertebrates demonstrates a tightly correlated relationship between sea-level rises and range expansion, dispersal events, and species invasions. The predominance of range expansion and species invasions during the Late Devonian reduced opportunities for vicariant speciation during this interval. Comparison of phylogenetic biogeographical patterns between Cambrian and Devonian trilobites allows discernment of the relative roles of tectonics and eustacy in driving biogeographical patterns.
Main conclusions  GIS analysis and phylogenetic biogeography are powerful tools for analysing the coevolution of the Earth and its biota. Analyses can identify episodes of vicariance and geo-dispersal and produce testable hypotheses for further analysis within the fossil record.     

Predation by early Cambrian trilobites on infaunal worms - evidence from the Swedish Mickwitzia Sandstone

Evidence of early Cambrian predation is found in trace fossils from the Mickwitzia Sandstone of VastergÖtland, Sweden. The arthropod burrow Rusophycw dispar was dug down to spreite burrows in such a manner that mere coincidence of place is unlikely. The predator is identified as an olenellacean trilobite, mainly through the presence of a cephalic impression. The prey, a worm that may have been a priapulid, was localized visually or chemically at the spreite-burrows apertures. The predator lacked legs specialized for seizing prey, as indicated by the parallel, but laterally displaced position of the Rusophycus above the spreite burrow. In this stance prey could be gripped by flexing the supposedly spinose legs of one side around it. When a spreite burrow of limited horizontal extension was encountered, parallel orientation was obtained prior to digging, probably through chemical sensing, whereas parallel orientation to an extended burrow required a number of diggings. These traces are important evidence of early Cambrian predation and of the predatorial capability of trilobites. □ Lower Cambrian, Sweden predation, trilobites, priapulids , Rusophycus.     

Abnormalities in early Paleozoic trilobites from central and eastern China

Malformations are common in trilobites, but the majority of described specimens are from Europe and North America. Only a few abnormal trilobites have been reported from China. Ten abnormal trilobites from Cambrian, Ordovician and Silurian strata in central and eastern China are documented. The abnormal Ordovician trilobites are found for the first time in China. All malformations occur in the thoraxes and pygidia, and were caused by a sub-lethal predatory attack, genetic or embryological malfunction, or injury sustained during molting. It is difficult to identify the predators of the six injured trilobites, but potential predators include Cambrian non-trilobite arthropods, Ordovician cephalopods, and Silurian eurypterids, chondrichthyes or cephalopods, even cannibalistic trilobites. Abnormal specimens caused by sub-lethal predatory attacks mainly occur in Cambrian strata in China and other areas in the world, and are rare in post-Cambrian strata. This pattern may reflect the rise of predators and increased predation in the post-Cambrian, which led to an increased trilobite fatality rate, thus reducing the probability that injured specimens would become fossilized.     

中国寒武纪三叶虫生物地理趋议

寒武纪三叶虫除北疆和藏南尚无可靠报道外,在中国各地均有广泛分布。三叶虫证据表明,除兴安区的额尔古纳一兴安地体和早寒武世中天山一北山地体显示了与西伯利亚和劳伦陆块在动物群方面的密切相关外,中国寒武纪的所有板块和大部分地体（包括中及晚寒武世的中天山～北山地体）均系当时东泛冈瓦纳的重要组成部分,在生物地理上彼此密切关联。对这些属于东泛冈瓦纳的中国区块的生物地理格局的进一步探讨,主要侧重于它们台地相浅水三叶虫动物群之间的对比,特别是立足于来自动物群纪录比较完整的华南、塔里木和华北板块的证据。综合分析研究表明这些板块和有关的地体在中及晚寒武世应归属同一生物地理区,而在早寒武世则可划分为两个生物地理亚区：一个由华南、塔里木板块和安南或印支地体组成,另一仅以华北板块为代表。     

Controls on gut phosphatisation: the trilobites from the Weeks Formation Lagerstätte (Cambrian; Utah)

