Chapter 10 Biogeography of Ordovician linguliform and craniiform brachiopods

Abstract The biogeographical patterns shown by Ordovician linguliform and craniiform brachiopods are greatly influenced by their dominance in low-diversity associations in marginal environments. This is particularly evident in the Early Ordovician, when linguliform-dominated dysaerobic assemblages are widely distributed along the deep shelves of Gondwana, the Kazakhstanian terranes and in Baltica. By the Darriwilian, micromorphic linguliforms are characteristic components of the pantropical climatic-controlled faunas of Laurentia, Cuyania and Kazakhstanian terranes, which – in spite of separation by extensive oceans – retain a distinct similarity. Analysis of craniiform biogeographical distribution is impeded significantly by the poor state of craniide taxonomy and lack of reliable data from most regions. However, in general their biogeographical dispersion is similar to other groups of the Palaeozoic Evolutionary Fauna. Unlike the linguliforms, which are important members of the Cambrian Evolutionary Fauna, there is no convincing Cambrian craniiform record; they may have evolved and dispersed from Gondwana and associated microcontinents and island arcs. The earliest well-established record is from the late Tremadocian of temperate to high-latitude peri-Gondwana. During most of the Ordovician, they have a peri-Iapetus distribution. They are very rare or absent in tropical Gondwana, South China and Kazakhstanian terranes and are not yet documented from Siberia. The trimerellides probably evolved in tropical peri-Gondwanan island arc settings. Their dispersion and major features of biogeography mirror those of atrypides.

Linguliform and craniiform brachiopods comprise a relatively minor, but distinctive component of Ordovician benthic faunas. Linguliforms are recognized as one of the major components of the Cambrian Evolutionary Fauna, together with trilobites and hexactinellide sponges (Sepkoski 1981), whereas the existing Cambrian record of craniiforms is sparse (Popov et al. 1999a), and in terms of three evolutionary faunas recognized by Sepkoski (1981) they can be considered as a minor component of the Palaeozoic Evolutionary Fauna (Harper et al. 2004). The biogeography of the Ordovician linguliforms and craniiforms has not previously been the subject of a separate comprehensive analysis, but has generally been considered together with biogeographical studies of rhynchonelliform brachiopods. Ordovician linguliform and craniiform brachiopods proliferated in marginal marine environments where they formed low-diversity associations often dominated by a single taxon. Some opportunistic linguliform-dominated faunal associations, which are associated with intervals with significant environmental changes, in tandem with trimerellide associations, reveal characteristic distributional patterns that have proved helpful in palaeogeographical reconstructions . Craniiforms, in particular the trimerellides, have also proven biogeographically important (Popov et al. 1997). However, because of the low diversity of these associations, the conventional methods (such as cluster and principal component analyses) have a limited application here.

Linguliform brachiopods
The life strategies and evolutionary history of linguliforms differ significantly from those of other brachiopod subphyla ). These differences are clearly reflected in their distinctive patterns of biogeographical distribution during the Ordovician, as discussed below. Recent linguliforms have a planktotrophic larva with a prolonged free-swimming stage in their ontogeny, which was retained from ancestral Cambrian stocks (Freeman & Lundelius 1999). By the beginning of the Ordovician they became adapted to a wide spectrum of marine environments from near-shore to abyssal depths Tolmacheva et al. 2004). In the Ordovician, linguliform brachiopods played a significant role in faunal assemblages that were characteristic of marginal environments, for example in near-shore mobile sands and dysaerobic conditions, which explains the special biogeographical patterns shown by the group. The Ordovician also is characterized by distinctive benthic associations of micromorphic linguliform brachiopods and there is growing evidence (Mergl 2002;Holmer et al. 2005) that they commonly had symbiotic relationships with sponges, a life habit which clearly affected the pattern of their biogeography. In addition, a wide range of different linguliform biogeographical patterns are evident in cases where they form minor components of various benthic communities that are dominated by other filter feeders, for example rhynchonelliform brachiopods and bryozoans.
