Linguliform Brachiopods first appeared at the beginning of the Cambrian, and they represent one of the key components of benthic communities during the Cambrian evolutionary radiation. The Linguliform order Acrotretida, which is uniquely characterised by a diminutive shell size and coniform ventral valve, had a relatively late origination, but subsequently diversified rapidly and became extremely abundant with a cosmopolitan distribution during the Cambrian and Ordovician. However, many questions concerning their early origin, divergence and phylogenetic relationships with other Brachiopod groups remain uncertain. To date, only six Acrotretide genera, including Eohadrotreta, Linnarssonia, Prototreta, Vandalotreta, Kuangshanotreta and Hadrotreta, are known from Cambrian Series 2. Recently published works show that Eohadrotreta rapidly achieved a wide distribution over key palaeocontinents (South China, East Gondwana, West Gondwana, North China), soon after their first appearance in Cambrian Epoch 2. Two species of Eohadrotreta (Eohadrotreta zhenbaensis and Eohadrotreta? zhujiahensis) were originally described from South China, but the diagnostic features to discriminate these taxa were not clearly defined, making it difficult to evaluate the biostratigraphy and phylogeny of this important genus. A subsequent detailed quantitative analysis of Brachiopod ontogeny demonstrated that the developmental pattern of the metamorphic shell was quite different in these two species, revealing heterochrony in the ontogeny of the earliest acrotretide representatives during the Cambrian radiation. Numerous morphological differences between Eohadrotreta? zhujiahensis and Eohadrotreta zhenbaensiss indicate that they may belong to separate genera.
In a paper published in the Journal of Systematic Palaeontology on 7 August 2020, Zhiliang Zhang of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments, Early Life Institute, and Department of Geology at Northwest University, the Department of Biological Sciences at Macquarie University, and Institute of Earth Sciences at Uppsala University, Lars Holmer, also of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments, Early Life Institute, and Department of Geology at Northwest University, and the Institute of Earth Sciences at Uppsala University, and Feiyang Chen and Glenn Brock, again of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments, Early Life Institute, and Department of Geology at Northwest University, and the Department of Biological Sciences at Macquarie University, describe a new Acrotretide genus from the Cambrian Series 2 Shuijingtuo Formation from southern Shaanxi and western Hubei provinces of South China.
Neoproterozoic to Palaeozoic sedimentary successions are well developed and distributed on the northern margin of the Yangtze Platform, South China. The Acrotretide Brachiopods described here come from the lower part of the Shuijingtuo Formation of South China. In southern Shaanxi, Precambrian to Cambrian strata are well exposed at the Xiaoyangba section, approximately 106 km south-east of Hanzhong City, including (in ascending order) the Ediacaran Dengying Formation, the Cambrian Xihaoping Member of Dengying Formation, the Shuijingtuo Formation and the Shipai Formation. The Shuijingtuo Formation (unconformably overlying the Xihaoping Member of the Dengying Formation) is characterised by organic-rich and nodular limestones at the lower boundary. The 31 m thick lower Shuijingtuo Formation is comprised of dark, thin silty limestones, while the 85 m thick upper part consists of siltstones interbedded with thin limestones. Large quantities of phosphatised shelly fossils have been reported from the lower part of the Shuijingtuo Formation at the Xiaoyangba section, which are dominated by the Eodiscoid Trilobite, Hupeidiscus orientalis, the Hyolith, Paramicrocornus zhenbaensis and Bradoriids. Associated fossils include the Brachiopods, Eohadrotreta, Palaeobolus, Botsfordia, and Lingulellotreta, the Tommotiid, Kelenella, Hyoliths, and Molluscs, along with Chancelloriids, Echinoderm plates, and Sponge spicules.