Despite being internal organs, digestive structures are frequently preserved in Cambrian Lagerstätten. However, the reasons for their fossilisation and their biological implications remain to be thoroughly explored. This is particularly true with arthropods – typically the most diverse fossilised organisms in Cambrian ecosystems – where digestive structures represent an as-yet underexploited alternative to appendage morphology for inferences on their biology. Here we describe the phosphatised digestive structures of three trilobite species from the Cambrian Weeks Formation Lagerstätte (Utah). Their exquisite, three-dimensional preservation reveals unique details on trilobite internal anatomy, such as the position of the mouth and the absence of a differentiated crop. In addition, the presence of paired pygidial organs of an unknown function is reported for the first time. This exceptional material enables exploration of the relationships between gut phosphatisation and the biology of organisms. Indeed, soft-tissue preservation is unusual in these fossils as it is restricted to the digestive structures, which indicates that the gut played a central role in its own phosphatisation. We hypothesize that the gut provided a microenvironment where special conditions could develop and harboured a source of phosphorus. The fact that gut phosphatization has almost exclusively been observed in arthropods could be explained by their uncommon ability to store ions (including phosphorous) in their digestive tissues. However, in some specimens from the Weeks Formation, the phosphatisation extends to the entire digestive system, suggesting that trilobites might have had some biological particularities not observed in modern arthropods. We speculate that one of them might have been an increased capacity for ion storage in the gut tissues, related to the moulting of their heavily-mineralised carapace.     

Reappraising the early evidence of durophagy and drilling predation in the fossil record: implications for escalation and the Cambrian Explosion

The Cambrian Explosion is arguably the most extreme example of a biological radiation preserved in the fossil record, and studies of Cambrian Lagerstätten have facilitated the exploration of many facets of this key evolutionary event. As predation was a major ecological driver behind the Explosion – particularly the radiation of biomineralising metazoans – the evidence for shell crushing (durophagy), drilling and puncturing predation in the Cambrian (and possibly the Ediacaran) is considered. Examples of durophagous predation on biomineralised taxa other than trilobites are apparently rare, reflecting predator preference, taphonomic and sampling biases, or simply lack of documentation. The oldest known example of durophagy is shell damage on the problematic taxon Mobergella holsti from the early Cambrian (possibly Terreneuvian) of Sweden. Using functional morphology to identify (or perhaps misidentify) durophagous predators is discussed, with emphasis on the toolkit used by Cambrian arthropods, specifically the radiodontan oral cone and the frontal and gnathobasic appendages of various taxa. Records of drill holes and possible puncture holes in Cambrian shells are mostly on brachiopods, but the lack of prey diversity may represent either a true biological signal or a result of various biases. The oldest drilled Cambrian shells occur in a variety of Terreneuvian‐aged taxa, but specimens of the ubiquitous Ediacaran shelly fossil Cloudina also show putative drilling traces. Knowledge on Cambrian shell drillers is sorely lacking and there is little evidence or consensus concerning the taxonomic groups that made the holes, which often leads to the suggestion of an unknown ‘soft bodied driller’. Useful methodologies for deciphering the identities and capabilities of shell drillers are outlined. Evidence for puncture holes in Cambrian shelly taxa is rare. Such holes are more jagged than drill holes and possibly made by a Cambrian ‘puncher’. The Cambrian arthropod Yohoia may have used its frontal appendages in a jack‐knifing manner, similar to Recent stomatopod crustaceans, to strike and puncture shells rapidly. Finally, Cambrian durophagous and shell‐drilling predation is considered in the context of escalation – an evolutionary process that, amongst other scenarios, involves predators (and other ‘enemies’) as the predominant agents of natural selection. The rapid increase in diversity and abundance of biomineralised shells during the early Cambrian is often attributed to escalation: enemies placed selective pressure on prey, forcing phenotypic responses in prey and, by extension, in predator groups over time. Unfortunately, few case studies illustrate long‐term patterns in shelly fossil morphologies that may reflect the influence of predation throughout the Cambrian. More studies on phenotypic change in hard‐shelled lineages are needed to convincingly illustrate escalation and the responses of prey during the Cambrian.     

Evolutionary Trends in the Ornamentation of Cambrian Solenopleuropsine Trilobites

A phylogeny of the Cambrian solenopleuropsine trilobites is constructed to evaluate the importance of differential speciation, extinction and developmental constraints in the evolutionary history of the clade. The transformation of the Pardailhania–Solenopleuropsis ( Manublesia )– Solenopleuropsis ( Solenopleuropsis ) lineages occurred during the mid Caesaraugustian–early Languedocian interval, with all measured morphological variables exhibiting continuous and gradual changes. Several evolutionary trends show tendencies towards an increase in tuberculation and its complexity on the anterior part of the cephalon. A mid Caesaraugustian anagenetic tendency is related to the influence of constant environmental conditions, whereas a late Caesaraugustian–early Languedocian cladogenetic tendency seems to have been an adaptative response to unstable environments. The Solenopleuropsinae clade has a well-preserved continuous fossil record over its full biogeographical range in the Mediterranean region. The sympatric evolution in centrally located populations (Mediterranean area) follows the model of phyletic gradualism, whilst a peripheral population in Avalonia gave rise (by allopatric speciation) to a distinct species, characterized by a variation in the relative size of the palpebral lobes, which may be evidence of a parallel evolution in the trends of ornamentation.     

