Conodontophorid biodiversification during the Ordovician in South China

Wu, R., Stouge, S. & Wang, Z. 2012: Conodontophorid biodiversification during the Ordovician in South China. Lethaia, Vol. 45, pp. 432–442. Analysis of the Ordovician conodontophorid diversity pattern for South China using normalized and total diversity measures reveals that diversity peaks occurred in the mid‐Tremadocian, mid‐late Floian, early Dapingian and mid‐Darriwilian periods. The conodontophorids radiated during the Floian, maintaining relatively high diversity into the early part of the Middle Ordovician until a significant diversity decrease occurred in the late Dapingian. A relatively low diversity level prevailed in the Late Ordovician. Three diversification intervals based on origination, extinction and turnover rates have been identified i.e. (1) Tremadocian to mid‐late Floian, (2) early Dapingian and (3) late Dapingian to early Darriwilian. Diversity curves for conodontophorids, brachiopods, graptolites, acritarchs and trilobites from South China are comparable during the Early Ordovician, although differences are apparent in the Middle and Late Ordovician. In South China, conodontophorid diversity reacted primarily to sea‐level changes during the Early and Middle Ordovician, when the peak of this biodiversification generally coincided with a transgression. Climate changes – especially the global cooling that occurred during the Late Ordovician glaciation – and sea‐water chemistry were also important controlling factors. □Biodiversification, conodonts, Ordovician, South China.     

Cambrian biostratigraphy of the Bowers back-arc basin,Northern Victoria Land,Antarctica — A review

The Bowers Mountains in Northern Victoria Land contain the richest Cambrian Series 3 (Miaolingian, middle Cambrian) and Cambrian Series 4 (Furongian, late Cambrian) fossiliferous successions in Antarctica. Almost all the fossils are found within the Bowers Supergroup, which outcrops within the Bowers Terrane, a fault-bounded northwest-southeast oriented strip in Northern Victoria Land. The fossils provide the main age control on the history and evolution of the Bowers volcanic arc and back-arc basin. The great bulk of the fossils occur within the Spurs Formation. The fossil assemblages are dominated by agnostoids and polymerid trilobites with most ranging in age from Drumian to Paibian, although one fauna is of Jiangshanian age. Over 40 agnostoid taxa and over 100 polymerid trilobite taxa have been recorded from the rocks of the Bowers Supergroup. The youngest fauna occurs within the adjacent Robertson Bay Terrane, where a limited fauna of polymerid trilobites and conodonts from within a limestone olistolith have a very late Cambrian or early Ordovician age. Faunal affinities are mainly with Australia, New Zealand, North and South China and the Himalaya with lesser ties to Iran, Kazakhstan, Siberia and Laurentia.     

Deep Endichnial Cruziana from the Lower-Middle Ordovician of Spain — A Unique Trace Fossil Record of Trilobitomorph Deep Burrowing Behavior

Full reliefs of Cruziana furcifera from the Lower-Middle Ordovician quartzite sandstone beds (Pochico Formation, southern Spain) points to deep, infaunal burrowing of trilobites. Some specimens show an unusual vertical extension with a wider lower part and a narrower upper part in cross section. They are referred to trilobites, which burrowed deeply in the sediment and were oriented obliquely head down and tail up. Deep burrowing seems to be common for other members of the Cruziana rugosa group, foremost C. rugosa and C. furcifera, less for C. goldfussi. The deep burrowing recorded in the discussed trace fossils can be referred to the earliest common infaunalization caused by trilobites and other arthropods during the Ordovician, probably in a response to a food competition on the sea floor, which promoted a behavioral plasticity within the same taxon or closely related taxa of trilobites.     

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.     

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.     