The quality of published global data on the biodiversity of Ordovician linguliforms is variable. It is relatively good for Baltica and high-to temperate-latitude Gondwana, and there are also good data compiled for Laurentia, the Australasian sector of low latitude Gondwana, Avalonia and the Kazakhstanian terranes. By contrast, there is only a single publication on the Mid Ordovician micromorphic linguliforms from South China, and no available data from Siberia and North China. Moreover, a considerable amount of data on the Ordovician linguliforms unfortunately remains unpublished, including descriptions of Late Ordovician microbrachiopods from Ireland (Avalonia), presented in an unpublished PhD thesis by McClean (Wright & McClean 1991), and of the Mid Ordovician faunas of the Argentinean pre-Cordillera.

Early Ordovician (Tremadocian -Floian)
The end of the Furongian (Late Cambrian) Epoch saw a major decline in the generic diversity of linguliform brachiopods. The decline is best seen in outer shelf and basinal faunal associations , which in the early Tremadocian Age were occupied mainly by the low-to medium-diversity Broeggeria Association. This association was distributed widely along the deep shelves of Gondwana, the Kazakhstanian terranes and in Baltica . The Broeggeria Association often tracks the distribution of dysaerobic environments, which are characterized, in particular, by the olenid trilobite biofacies. The Tremadocian micromorphic brachiopod associations mostly have a low diversity and they are dominated by Eurytreta and Ottenbyella, which have almost cosmopolitan distribution. Similar Tremadocian micromorphic brachiopod associations have been documented from Baltica , Laurentia (Popov et al. 2002;Holmer et al. 2005) and Kazakhstanian terranes Holmer et al. 2001), and from various peri-Gondwanan regions Streng et al. 2011).
In the transitional period from the Furongian to the early Tremadocian, a number of low-diversity obolid associations can be found on the shallow clastic shelves of Baltica and high-to temperatelatitude Gondwana, including the Obolus Association in Baltica , and the Tunisiglossa Association in the North African sector of Gondwana (Havlíček 1989;Fig. 10.1).
The late Tremadocian to early Floian transition (Paltodus deltifer to Prioniodus elegans biozones) is associated with an increased turnover and a significant proliferation of lingulate brachiopod faunas . In the shallow temperate-latitude clastic shelves of Gondwana and Baltica, the early Tremadocian obolid associations were replaced mainly by the Hyperobolus Association (Bohemia, Australasian sector of Gondwana and the South Urals; Havlíček 1982Havlíček , 1989Percival & Engelbretsen 2007) and later by the Thysanotus-Leptembolon Association ( Fig. 10.1), which then spread widely in Bohemia, the East Baltic, South Urals, Alborz and Central Iran (Mergl 1986(Mergl , 1996(Mergl , 1997Ghobadi Pour 2008;Popov et al. 2008;Fig. 10.1). In addition to epibenthic obolids, which form the core of the association, there are also some infaunal taxa (e.g. Leptembolon and Lingulella) together with epibenthic spinose siphonotretides (e.g. Eosiphonotreta and Siphonobolus).
The expansion of the Thysanotus-Leptembolon Association was diachronous and associated with significant environmental changes. In Baltoscandia and the Alborz peri-Gondwanan terrane, this expansion occurred just prior to the onset of temperate-latitude carbonate sedimentation, which coincided in both regions with proliferation of benthic associations with clear characteristics of the Palaeozoic Evolutionary Fauna (Bassett et al. 2002;Popov et al. 2008). In Bohemia, the Thysanotus -Leptembolon fauna appeared because of immigration after a period of non-deposition corresponding to the entire Furongian Epoch (Mergl 1986). Bednarczyk (1999) suggested that the Thysanotus Leptembolon fauna may have been of a peri-Gondwanan origin, and this is now supported by the earliest known occurrence of Thysanotus from the Shirgesht Formation of Derenjal Mountains in Central Iran, where it is associated with the billingsellide brachiopod Protambonites and the conodont Cordylodus angulatus .