Gut evolution in early Cambrian trilobites and the origin of predation on infaunal macroinvertebrates: evidence from muscle scars in Mesolenellus

Trilobites are particularly common Cambrian fossils, but their trophic impact on the rapidly evolving marine ecosystems of that time is difficult to assess, due to uncertainties on how diverse their feeding habits truly were. Gut anatomy might help to constrain inferences on trilobite feeding ecology, but preservation of digestive organs is exceedingly rare. Muscle scars on the glabella, known as ‘frontal auxiliary impressions’ (FAIs), have been interpreted as evidence of the evolution of a pouch‐like organ with powerful extrinsic muscles (i.e. a crop) in some trilobites. Here we describe FAIs in Mesolenellus hyperboreus from Cambrian Stage 4 strata of North Greenland, which represents the oldest example of such structures and their first report in the Suborder Olenellina. Mesolenellus FAIs suggest that the crop in trilobites was clearly differentiated from the rest of the digestive tract, and essentially located under a hypertrophied glabellar frontal lobe. Reviews of the digestive anatomy of trilobite sister‐taxa and the glabellar morphology of the oldest‐known trilobites suggest that the gut of the trilobite ancestor was an essentially simple tract (i.e. no well‐differentiated crop) flanked laterally by numerous midgut glands. A crop first evolved in the Cambrian in groups like olenelloids and (later) paradoxidoids. Using ichnological evidence, we hypothesize that the emergence of olenelloids yields evidence for the evolution of predatory inclinations in a group of arthropods originally dominated by surface‐deposit‐feeders. By allowing the exploitation of a rapidly developing food source, infaunal animals, the diversification of feeding strategies in trilobites might partially explain their unparalleled evolutionary success.     

Complete record of Furongian polymerid trilobites and agnostoids of Scandinavia – a biostratigraphical scheme

Terfelt, F., Ahlberg, P. & Eriksson, M.E. 2011: Complete record of Furongian polymerid trilobites and agnostoids of Scandinavia – a biostratigraphical scheme. Lethaia, Vol. 44, pp. 8–14. So far, 112 polymerid trilobite species/subspecies and 13 agnostoid species/subspecies have been recorded from the Furongian (upper Cambrian) of Scandinavia. For the first time, their zonal occurrences are summarized in a biostratigraphical scheme serving as a practical synopsis for students of this interval in time. Ninety‐six of the recorded polymerid trilobite species/subspecies belong to the family Olenidae whereas the remaining 16 are distributed across eight other families. Levels of increased speciation and low diversity (including stratigraphical range gaps) are conspicuous and these may be correlated with recorded physical and chemical anomalies. □Agnostoids, biostratigraphy, Cambrian, Furongian, polymerids, Scandinavia, trilobites.     

中国贵州东部乌溜-曾家崖剖面早—中寒武世凯里组疑源类(英文)

我国贵州东部乌溜-曾家崖凯里组剖面作为世界早-中寒武世界线层型侯选剖面,产出良好保存的大型无脊椎动物化石(尤其是三叶虫)。因此,更加详细研究该剖面海洋疑源类的生物地层非常重要,将提供围绕界线的生物和环境变化的细微记录。乌溜-曾家崖剖面凯里组的疑源类划分两个组合,它们是凯里组0-52m的Leiomarginata si mplex-Fi mbriaglomerella membranacea组合和52-140m的Cristallinium cambriense-Heliosphaeridiumnodosum-Globosphaeridiumcerinum组合。140-214m仅有很少疑源类标本,可能因为凯里组上部以白云岩为主,不适宜有机壁微体化石的保存。距凯里组底部往上约52m处疑源类组合出现重大变化,无疑为解释沉积环境提供重要资料。两疑源类组合间界线,位于52.3-52.7m,此稍低于被三叶虫Oryctocephalusindicus首现所指示的全球寒武系第5阶潜在层型剖面界线。     