Ontogeny of the Furongian (late Cambrian) trilobite Proceratopyge cf. P. Lata Whitehouse from northern Victoria Land,Antarctica, and the evolution of metamorphosis in trilobites

There were multiple origins of metamorphosis‐undergoing protaspides in trilobite evolution: within the superfamilies Remopleuridioidea, Trinucleoidea, and within the Order Asaphida. Recent studies have revealed that the protaspides of the Cambrian representatives of the Remopleuridioidea and the Trinucleoidea did not undergo metamorphosis. However, ontogeny of the Cambrian members of the Order Asaphida has remained unknown. This study documents the ontogeny of the Furongian asaphoidean ceratopygid trilobite, Proceratopyge cf. P. lata Whitehouse, from northern Victoria Land, Antarctica. Two stages for the protaspid phase, five developmental stages for the post‐protaspid cranidia, and ten stages for the post‐protaspid pygidia have been identified. Interestingly, the protaspis directly developed into a meraspis without metamorphosis. A new cladistic analysis resulted in a single most parsimonious tree, according to which the presence of the bulbous commutavi protaspis turns out to be a synapomorphy for Asaphidae + Cyclopygoidea, not a synapomorphy for the Order Asaphida as previously suggested. In addition, it is inferred that there was convergent evolution of indirectly‐developing commutavi protaspides during the Furongian and Early Ordovician. Metamorphosis‐entailing planktonic larvae evolved in many different metazoan lineages near the Cambrian–Ordovician transition, due to the escalating ecological pressure of the Great Ordovician Biodiversification Event. Since the bulbous commutavi protaspid morphology is thought to be an adaptation for a planktonic life mode, the convergent evolution of the indirect development in the three trilobite lineages at this period might have been a result of adaptation to the early phase of the Great Ordovician Biodiversification Event.     

Origin of planktotrophy--evidence from early molluscs

The size of early ontogenetic shells (protoconchs) of ancient benthic molluscs suggests that feeding larvae occurred at about 490 myr (approximately, transition from Cambrian to Ordovician). Most studied Ordovician protoconchs were smaller than Cambrian ones, indicating smaller Ordovician eggs and hatchlings. This suggests substitution of nutritious reserve matter such as yolk by plankton as an energy source for larvae. The observed size change represents the first direct empiric evidence for a late Cambrian to Ordovician switch to planktotrophy in invertebrate larvae. It corroborates previous hypotheses about a possible polyphyly of planktotrophy. These hypotheses were primarily based on molecular clock data of extant clades with different types of larva, change in the overall body size, as well as increasing predation pressure on Early Paleozoic sea floors. The Early Ordovician is characterized by an explosive radiation of benthic suspension feeders and it was suggested that planktotrophy would prolongate escape from benthic predation on hatchlings. This biological escalation hypothesis does not fully explain why planktotrophy and suspension feeding became important at the same time, during a major biodiversification. An additional factor that probably included availability of nutrients must have played a role. We speculate that an increasing nutrient supply and availability of photoautotrophic plankton in world oceans have facilitated both planktotrophy and suspension feeding, which does not exclude a contemporaneous predation-driven escalation. It is very likely that the evolution of planktotrophy as well as increasing predation contributed to the Ordovician radiation.     

Trace fossils from arenig flysch sediments of eire and their bearing on the early colonisation of the deep seas

A sequence of Lower Ordovician (Arenig) turbidites in Co. Wexford, Eire, has yielded one of the earliest diverse ichnofaunas yet recorded from deep water sediments comprising: Chondrites, Glockerichnus, Gordia, Helminthopsis, Lorenzinia, Neonereites, Palaeophycus, Paleodictyon, Planolites, Sublorenzinia, Taenidium, Taphrhelminthopsis, Teichichnus and Tomaculum. This ichnofauna is critical in any analysis of the colonisation of the deep seas by trace fossil‐producing animals.

A world‐wide review shows that the earliest trace fossils are mainly from Late Precambrian shelf sea environments, but many more evolved during very rapid diversification in the pre‐trilobite Lower Cambrian.