The Furongian to early Tremadocian interval in Baltica and temperate-latitude Gondwana was a time of proliferation of spinose siphonotretides (Family Siphonotretidae), which show a distinctive geographical distributional pattern (Popov et al. 2009a; Fig. 10.1). The earliest spinose siphonotretides are confined to Gondwana (Bohemia, Alborz, Central Iran). In Baltoscandia, the siphonotretides are represented by the enigmatic genus Helmersenia, which is characterized by having a coarsely punctuate shell without spines (Williams et al. 2004) and forms almost monotaxic associations. The equally enigmatic siphonotretide Schizambon lacks both spines and perforations; it is found on both sides of Iapetus, but is more abundant in Laurentia (Holmer et al. 2005;Popov et al. 2009a). By the end of the Tremadocian, spinose siphonotretides replaced Helmersenia in Baltica and also spread to Laurentia (Holmer et al. 2005).
The Middle Ordovician interval from the Lenodus variabilis to Pygodus anserinus biozones is the best known interval in terms of the biogeographical distribution of lingulate microbrachiopod assemblages. They are represented on the eastern (Alabama) and western (Nevada) margins of Laurentia (Cooper 1956;Krause & Rowell 1975), in several Kazakhstanian terranes (Popov 2000a), Iran , New Zealand ) and South China (Zhang 1995), and they are widely spread in Baltoscandia (Gorjansky 1969;Holmer 1989). In addition, significant unpublished data are available for the Cuyania Terrane in the Argentinean Precordillera.
The major features of temperate-latitude lingulate brachiopod faunas can be typified by those described from the Mid Ordovician of Baltica (Gorjansky 1969;Holmer 1989), Iran ) and South China (Zhang 1995). They are characterized by a low taxonomic diversity and abundance of ephippelasmatids belonging to the genera Aipyotreta, Myotreta and Numericoma. Other characteristic genera are Scaphelasma (Scaphelasmatidae), Torynelasma (Torynelasmatidae), Biernatia and Eoconulus (Eoconulidae). Most of these genera, with the exception of Aipyotreta, Myotreta and Sasyksoria, are cosmopolitan. Torynelasmatids are present in the most of known faunas of that age, but absent in Avalonia, Bohemia and Alborz, which were located at temperate latitudes, and they are also absent from the Cuyania Terrane. The torynelasmatids are the most diverse in the Kazakhstanian terranes, where they are represented by the endemic genera, such as Christicoma, Issedonia and Polylasma. The ephippelasmatid Lurgiticoma is known from Kazakhstan and New Zealand (Nazarov & Popov 1980;Percival et al. 2009) and was probably tropical peri-Gondwanan. A remarkable feature of the Chinese fauna is the lack of the lingulids Elliptoglossa and Paterula, an almost complete absence of the family Acrotretidae (e.g. Acrotreta, Conotreta, Cyrtonotreta and Spondylotreta), which are rather characteristic of the contemporaneous faunas in Baltica, Laurentia and Kazakhstanian terranes, and the abundance of Myotreta, which is known otherwise only from Baltica ( Fig. 10.2). The co-occurrence of Myotreta indicates that the Chinese fauna has distinct links to the contemporaneous Baltic fauna, which is also shown by the joint occurrence of common species like Numericoma perplexa, N. spinosa, Acanthambonia cf. delicata and Scaphelasma mica. Another characteristic Baltic genus is Biernatia, and it is remarkable that biernatids are restricted in geographical distribution to Baltica during the Early Ordovician, whereas in the Dapingian and most of the Darriwilian they occur only in Baltica and Alborz Fig. 10.2). Only from the late Darriwilian -early Sandbian interval (Pygodus serra -Pygodus anserinus biozones) did the Biernatidae become almost cosmopolitan in distribution, and by that time the family had spread to Laurentia, Cuyania, New Zealand (Australasian sector of Gondwana) and Kazakhstanian terranes, as well as to South China. The Biernatidae is also represented by two endemic genera in Avalonia .
The late Darriwilian was a time of significant changes in taxonomic composition of microbrachiopod assemblages, with the number of cosmopolitan taxa increased, whereas the biogeographical differences between individual faunas decreased. In South China the generic diversity of micromorphic lingulates nearly doubled in the Pygodus anserinus Biozone and such genera as 30°3 0°6 0°0°U ra ls B a lt ic a Sib eri a L a u r e n t ia G o n d w a n a P e ru n ic a H e ll e n ic C e n tr a l Ir a n  (Fig. 10.3). One includes faunas from South China, Alborz, Bohemia and Avalonia. In the Darriwilian, all of these regions were situated in high or temperate latitudes in relative proximity, or within a Gondwanan margin affected by the influence of the cool Southern Subpolar Current (Fig. 10.4).