Biomechanical analyses of Cambrian euarthropod limbs reveal their effectiveness in mastication and durophagy

Durophagy arose in the Cambrian and greatly influenced the diversification of biomineralized defensive structures throughout the Phanerozoic. Spinose gnathobases on protopodites of Cambrian euarthropod limbs are considered key innovations for shell-crushing, yet few studies have demonstrated their effectiveness with biomechanical models. Here we present finite-element analysis models of two Cambrian trilobites with prominent gnathobases—Redlichia rex and Olenoides serratus—and compare these to the protopodites of the Cambrian euarthropod Sidneyia inexpectans and the modern American horseshoe crab, Limulus polyphemus. Results show that L. polyphemus, S. inexpectans and R. rex have broadly similar microstrain patterns, reflecting effective durophagous abilities. Conversely, low microstrain values across the O. serratus protopodite suggest that the elongate gnathobasic spines transferred minimal strain, implying that this species was less well-adapted to masticate hard prey. These results confirm that Cambrian euarthropods with transversely elongate protopodites bearing short, robust gnathobasic spines were likely durophages. Comparatively, taxa with shorter protopodites armed with long spines, such as O. serratus, were more likely restricted to a soft food diet. The prevalence of Cambrian gnathobase-bearing euarthropods and their various feeding specializations may have accelerated the development of complex trophic relationships within early animal ecosystems, especially the ‘arms race'' between predators and biomineralized prey.     

Pholcid spider molecular systematics revisited,with new insights into the biogeography and the evolution of the group

We analysed seven genetic markers sampled from 165 pholcids and 34 outgroups in order to test and improve the recently revised classification of the family. Our results are based on the largest and most comprehensive set of molecular data so far to study pholcid relationships. The data were analysed using parsimony, maximum‐likelihood and Bayesian methods for phylogenetic reconstruction. We show that in several previously problematic cases molecular and morphological data are converging towards a single hypothesis. This is also the first study that explicitly addresses the age of pholcid diversification and intends to shed light on the factors that have shaped species diversity and distributions. Results from relaxed uncorrelated lognormal clock analyses suggest that the family is much older than revealed by the fossil record alone. The first pholcids appeared and diversified in the early Mesozoic about 207 Ma ago (185–228 Ma) before the breakup of the supercontinent Pangea. Vicariance events coupled with niche conservatism seem to have played an important role in setting distributional patterns of pholcids. Finally, our data provide further support for multiple convergent shifts in microhabitat preferences in several pholcid lineages. Our findings suggest that both adaptive and non‐adaptive speciation may have played an important role in the diversification of pholcid lineages.     

Cambrian agnostid communities in Tasmania

Jago, J. B.: Cambrian agnostid communities in Tasmania.
Two or possibly three different agnostid trilobite assemblages can be distinguished in the late Middle and early Upper Cambrian sequences of northern and western Tasmania. This is significant because in recent years agnostid trilobites have been widely used in local and international correlations of Middle and Upper Cambrian rocks. The three assemblages recognized are (1) an agnostid assemblage in which polymerid trilobites are abscnt, rare or present as thanatocoenotic fossils, (2) a ptychagnostid-non-nepeid assemblage, and (3) a nepeid-clavagnostid-peronopsid assemblage in which non-agnostid trilobites are abundant but ptychag-nostids are absent. It is proposed that assemblage (1) represents an open sea fauna, with assemblages (2) and (3) occurring in progressively shallower water.     

Microgeographic evolution of snail shell shape and predator behavior

Genetic divergence in geographically isolated populations is a prerequisite for allopatric speciation, one of the most common modes of speciation. In ecologically equivalent populations existing within a small, environmentally homogeneous area, an important role for environmentally neutral divergence is often found or inferred. We studied a species complex of conspicuously shaped Opisthostoma land snails on scattered limestone outcrops within a small area of lowland rainforest in Borneo. We used shell morphometrics, mitochondrial and nuclear DNA sequences, and marks of predation to study the factors involved in allopatric divergence. We found that a striking geographic divergence exists in shell morphology, which is partly associated with neutral genetic divergence. We also found geographic differentiation in the behavior of the snails' invertebrate predator and evidence of an evolutionary interaction between aspects of shell shape and predator behavior. Our study shows that adaptation to biotic aspects of the environment may play a more important role in allopatric speciation than previously suspected, even on a geographically very small scale.