There was little increase in diversity in shallow water after the Lower Cambrian but a progressive colonisation of the deep ocean took place and this accelerated during the Ordovician, when the main lineages of deep sea trace fossils were established there. Rosetted, patterned, meandering and simple spiral forms evolved in shallow water in the Upper Precambrian and pre‐trilobite Lower Cambrian and only later migrated into the deep sea, whereas complex, closely programmed, spiral traces may have evolved there.     

Reassessment of the mode of life of Pagetia Walcott, 1916 (Trilobita: Eodiscidae) based on a cluster of intact exuviae from the Kaili Formation (Cambrian) of Guizhou, China

Pagetia is the most abundant trilobite in the Kaili Formation (lower to middle Cambrian). During the course of studying the museum collections of Kaili trilobites ( n  > 1000), a cluster containing 22 pagetiid individuals at various growth stages is noted. Specimens record the growth range from the degree 0 meraspid to late holaspid phases. Based on the relative completeness of the moults except for missing free cheeks, these specimens are interpreted as intact exuviae that had undergone minimal transport prior to burial. If Kaili pagetiids had pelagic or planktonic living habits as previously suggested, it would be difficult to explain the presence of intact exuviae in clusters. Therefore, Kaili pagetiids are interpreted here as having a benthic mode of life after the onset of the meraspid phase.     

Testing for a decline in diversity prior to extinction: Languedocian (latest mid‐Cambrian) distribution of cinctans (Echinodermata) in the Iberian Chains,NE Spain

Abstract: The middle Cambrian strata of the Iberian Chains (north‐eastern Spain) and the Montagne Noire (southern France) record an adaptative radiation of cinctans and trilobites, which spanned the Leonian–early Languedocian interval. A diachronous diversity peak was reached by both benthic groups when favourable palaeoenvironmental conditions (clayey vs. silty substrates) were established. The acme in diversity was followed by a gradual decline and a barren interval associated with the onset of the mid‐Languedocian regression, well constrained throughout the western Mediterranean region. For trilobites, the aftermath of the regression is characterized by a late‐Languedocian major faunal turnover of families, followed by a renewed Furongian–early Tremadocian radiation related to the stepwise immigration of trilobite invaders from northern and eastern Gondwana, under persistent transgressive conditions. In contrast, the cinctans reappeared only patchily in late‐Languedocian monospecific coquinas and finally disappeared before the Furongian. Thus, the late Languedocian is a crucial interval in which to analyse the decline in diversity and final extinction of cinctans in the aftermath of the mid‐Languedocian regression.     

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.     

Ordovician and Silurian brachiopods from graptolitic shales and related deep-water argillaceous rocks

Ordovician and Silurian graptolitic shales and deep-water mudstones contain a sparse fauna of clustered, minute shells which are commonly believed to have been epiplankton attached to seaweed. Modern deep-water organisms may preferentially attach to local firm areas on the soft sediment. It is suggested that the Ordovician and Silurian shells may also have been benthic animals attached to local firm regions of the sea floor. These substrates might have included algal fronds which had fallen to the bottom.     

Dynamic palaeoredox and exceptional preservation in the Cambrian Spence Shale of Utah