In the Three Gorges area of western Hubei, the Aijiahe section, approximately 25 km north-west of Yichang City, has been well studied. The 68 m thick Shuijingtuo Formation (unconformably overlying the Yanjiahe Formation) is composed of three parts. The 10 m thick basal part consists of condensed black shale intercalated with black limestone concretions, yielding the oldest known Trilobite, Tsunyidiscus actus, from this region. The 40 m thick middle part of the succession is dominated by alternating black shale and thin limestones, while the 18 m thick upper part consists of greyish-black argillaceous limestones. Abundant phosphatised shelly fossils have been described from the middle and upper parts of the Shuijingtuo Formation at the Aijiahe section with the Acrotretide Brachiopod, Eohadrotreta zhenbaensis, the Eodiscoid Trilobite, Tsunyidiscus actus, and Hyoliths are the most dominant groups. Other associated fossils include the Brachiopods, Palaeobolus, Spinobolus and Lingulellotreta, Archaeocyaths, Molluscs, Bradoriids, Chancelloriid sclerites and Sponge spicules.
All specimens described and illustrated by Zhang et al. are from the Shuijingtuo Formation (Cambrian Series 2) of South China. The specimens were obtained through acetic acid (10%) maceration of samples from thin-bedded limestones following standard techniques.
The new genus is given the name Palaeotreta, which derives from 'Palaeo', meaning ‘old’, from the Greek palaiós, describing an ancestral representative of Acrotretide Brachiopods, with the ending '–treta', meaning ‘perforated’ from the Greek trētos.
The shells of Palaeotreta are ventribiconvex, sub-circular to transversely oval, ornamented with evenly distributed pits, about 1 mm in diameter covering the metamorphic shell, and with finely concentric growth lines on the postmetamorphic shell. Ventral valve cap-like to low conical, pseudointerarea vestigial, from catacline to apsacline, with very short intertrough. Incipient pedicle notch is open during the long pedicle foramen-developing stage. Adult pedicle foramen located outside of the metamorphic shell. Growth lines developed on a separate area between the metamorphic shell and pedicle foramen. Apical process vestigial, very close to the pedicle foramen. Cardinal muscle scars and vascula lateralia weakly impressed. Dorsal valve slightly convex. Pseudointerarea orthocline, short with narrow subtriangular median groove. Median buttress generally well developed, while median septum vestigial. Cardinal muscle scars weakly impressed. Secondary shell layer columns relatively short.
The new genus is established for Acrotretide species that are similar to Eohadrotreta and Linnarssonia. However, the new genus differs from all other (especially from Cambrian Epoch 2) Acrotretides in having the following characters: (1) a catacline to apsacline inclination of ventral pseudointerarea with short intertrough; (2) a pedicle foramen that is located mostly outside the metamorphic shell; (3) a vestigial apical process and median septum; and (4) weakly impressed cardinal muscle scars. Eohadrotreta, Prototreta, Vandalotreta, Hadrotreta and Kotylotreta mainly have procline pseudointerarea with a relatively long intertrough. Although the Apsacline ventral pseudointerarea and internal structures (apical process, median septum) of Palaeotreta can be compared with Cambrian Epoch 3 taxa, such as Aphelotreta and the Furongian, Linnarssonella, the positions of pedicle foramen and ventral pseudointerarea are different. The ventral apical process and dorsal septum of Palaeotreta are very weakly defined, while Linnarssonia develops a high, boss-like apical process and has a high median septum. Eohadrotreta has a relatively more well-developed apical process and median septum. Vandalotreta has an apical process that forms a boss-like thickening anterior to the internal foramen and Hadrotreta develops apical pits that are situated close to the apical process. Kuangshanotreta is difficult to compare with the new genus in the absence of data on shell morphology and ornamentation.
Palaeotreta is the third genus of the Acrotretida that has been described from the Cambrian Series 2 of South China, and this region has a higher diversity of coeval Acrotretides as compared to South Australia, Laurentia, Antarctica and Siberia. The most unique character of Palaeotreta is the position of the pedicle foramen, which is located mostly outside of the metamorphic shell from the T2 ontogenetic stage. In this feature it has similarities to some Cambrian Ceratretidae and Ordovician Scaphelasmatidae species; but to date, this character is not known from early Cambrian Acrotretidae.
