Palaeontology, Palaeoecology And Palaeobiogeography Of Tentaculitoids In The Middle Palaeozoic

Tentaculitoids, an enigmatic group of marine invertebrate, occur in great numbers in the Silurian and Devonian. Most investigations of these organisms have focused mainly on their taxonomy, such as descriptions of new taxonomic units, establishment of classification schemes and speculations on their biological affinities. Meanwhile, biostratigraphy of tentaculitoids has also attracted much attention. Biostratigraphic zones of Devonian tentaculitoids have been accepted generally for global and regional stratigraphic division and correlation. However, detail studies have never been on their palaeobiogeography, palaeoecology and evolution.Qualitative data on global tentaculitoids from the Llandovery to the Famennian, have been reviewed as to their stratigraphic, geographic distribution and diversity at the generic level. Seven conclusions have been released as follows:(1) From the Llandovery to the Wenlock, the geographic range of tentaculitoids expanded, and remained constant in the Ludlow and Pridoli. Afterwards, it increased dramatically having attained a maximum in the Early Devonian, and thereafter decreased continuously in the Middle and Late Devonian. The variation in geographic range is coincident’ with that of the normalized generic diversity of tentaculitoids from the Llandovery to the Famennian;(2) There was a negative correlation between sea level and the normalized generic diversity of tentaculitoids. A lowering of global sea level was coincident with increase of the normalized generic diversity of tentaculitoids, whereas rise in sea level is coincident with decline of the normalized generic diversity of these organisms;(3)Tentaculitoids were environmentally sensitive and stenotopic organisms which were restricted exclusively to tropical latitudes;(4) Gotlandellites is here regarded as the oldest known genus of tentaculitoid originated from the Llandovery in Europe and disperse to Saudi Arabia and Australia in the Wenlock;(5)The migration of tentaculitoid resulted from the gyre in the Palaeo Pacific Ocean and in the Palaeo-Tethys in conjunction with the movement of the plates and rise in global sea level;(6) Tentaculitoids disappeared in the early Famennian. It was a normal extinction. Invasive species is proposed to be a considerable threat for the extinction of tentaculitoids in the Late Devonian. Ecological pressure created by the indigenous organisms should also be in consideration as a dynamic for the extinction of tentaculitoids.Devonian tentaculitoids are abundant and widely distributed in South China and Sibumasu. Based on an extensive literature review and our data, a significant fluctuation on tentaculitoid geographical and stratigraphic distribution, as well as generic diversity from the Lochkovian to the Famennian in these two plates are demonstrated as follows:(1) Tentaculitoids expanded their geographic range from the Lochkovian to the Emsian in Sibumasu and South China. After that, it decreased progressively until the end of the Frasnian in Sibumasu, while remained constant throughout the Eifelian and the Fransian in South China;(2) Normalized diversity of tentaculitoids climbed up at genus and species levels from the Lochkovian to the Pragian in Sibumasu, and decreased continuously until the end of Frasnian. Meanwhile, it increased at genus and species levels generally in South China throughout the Lochkovian and Emsian, and declined constantly until the early Famennian;(3)There was a positive relationship on the whole between global sea-level and the normalized diversity at genus and species level of Devonian tentaculitoids in South China. The variation in geographic range of Devonian Tentaculitoids in Sibumasu was coincident with that of the normalized diversities at both generic and specific levels, but opposite to that of global sea-level.Until now, no detailed studies of tentaculitoids have focused on their taphonomy or geobiology. Meanwhile, there are conflicts over their ecological habit all along. Some data noted them as plankton, but some others treated them as benthos, nektobenthos or hemiendobenthos. With the help of field work and polished thin sections, ecology of tentaculitoids is investigated by means of close and constant observation on their state of preservation and conch malformation in South China, three opinions concluded:(1) Tentaculitoids could be preserved in body fossils, mold and cast fossils. The calcitic conchs can be retained or eroded, and in some cases they were even pyritized or silicified. Most tentaculitoids were preserved more or less parallel to bedding of strata that enclose them, few others were vertically or obliquely;(2) Tentaculitids probably inhabited in the aerated and muddy shallow as an epibenthos with its aperture down vertically. It could swim in a short range with the aid of imperforate septa;(3) Dacryoconarids and homoctenids were marine plankton living in the upper ocean with their aperture up vertically;(4) Tentaculitoids are inferred to be filter-feeding animals, instead of being omnivores or carnivores for reasons of its tiny shell and limited locomotivity. As to classification of tentaculitoids, many schemes had been introduced in Russian, English, French and Chinese respectively. The publications, as a language barrier, hampered communication and resulted in considerable repetitive work. For instance, several new taxonomic units, those established in German, were overlooked totally in many English publications. Based on extensive review and out data, a new classification scheme of these neglected organisms at the generic level is introduced in this paper. Tentaculitoids are classified into three orders according to their larval morphology. Two superfamilies in order Dacryoconarida are distinguished by thickness of conchs (whether interior conch wall is parallel to the exterior one). Eighteen families are established on the basis of their conch symmetry (bend or straight) and ornamentation types, and sixty four genera are separated in the light of ornamentation shape and their arrangement. Eleven synonyms, eleven homonyms, and thirty eight genera, including two subgenera without sufficient evidences, are abandoned. Two families and two genera are newly introduced in the opinion of the new classification schemes. They are Styliacudae Fam. nov., Albertidae Fam. nov., Alberticus gen. nov., Cyrtolarvacus gen. nov. Tentaculitoids are assigned to be one of the most primitive branches originated from mollusk on account of their similarities in morphology, composition, microstructure of the shell, and resemblances in their ecology and ontogeny between molluscs and the oldest Gotlandellites.With the aid of polished thin sections, heating-acid-digestion experiment and scanning electron microscope, three new genera and ten new species are established at three sections (Huangmaoping section, Taihecun section and Wuxiangling section) in South China. They are Tentaculites tenuiparva sp. nov., Seretites caltulinus sp. nov., Podolites cylicus sp. nov., Odessites auresites sp. nov., Bicingulites nanningensis sp. nov., Singlering varing gen. et sp. nov., Singlering hamonites gen. et sp. nov., Sensimeudio sharpante gen. et sp. nov., Sensimeudio undansantes gen. et sp. nov., Costelix xinpoensis gen. et sp. nov..Hitherto, no deep research has been tested to the evolution of tentaculitoids except for some speculations among some species or subgenera in Nowakia and Procorniculina. Evolutionary processes have been examined in the light of their stratigraphic and geographic distribution, and observation on their morphological change of conchs at genus level from the Silurian to the Devonian. Some ideas are summarized as follows:(1) Tentaculitids were dominated in the Silurian. They had gone through a process of evolution by converting their strongly curved conchs into straight ones from the Llandovery to the Early Devonian;(2) Tentaculitids exterior ornamentations and interior structures diversified generally during the Llandovery and the Early Devonian;(3) Dacryoconarids flourished in the Early and Middle Devonian. Their conchs gradually evolved from being bilaterally to radially symmetric;(4) Larval bulb of tentaculitoids was conical in the Silurian. Thereafter, trumpet-like initial chamber top with apical spine, slightly inflated tube initial chamber with apical spine ahead, and drop-like or rounded initial chamber with or without short apical spine occurred in swift succession in the Lochkovian. In other words, homoctenids were originated from tentaculitids, and evolved into dacryoconarids. Namely, their larva underwent a planktotrophic-mixtrophic-autotrophic evolutionary process in succession;(5) Larval bulb of tentaculitoids evolved from being bilaterally to radially symmetrical in the Early Devonian.