Garson, D.E., Gaines, R.R., Droser, M.L., Liddell, W.D. & Sappenfield, A. 2011: Dynamic palaeoredox and exceptional preservation in the Cambrian Spence Shale of Utah. Lethaia, Vol. 45, pp. 164–177. Burgess Shale‐type faunas provide a unique glimpse into the diversification of metazoan life during the Cambrian. Although anoxia has long been thought to be a pre‐requisite for this particular type of soft‐bodied preservation, the palaeoenvironmental conditions that regulated extraordinary preservation have not been fully constrained. In particular, the necessity of bottom water anoxia, long considered a pre‐requisite, has been the subject of recent debate. In this study, we apply a micro‐stratigraphical, ichnological approach to determine bottom water oxygen conditions under, which Burgess Shale‐type biotas were preserved in the Middle Cambrian Spence Shale of Utah. Mudstones of the Spence Shale are characterized by fine scale (mm‐cm) alternation between laminated and bioturbated intervals, suggesting high‐frequency fluctuations in bottom water oxygenation. Whilst background oxygen levels were not high enough to support continuous infaunal activity, brief intervals of improved bottom water oxygen conditions punctuate the succession. A diverse skeletonized benthic fauna, including various polymerid trilobites, hyolithids, brachiopods and ctenocystoids suggests that complex dysoxic benthic community was established during times when bottom water oxygen conditions were permissive. Burgess Shale‐type preservation within the Spence Shale is largely confined to non‐bioturbated horizons, suggesting that benthic anoxia prevailed in intervals, where these fossils were preserved. However, some soft‐bodied fossils are found within weakly to moderately bioturbated intervals (Ichnofabric Index 2 and 3). This suggests that Burgess Shale‐type preservation is strongly favoured by bottom water anoxia, but may not require it in all cases. □Anoxia, Burgess Shale, Burgess Shale type‐preservation, Langston Formation, Spence Shale Member, Utah.     

Why barren intervals? A taphonomic case study of the Scandinavian Alum Shale and its faunas

The Scandinavian Alum Shale Formation (Middle Cambrian to Lower Ordovician) accumulated under generally low oxygen concentrations. Syndepositional changes in the oxygen concentrations of the bottom water are reconstructed on the basis of the fossil fauna. Under relatively high oxygen concentrations, brachiopods and non-olenid polymerid trilobites inhabited the sea floor. Under lower oxygen concentrations the fauna was dominated by agnostids and at lowest oxygen levels by olenid trilobites. The enrichments of vanadium relative to nickel, as well as the enrichment of sulphur, match these faunal changes. A geochemical classification of the dysoxic environment is presented. The abundance of calcareous fossils decreases with increasing bottom water oxygen concentrations, indicating that the preservation of calcareous hard parts is most likely at the lowest oxygen concentrations. The poor preservation of calcareous fossils at relatively high oxygen concentrations is explained by the generation of corrosive pore waters during the reoxidation of sulphide compounds. Trilobitic and non-trilobitic intervals alternate up through the shale. Non-trilobitic intervals are either barren or contain non-calcareous fossils (phosphatic brachiopods, phosphatic 'ostracodes' and graptolites). The stratigraphical variation of trilobitic and non-trilobitic intervals is interpreted to reflect major changes in oxygen levels that might be linked to sea-level and climatic fluctuations.     

The origin of the animals and a ‘Savannah’ hypothesis for early bilaterian evolution

The earliest evolution of the animals remains a taxing biological problem, as all extant clades are highly derived and the fossil record is not usually considered to be helpful. The rise of the bilaterian animals recorded in the fossil record, commonly known as the ‘Cambrian explosion’, is one of the most significant moments in evolutionary history, and was an event that transformed first marine and then terrestrial environments. We review the phylogeny of early animals and other opisthokonts, and the affinities of the earliest large complex fossils, the so‐called ‘Ediacaran’ taxa. We conclude, based on a variety of lines of evidence, that their affinities most likely lie in various stem groups to large metazoan groupings; a new grouping, the Apoikozoa, is erected to encompass Metazoa and Choanoflagellata. The earliest reasonable fossil evidence for total‐group bilaterians comes from undisputed complex trace fossils that are younger than about 560 Ma, and these diversify greatly as the Ediacaran–Cambrian boundary is crossed a few million years later. It is generally considered that as the bilaterians diversified after this time, their burrowing behaviour destroyed the cyanobacterial mat‐dominated substrates that the enigmatic Ediacaran taxa were associated with, the so‐called ‘Cambrian substrate revolution’, leading to the loss of almost all Ediacara‐aspect diversity in the Cambrian. Why, though, did the energetically expensive and functionally complex burrowing mode of life so typical of later bilaterians arise? Here we propose a much more positive relationship between late‐Ediacaran ecologies and the rise of the bilaterians, with the largely static Ediacaran taxa acting as points of concentration of organic matter both above and below the sediment surface. The breaking of the uniformity of organic carbon availability would have signalled a decisive shift away from the essentially static and monotonous earlier Ediacaran world into the dynamic and burrowing world of the Cambrian. The Ediacaran biota thus played an enabling role in bilaterian evolution similar to that proposed for the Savannah environment for human evolution and bipedality. Rather than being obliterated by the rise of the bilaterians, the subtle remnants of Ediacara‐style taxa within the Cambrian suggest that they remained significant components of Phanerozoic communities, even though at some point their enabling role for bilaterian evolution was presumably taken over by bilaterians or other metazoans. Bilaterian evolution was thus an essentially benthic event that only later impacted the planktonic environment and the style of organic export to the sea floor.     