The first species assigned to the new genus is Palaeotreta shannanensis, a new species from the Shuijingtuo Formation (layer S4-3, 0.7 m above the base) at Xiaoyangba section in Zhenba County, south-eastern Shaanxi. A total of 21 ventral valves and nine dorsal valves were recovered from this locality. The name 'shannanensis' means 'from Shannan' in reference to the occurrence of the species in Shannan, southern Shaanxi Province.
The shell of Palaeotreta shannanensis is ventribiconvex, sub-circular in outline with round posterior margin. 1.1 μm hemispherical pits evenly distributed on the whole metamorphic shell surface without overlapping, while the post-metamorphic shell is covered by finely circular growth lines and drape structures. Shell structure consists of thin-lamella (2 μm) primary layer and thin-lamina (5 μm) secondary columnar layers.
Ventral valve sub-circular, on average 89% as long as wide with maximum width at mid-length. The valve is convex, with a cap-like shape, on average 21% as deep as long, with maximum height almost at midvalve. Metamorphic shell pronounced at the apex, occupying 13% of valve length. Pseudointerarea weakly developed, catacline to apsacline. Intertrough poorly defined, very short, occupying on average about 2% of the length and 6% of the width of the valve. Apical process vestigial, only observed in adult
specimens, close to pedicle foramen, occupying 20% of valve length. Enclosed pedicle foramen almost circular, about 50 μm in diameter, located mostly directly outside of the metamorphic shell, until valve reaches about 1100 μm in length. Growth lines developed t the posterior margin of the metamorphic shell and the lateral side of the pedicle foramen. Cardinal muscle scars and vascula lateralia weakly impressed.
Dorsal valve sub-circular, on average 87% as long as wide, with maximum width almost at mid-valve. It is slightly convex, on average about 19% as deep as long. Pseudointerarea small, orthocline, occupying 6% of the valve length and 36% of valve width. Median groove sub-triangular, short, about 31% of the pseudointerarea width. Median buttress generally developed, fading anteriorly. Median septum vestigial, only developed in adult valve, extending anteriorly for 59% of valve length. Cardinal muscle scars weakly impressed, occupying 17% of the length and 45% of the width of the valve.
Palaeotreta shannanensis shares a similar morphology in outline with Eohadrotreta zhenbaensis (from the same locality). However, the new species has a low cap-like shape, small pseudointerarea with very short intertrough, apsacline pseudointerarea in the adult valve, prolonged pedicle foramen-forming stage and a pedicle foramen that is located outside of the metamorphic shell. Furthermore, the new species has a less developed intertrough as compared with Eohadrotreta zhenbaensis; in the latter species the intertrough is becoming remarkably prominent in stage T3, which does not happen in the new species. The shell structure of Palaeotreta shannanensis is similar to that of Eohadrotreta zhenbaensis. However, the former species has a very thin primary layer about 2 μm thick, and very thin secondary columnar layers. The thickness of columns in Palaeotreta shannanensis is variable in different parts of the shell, ranging from 4 μm to 8 μm, and are quite short compared to the columns of Eohadrotreta zhenbaensis. The secondary layer insignificantly increases the overall thickness of the ventral valve, being composed of one to two columnar laminae, resulting in a low cap-like shape in Palaeotreta shannanensis.
Palaeotreta shannanensis differs from Vandalotreta djagoran and Linnarssonia rowelli in having a lower ventral valve, weakly developed apical process and median septum, apsacline ventral pseudointerarea, as well as a pedicle foramen that is mostly located outside the metamorphic shell. Palaeotreta shannanensis has a similar inclination of the ventral pseudointerarea as Aphelotreta khemangarensis, while the ornamentation of the metamorphic shell, pedicle foramen size and the median groove allow the distinction of the two species.
At the Xiaoyangba section, Palaeotreta shannanensis occurs at the base of the Shuijingtuo Formation, while Eohadrotreta zhenbaensis first occurs 60 cm higher. Therefore, Palaeotreta shannanensis represents the oldest known Acrotretide Brachiopod in southern Shaanxi. The diachronous nature of the stratigraphic hiatus at the base of Cambrian Series 2 across southern Shaanxi and western Hubei makes the biostratigraphic correlation between these regions imprecise, and more index fossils from new sections are required to better constrain the time gap and better resolve correlation.
