Early Triassic benthic invertebrates from the Great Bank of Guizhou, South China: systematic palaeontology and palaeobiology

Abstract

To further our understanding of the evolution, selectivity and ecological composition of marine communities following the latest Permian mass extinction, new collections from underrepresented regions in the immediate extinction aftermath are required. Here, we provide new systematic data and the first palaeobiological account of the benthic invertebrate community from the Great Bank of Guizhou, South China. We systematically describe three brachiopod species, 26 bivalve species, 11 gastropod species, 1 microconchid and 1 crinoid species. The descriptions include 5 new species; 2 bivalve species (Hoernesia? danisae, Atomodesma? hautmanni) and 3 gastropod species (Donaldina erwini, Cossmannina alfischeri and Vernelia samae). This is the most species-rich benthic community known so far from the extinction aftermath, which is typically characterized by a high proportion of Permian holdover genera and cosmopolitan taxa. Taxonomically, this community is different from coeval faunas with dissimilarity values >60%. Ecologically, however, this fauna is similar to faunas from the Dolomites (Italy) and East Greenland. This new data, therefore, suggests that the lower Griesbachian invertebrate faunas were taxonomically heterogeneous, whereas ecologically they were relatively homogenous. The marine community on the Great Bank of Guizhou records genera that survived the mass extinction event with some, but not all, recording a size reduction, that is, the Lilliput effect. The absence of large body fossils and the preferential survival of small species suggest that the mass extinction event was size-selective.

Facies selectivity of benthic invertebrates in a Permian/Triassic boundary microbialite succession: Implications for the “microbialite refuge” hypothesis

“…Studies of living microbialites have observed metazoans exploiting local oxygen refugia within the microbialites (e.g., Gingras, Lalond, Amskold, & Konhauser, ; Rishworth, Perissinotto, & Bird, ; Tarhan, Planavsky, Laumer, Stolz, & Reid, ). These metazoans, however, are typically soft‐bodied, infaunal, and motile, which is not consistent with the epifaunal, stationary lifestyles of the benthic communities reported from the Great Bank of Guizhou (Foster et al., ). In addition, if the microbial mats were a preferable environment, we would expect that the microbial facies would contain a greater species richness and abundance than the surrounding non‐microbial rocks, which was not observed in this study (Figures and ).…”

“…Therefore, this explains why the microbialites record a benthic community and why the non‐microbial fabric contains significantly more bioclasts than the microbial fabric. Systematic studies of the species that occur in the microbialites also find that at both the genus and species levels the microbialites did not contain a unique suite of benthic invertebrates, and these taxa are also mainly characterized as cosmopolitan taxa (Foster et al., ; Hautmann et al., ). This observed difference between microbial and non‐microbial facies contradicts the definition of a refugium (i.e., an area which a population can survive through periods of unfavorable conditions), implying that microbialites were not a refuge in the aftermath of the latest Permian mass extinction.…”

Suppressed competitive exclusion enabled the proliferation of Permian/Triassic boundary microbialites

“…The presence of such obligate aerobes was not used to falsify anoxic conditions, but instead, the microbial mat communities that precipitated the buildups were interpreted as unique oxygenated refugia in otherwise anoxic settings; this is known as the microbialite refuge hypothesis (Forel et al , ). Even though microbial mats can provide oxygenated refugia in hypoxic and anoxic environments (Gingras et al , ), the fauna that occurs in the Early Triassic microbialites are mostly not animals that can live within a microbial mat (Foster et al , ). In addition, the fossils within the microbialites are allochthonous and have been transported around and into the microbial frameworks, thus the animals did not apparently live within the microbial mats (Foster et al , ) (Figure ).…”

“…The microbial buildups on the Great Bank of Guizhou occur within a narrow range of environmental conditions. The thickest microbialite successions occur on the inner platform and are intercalated with oolitic and skeletal packstones and grainstones that contain a diverse, para‐autochthonous assemblage (Foster et al , ). Cross‐bedding can be recognized in the grainstones that drape over the microbialites.…”

Echinoids from the Tesero Member (Werfen Formation) of the Dolomites (Italy): implications for extinction and survival of echinoids in the aftermath of the end-Permian mass extinction

“…This is exemplified by the fact that there are currently only three species known from relatively well-preserved echinoid tests from Lower Triassic strata (Linck, 1955; Kier, 1968b; Godbold et al, 2017). Despite the rarity of well-preserved specimens, disarticulated echinoid plates and spines are abundant throughout Lower Triassic strata (Broglio Loriga, Masetti & Neri, 1983; Schubert & Bottjer, 1995; Moffat & Bottjer, 1999; Rodland & Bottjer, 2001; Fraiser & Bottjer, 2004; Nützel & Schulbert, 2005; Mata & Woods, 2008; McGowan, Smith & Taylor, 2009; Hofmann et al, 2013; Pietsch et al, 2019; Foster et al, 2018; Foster et al, 2019a; Foster et al., 2019b), though they are typically identified in thin section (e.g., Noé, 1987; Krystyn et al, 2003; Posenato, 2009; Foster et al, 2019a; Foster et al., 2019b). A potential source of “hidden diversity” comes from this disarticulated material (Twitchett & Oji, 2005; Pietsch et al, 2019) as echinoid coronal plates, like the ossicles of other echinoderm groups, display morphological features characteristic of both higher, and lower, taxonomic levels (Gordon & Donovan, 1992; Nebelsick, 1992; Nebelsick, 1995; Nebelsick, 1996; Donovan, 2001; Thompson & Denayer, 2017).…”