Morphology and phylogenetic interpretation of a new Cambrian edrioasteroid (Echinodermata) from Spain

Abstract: The edrioasteroid, Aragocystites belli gen. nov. sp. nov. from the middle Cambrian Murero Formation of Spain, is described based on a small number of very well‐preserved specimens. Important anatomical characteristics include star‐shaped and pseudoclavate theca, rare or absent epispires, well‐developed interradially positioned oral plates and several unorganized cover plates associated with each widely exposed flooring plate. A phylogenetic analysis including several Cambro–Ordovician species shows it is more derived than Stromatocystites and Totiglobus but is a sister group to a clade comprising Cambraster and Edriodiscus. Ontogenetic observations based on juveniles of 5 mm in diameter suggest that this species changed thecal shape markedly during growth. A. belli gen. nov. sp. nov. probably lived in quiet environments where it attached directly to the sea floor on stabilized substrates.     

PATTERNS OF EVOLUTION AND EXTINCTION IN THE LAST HARPETID TRILOBITES DURING THE LATE DEVONIAN (FRASNIAN)

Abstract: Late Devonian (Frasnian) harpetid trilobites have hitherto only been described from the western side of the Protethys Ocean, in what is now Europe and North Africa, as well as from Gondwana‐derived northwestern Kazakhstan (Mugodjar). However, late Frasnian strata in the Canning Basin, Western Australia, that were deposited on the eastern side of this ocean, contain a rich harpetid fauna. Described herein are two new harpetids: Eskoharpes gen. nov. and Globoharpes gen. nov., within which are placed six species: E. palanasus sp. nov., E. wandjina sp. nov., E. boltoni sp. nov., E. guthae sp. nov., G. teicherti sp. nov. and G. friendi sp. nov. The ontogenetic development of E. palanasus, E. wandjina and G. teicherti are described, including the first unequivocal harpetid protaspis. Globoharpes exhibits evidence of sexual dimorphism in the development of a pronounced preglabellar boss in some specimens. This structure is thought to have functioned as a brood pouch. Such structures have previously only been described in Cambrian and Ordovician trilobites, and never before in harpetids. It is suggested that the characteristic harpetid fringe functioned as a secondary respiratory structure. The Eskoharpes lineage shows evolutionary trends that mirror changes seen in ontogenetic development of the youngest species, suggesting the operation of peramorphic processes. This is the first record of heterochrony in harpetids and the first documented example of peramorphosis in Devonian trilobites. These harpetids demonstrate a stepped pattern of extinction during the late Frasnian, probably related to the effects of the two Kellwasser biocrises that have been well documented in European Frasnian sections. Highly vaulted species of Eskoharpes and the strongly vaulted Globoharpes became extinct at the Lower Kellwasser Event. The flatter species of Eskoharpes became extinct at the base of the Upper Kellwasser Event shortly prior to the Frasnian/Famennian boundary. The extinction of these harpetids, along with contemporaneous forms from Europe, which are also discussed herein, marks the end of the trilobite order Harpetida worldwide.     