The ontogeny of Palaeotreta shannanensis includes the formation of an enclosed pedicle foramen that is mostly located outside of the metamorphic shell, as well as an apsacline ventral pseudointerarea during later ontogenetic stages, which are unlike any other Acrotretides from Cambrian Epoch 2. The problematic Eohadrotreta? zhujiahensis is also referred to the new genus Palaeotreta. The new information on the development of the pedicle foramen, pseudointerarea, apical process and median septum is also shown to be important for deciphering phylogeny. Three ontogenetic stages (e.g. pedicle foramen-forming stage (T1), pedicle foramen-enclosing stage (T2) and intertrough-increasing stage (T3)) were identified for Eohadrotreta? zhujiahensis and can also be identified in Palaeotreta, with the species thus being renamed Palaeotreta zhujiahensis.
The shell of Palaeotreta zhujiahensis ventribiconvex, transverse oval in outline with slightly straightened posterior margin. 1.3 μm hemispherical pits evenly distributed on the whole metamorphic shell surface without overlapping while post-metamorphic shell covered by finely circular growth lines and drape structures. Shell structure consists of thin-lamella (2 μm) primary layer and thin-lamina (5–10 μm) secondary columnar layers.
Ventral valve sub-circular, on average 83% as long as wide with maximum width at the posterior half of valve. It is convex, with a low conical shape, on average 28% as deep as long, with a maximum height almost at mid-valve. Metamorphic shell pronounced at the apex, occupying 31% of the valve length. Pseudointerarea weakly developed, almost catacline, divided by a very short intertrough, which is on average about 5% of the length and 11% of the width of the valve. Apical process weakly developed, occupying on average 30% of valve length, close to pedicle foramen. Pedicle foramen is relatively large, about 90 μm in diameter, enclosed and located directly outside the metamorphic shell until valve reaches about 650 μm in length. Growth lines distinctively developed at the posterior margin of the metamorphic shell. Cardinal muscle scars and vascula lateralia weakly impressed.
Dorsal valve transversely oval, on average 82% as long as wide, with maximum width almost at mid-valve. Slightly convex, on average 17% as deep as long. Pseudointerarea small, orthocline, occupying about 7% of valve length and 38% of valve width. Median groove subtriangular, short, on average 44% of pseudointerarea width. Median buttress moderately developed, fading anteriorly. Median septum vestigial, only developed in adult valve, extending anteriorly at mid-valve. Cardinal muscle scars gently impressed, occupying 22% of the length and 51% of the width of the valve.
Based on their similar morphology, Palaeotreta zhujiahensis was originally considered to represent a second species of Eohadrotreta. However, there are significant differences in ontogenetic growth between these two species. New material collected from western Hubei demonstrates that Palaeotreta zhujiahensis has a lower ventral valve, straightened posterior margin, relatively larger pedicle foramen, late enclosure of the pedicle foramen, smaller ventral pseudointerarea, much shorter intertrough, weakly developed growth lines, apical process and median septum, thinner secondary layers and more weakly impressed cardinal muscle scars than those of Eohadrotreta zhenbaensis. Furthermore, the most characteristic feature of Palaeotreta zhujiahensis is that the pedicle foramen is located directly outside of the metamorphic shell, which fits closely with the diagnosis of the new genus Palaeotreta. The valve shape and ontogenetic development of the pedicle foramen in Palaeotreta zhujiahensis can be compared with that of Palaeotreta shannanensis. However, the former has a catacline inclination of the ventral pseudointerarea and a relatively longer intertrough, which is two times longer than that of Palaeotreta shannanensis.
The shell structure of Palaeotreta zhujiahensis is comparable with that of Palaeotreta shannanensis. Both have a very thin primary layer about 2 mm thick, but Palaeotreta shannanensis has relatively thinner columnar layers. The thickness of columns in Palaeotreta zhujiahensis is variable in different shell regions, ranging from 5 μm to 10 μm, which is quite short compared to the columns in Eohadrotreta zhenbaensis.
At the Aijiahe section, Palaeotreta zhujiahensis co-occurs with Eohadrotreta zhenbaensis at the middle part of the Shuijingtuo Formation. Compared to the biostratigraphy with southern Shaanxi, Palaeotreta zhujiahensis is slightly younger than Palaeotreta shannanensis. Palaeotreta zhujiahensis is the second species discovered in southern Shaanxi and western Hubei (after Eohadrotreta zhenbaensis), but Eohadrotreta zhenbaensis has a much wider palaeogeographical distribution.
Recent study of the earliest ontogeny of Cambrian Epoch 2 Acrotretide Brachiopods has revealed that the primary larval body plan of Acrotretides is shared with most early linguliforms, and that the metamorphic shell is formed at the end of the planktotrophic stage. The same kind of early ontogenetic characters can also be observed in Palaeotreta shannanensis. The raised lobes at the posterior end of both the ventral and dorsal valves of Palaeotreta shannanensis are about 50 μm in size, which is comparable to the protegula of Eohadrotreta zhenbaensis. They are probably secreted initially as a bivalved organic shell by the larval mantle lobes. During subsequent peripheral growth of the brephic shell at the planktotrophic stage, two pairs of lobes are preserved as imprints of larval setal sacs on the dorsal side of the larval body. Pits with an average diameter of 1 μm evenly cover the whole surface of the metamorphic shell, which may represent imprints of mineralised tablets that potentially protected the larvae against ultraviolet radiation penetrating surface waters. The metamorphic shell is separated from the post-metamorphic shell by a pronounced halo when the shell reached about 200 μm in width, which likely represents the completion of larval metamorphosis. A narrow belt about 20 μm wide, outside the metamorphic shell, lacking drape structures indicates the secretion of the neanic shell. Subsequently, the mature shell is developed by accretionary growth outside the neanic shell, exhibiting all major characters of the adult shell including drape structures and a three-layer shell. The growth of the mature shell indicates the development of marginal mantle after completion of metamorphosis. Detailed investigation of the earliest ontogeny of Palaeotreta zhujiahensis has previously been documented. The closely similar features of metamorphosis between Eohadrotreta and Palaeotreta imply that Palaeotreta probably had a life cycle similar to the early Cambrian Acrotretide Eohadrotreta and Lingulide Eoobolus, which are interpreted to have had a relatively prolonged free-swimming stage.
For the later post-metamorphic ontogeny, a strong allometric growth pattern has been demostrated in Eohadrotreta zhenbaensis by quantifying the ontogenetic variations in size and shape in successive ontogenetic stages. This also revealed that Palaeotreta zhujiahensis experienced three stages during the entire ontogenetic sequence, which can be well compared with the almost coeval Eohadrotreta zhenbaensis. When compared to the ontogeny of Eohadrotreta zhenbaensis, however, only two ontogenetic stages (T1–T2) can be distinguished during the growth of Palaeotreta shannanensis. During the T1 pedicle foramen-forming stage (pedicle opening is not enclosed as a foramen and the ventral intertrough is not developed), the shape of the ventral valve remains low and cap-like and the cardinal muscle scars, apical process and median septum are vestigial, while the incipient pedicle notch changes from semicircular to circular. In the subsequent T2 pedicle foramen-enclosing stage (enclosure of pedicle notch to form a foramen and development of an incipient apical process and median septum), the marginal accretionary growth increases the length and width of both valves, while the ventral valve maintains a low cap-like form. At the same time, cardinal muscle scars are weakly impressed, while the apical process and median septum are weakly developed. The inclination of the ventral pseudointerarea changes from catacline to apsacline, and the accelerated shell growth along the posterior margin of the metamorphic shell produces a posterior migration of the enclosed pedicle foramen. It is important to note that the stage T2 of Palaeotreta shannanensis is much delayed until valves reach about 1100 mm in length, which is twice the size compared to Eohadrotreta zhenbaensis. Furthermore, the boundary delineating the pedicle foramen from the metamorphic shell is developed early in stage T1, and successive growth lines of the propareas near the posterior margin of the metamorphic shell and lateral sides of pedicle foramen are well developed in stage T2. These remarkable ontogenetic variations are also well demonstrated in the entire three ontogenetic stages of Palaeotreta zhujiahensis.