The cuticle of the enigmatic arthropod Phytophilaspis and biomineralization in Cambrian arthropods

Lin, J.‐P., Ivantsov, A.Y. & Briggs, D.E.G. 2011: The cuticle of the enigmatic arthropod Phytophilaspis and biomineralization in Cambrian arthropods. Lethaia, Vol. 44, pp. 344–349. Many non‐trilobite arthropods occur in Cambrian Burgess Shale‐type (BST) biotas, but most of these are preserved in fine‐grained siliciclastics. Only one important occurrence of Cambrian non‐trilobite arthropods, the Sinsk biota (lower Sinsk Formation, Botomian) from the Siberian Platform, has been discovered in carbonates. The chemical compositions of samples of the enigmatic arthropod Phytophilaspis pergamena Ivantsov, 1999 and the co‐occurring trilobite Jakutus primigenius Ivantsov in Ponomarenko, 2005 from this deposit were analysed. The cuticle of P. pergamena is composed of mainly calcium phosphate and differs from the cuticle of J. primigenius, which contains only calcium carbonate. Phosphatized cuticles are rare among large Cambrian arthropods, except for aglaspidids and a few trilobites. Based on recent phylogenetic studies, phosphatization of arthropod cuticle is likely to have evolved several times. □arthropod cuticle, Burgess Shale‐type preservation, fossil‐diagenesis, phosphatization.     

Life‐history predicts past and present population connectivity in two sympatric sea stars

Life‐history traits, especially the mode and duration of larval development, are expected to strongly influence the population connectivity and phylogeography of marine species. Comparative analysis of sympatric, closely related species with differing life histories provides the opportunity to specifically investigate these mechanisms of evolution but have been equivocal in this regard. Here, we sample two sympatric sea stars across the same geographic range in temperate waters of Australia. Using a combination of mitochondrial DNA sequences, nuclear DNA sequences, and microsatellite genotypes, we show that the benthic‐developing sea star, Parvulastra exigua, has lower levels of within‐ and among‐population genetic diversity, more inferred genetic clusters, and higher levels of hierarchical and pairwise population structure than Meridiastra calcar, a species with planktonic development. While both species have populations that have diverged since the middle of the second glacial period of the Pleistocene, most P. exigua populations have origins after the last glacial maxima (LGM), whereas most M. calcar populations diverged long before the LGM. Our results indicate that phylogenetic patterns of these two species are consistent with predicted dispersal abilities; the benthic‐developing P. exigua shows a pattern of extirpation during the LGM with subsequent recolonization, whereas the planktonic‐developing M. calcar shows a pattern of persistence and isolation during the LGM with subsequent post‐Pleistocene introgression.     

The oldest labechiid stromatoporoids from intraskeletal crypts in lithistid sponge–Calathium reefs

Early Ordovician (early Floian) reefs of South China include lithistid sponge–Calathium reefs with a three‐dimensional skeletal framework. These structures are among the first post‐Cambrian skeletal‐dominated reef structures and provides an opportunity to test how the novel metazoan builders changed the environments and increased topographic complexity within benthic communities. We document the oldest labechiid stromatoporoid (Cystostroma) in a lithistid sponge–Calathium reef of the Hunghuayuan Formation in southeastern Guizhou, South China. These earliest stromatoporoids may have originated in reefs, and we argue that the complex topography created by the hypercalcified sponge Calathium facilitated the emergence of stromatoporoids. Beyond Cystostroma, keratose sponges, Pulchrilamina (hypercalcified sponge) and bryozoans have also inhabited in the micro‐habitats (cavities and hard substrates) provided by Calathium. These findings suggest that ecosystem engineering by Calathium played an important role in the further diversification of reefs during the Ordovician.