The morphology of juvenile shells is quite similar between Palaeotreta and Eohadrotreta, which makes it difficult to distinguish them when a complete ontogeny is lacking. However, the heterochronic process probably occurred in later ontogenetic stages, resulting in quite different morphologies in adults of Palaeotreta and Eohadrotreta. A strong allometric growth pattern was verified in Eohadrotreta zhenbaensis especially during the T3 intertrough-increasing stage (complete development of ventral intertrough, apical process, median septum and imprint of vascula lateralia) by the rapid increase in the ventral intertrough length. Nonetheless, this ontogenetic variation in stage T3 for Palaeotreta shannanensis is completely lacking, resulting in a very small ventral pseudointerarea and short intertrough. As the growth of the intertrough is an important event in Acrotretide Brachiopods, the relatively slow development of the intertrough (about 1.6% length of ventral valve) in Palaeotreta shannanensis results in a change in inclination of the ventral pseudointerarea from catacline to apsacline, producing a low, cap-like valve shape. In contrast, the ventral valve of Eohadrotreta zhenbaensis becomes more conical with a very long intertrough (about 24.5% length of ventral valve) and the inclination of the ventral pseudointerarea changes from catacline to procline during stage T3. Compared with the slightly older Palaeotreta shannanensis, Palaeotreta zhujiahensis demonstrates a relatively rapid growth during ontogenetic stage T1, as the pedicle foramen is enclosed at an earlier time when the ventral valve reaches 650 μm in length, contrasting with 1100 μm in Palaeotreta shannanensis. Thus, there is an earlier termination of shape change in the younger Palaeotreta zhujiahensis during ontogenetic stage T1. Moreover, the growth of the ventral pseudointerarea greatly decreases during ontogenetic stage T3 (increasing 0.8% of the length and 3.5% of the width of the ventral valve), producing a cap-like shape, catacline ventral pseudointerarea and posterior migration of the pedicle foramen outside of the metamorphic shell as well. The morphological similarity between the adult valves (650–1600 μm) of Palaeotreta zhujiahensis and juvenile valves (smaller than 1110 μm) of Palaeotreta shannanensis indicates paedomorphosis (progenesis) of Palaeotreta zhujiahensis. Thus, heterochrony plays an important role in the differentiation of species in the new genus Palaeotreta.
Changes in shape through ontogeny and the different growth patterns in the two species of Palaeotreta may correspond to subtle modification in life mode during the gradual cumulative increase in size. However, the life habit of extinct acrotretides is still not well understood. Rare but exquisitely preserved fossils may suggest that Acrotretides attached to shell fragments or even to other benthic animals like sponges to achieve secondarily tiered niches. The low cap-like shape and apsacline ventral pseudointerarea indicate that Palaeotreta shannanensis is likely to have had a mode of life similar to Treptotreta with a thin pedicle anchorage at the apex of the ventral valve. The development of a slightly longer intertrough whilst maintaining a catacline pseudointerarea in Palaeotreta zhujiahensis may have aided stable attachment to the substrate.
The phylogenetic relationships of the Order Acrotretida to other groups within the Class Lingulata are still poorly resolved, though acrotretides possess many important unique characters, such as diminutive body size, conical shape of the ventral valve with enclosed pedicle foramen, simplified muscular system, developed apical process and often a raised median septum in the dorsal valve. Recent studies have revealed that bearing an incipient pedicle notch in the juvenile, pitted metamorphic shell and the presence of columnar shell structures in Acrotretides demonstrates a close relationship with the oldest known Lingulide Brachiopods. As the position of the developing pedicle foramen in relation to the metamorphic shell is one of the most taxonomically important characters in Acrotretides, investigation of the ontogeny of the pedicle foramen provides a new way of considering Acrotretide phylogeny. The incipient pedicle notch in juvenile specimens of Palaeotreta during ontogenetic stage T1 is interpreted to represent a similar ancestral type of pedicle opening between the ventral and dorsal valves that occurs in Linguliform Brachiopods. The weakly developed ventral pseudointerarea is comparable with the raised pseudointerarea above the valve floor in older lingulides such as Eoobolus. In forming a high conical ventral valve, the cardinal muscles move posteriorly to the raised pseudointerarea in Acrotretides. During later ontogeny, the slow growth of the intertrough results in a catacline pseudointerarea in Palaeotreta zhujiahensis, whilst the lack of major growth of the intertrough changes the pseudointerarea from catacline to apsacline in Palaeotreta shannanensis. In contrast, the rapid growth of the intertrough in Eohadrotreta zhenbaensis results in a significant gradual transition of the ventral valve pseudointerarea from catacline to procline. The new evidence presented by Zhang et al. supports the hypothesis that the ventral intertrough in Acrotretide Brachiopods evolved by the gradual and variable ‘rolling up’ of the Lingulide propareas along the posterior margin of the ventral valve in association with changes in the inclination of the ventral intertrough. A distinct evolutionary lineage from basal Botsfordiids that developed an open delthyrium and vestigial ventral pseudointerarea to more derived Acrothelids which have an enclosed pedicle foramen and raised ventral pseudointerarea probably indicates that this monophyletic clade is derived from the Lingulides.
There are three main positions of the pedicle foramen in the fossil record of Acrotretides concerning its position related to the metamorphic shell. Type 1 is not enclosed in the metamorphic shell, Type 2 is completely enclosed in the metamorphic shell and Type 3 is mostly located outside of the metamorphic shell. The first type, where the pedicle foramen is partly enclosed in the metamorphic shell, occurred widely across genera beginning in Cambrian Epoch 2, such as Linnarssonia, Prototreta (or Homotreta), Eohadrotreta, Hadrotreta and Vandalotreta. The second and third types (Type 2–3), where accelerated shell growth along the posterior margin of metamorphic shell resulted in anterior or posterior migration of the pedicle foramen respectively, become common later in Cambrian Epoch 3 and the Ordovician (e.g. Apsotreta and Scaphelasma). However, this is the first discovery of the Type 3 during Cambrian Epoch 2 in Palaeotreta from South China.
The morphological diversity amongst Acrotretide Brachiopods in Cambrian Stage 2 of South China includes great variation in the position of the pedicle foramen, shell growth patterns and inclination of the ventral pseudointerarea. The low cap-like shape, vestigial apical process and median septum, very short intertrough and longer pedicle foramen-forming stage (T1) probably imply that Palaeotreta shannanensis is one of the ancestral stock of Acrotretides. This is also supported by its early occurrence in the Shuijingtuo Formation of southern Shaanxi. Compared with the other two species Palaeotreta zhujiahensis and Eohadrotreta zhenbaensis, Palaeotreta shannanensis has a more restricted distribution and lower abundance, which may indicate this ancestral acrotretide was not suited to life in distal mixed clastic platforms. By contrast, Eohadrotreta zhenbaensis has more derived characters exemplified by faster growth rates during accretionary shell growth, especially during stage T3, enabling a wider geographical distribution and increased abundance in shallow carbonate environments. The evidence presented here suggests evolutionary modification to ontogenetic changes resulted in the early divergence of Palaeotreta and Eohadrotreta, and played an important role in the early evolution of Acrotretides. The early Cambrian widespread Acrotretides Hadrotreta and Prototreta, developing a long ventral intertrough and dorsal median septum, may have a similar heterochronic process to Eohadrotreta zhenbaensis during the T3 intertrough increasing stage. Heterochrony also probably drove rapid morphological variation in acrotretides, explaining their subsequent radiation and optimal adaptation across a wide range of shallow marine depositional environments. These adaptations contributed to the success of younger Acrotretide Brachiopods, especially during the transition from Cambrian Epoch 2 to Epoch 3 and their subsequent radiation during the Great Ordovician Biodiversification Event.
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