The second cluster is confined mainly to Laurentia, the tropical Kazakhstanian terranes and to Baltica, which was situated in temperate latitudes. Unlike the Early Ordovician Epoch, by the Darriwilian Baltica had moved on a considerable distance from Gondwana (Torsvik & Rehnström 2003) and supported essentially endemic faunas of rhynchonelliform brachiopods and trilobites (Fortey & Cocks 2003). By that time it had drifted away from the Southern Subpolar Current, which controlled faunal migrations along the western Gondwanan coast (Wilde 1991;Ghobadi Pour et al. 2010). This location may also suggest that there was a relatively free exchange of micromorphic linguliform brachiopod faunas across the Iapetus between Baltica and Laurentia during the Darriwilian.
Remarkably, Baltica forms a subcluster with the Darriwilian faunas from the Cuyania Terrane (yet to be formally described), which was located in proximity to the South American sector of Gondwana on the opposite side of the ocean in approximately the same southern latitudes (Fig. 10.3).
The low-latitude micromorphic linguliform brachiopod faunas are characterized in general by higher taxonomic diversity and the occurrence of pantropical genera like Rhysotreta and Undiferina (the latter genus makes its first appearance in South China only in the Pygodus anserinus Biozone, and in Baltoscandia it is known only in the Sandbian).
In general, the observed patterns of biogeographical distribution of micromorphic lingulate brachiopods through the Mid Ordovician Epoch suggest that this was controlled mostly by climatic factors. Thus, in spite of a considerable oceanic space separating Laurentia, Cuyania, the Kazakhstanian terranes and the Australasian sector of Gondwana, they retain a distinct similarity across the low latitudes.

Late Ordovician (Sandbian to Hirnantian)
Linguliforms from the Late Ordovician Epoch are the least well studied faunas, and there are only a few published studies from Baltica (Holmer 1986(Holmer , 1989(Holmer , 1991a, Kazakhstanian terranes (Nazarov & Popov 1980;Popov 2000a, b), Avalonia (Wright 1963) and the Australian sector of low-latitude Gondwana (Percival 1978;Percival et al. 1999). There is therefore no good basis for a detailed analysis of the biogeographical distribution of linguliforms within the Late Ordovician Epoch.
In Baltica, micromorphic linguliforms are comparatively well known, and by the Sandbian -early Katian the associations include cosmopolitan lingulides such as Paterula and Rowellella, and the equally cosmopolitan acrotretides Hisingerella, Biernatia, Opsiconidion and Acrotretella, together with the siphonotretide Acanthambonia (Holmer 1986;. There is also a moderate abundance of poorly known large lingulides, including several species of the infaunal Pseudolingula in Baltica plus several poorly known macroscopic linguliforms and siphonotretids (Gorjansky 1969;Holmer 1991a, b).
Linguliforms described from the Australian sector of low latitude Gondwana include both endemic taxa, such as Casquella and Anomaloglossa, and the cosmopolitan Paterula, Elliptoglossa   and Hisingerella (Percival 1978;Percival et al. 1999). Mid Katian microbrachiopod assemblages from the lower Malongulli Formation of New South Wales (Percival et al. unpublished data) contain characteristic pantropical acrotretide genera, for example, Torynelasma, Undiferina, Rhysotreta and Nushbiella, in addition to the numerous cosmopolitan genera, which constitute a background assemblage. Kazakhstanian terranes, which mainly occupied subequatorial positions west of the Australian sector of Gondwana (Popov et al. 2009b), also supported diverse micromorphic linguliform faunas (including Acrotreta, Biernatia, Dictyonites, Ephippelasma, Rowellella and Scaphelasma). There are a few endemic taxa, for example, acrotretoids Naimania and Tasbulakia, whereas Veliseptum is also known from the Katian of Baltica (Popov 2000a, b;Holmer 1986).
There is little data from late Katian and Hirnantian linguliform brachiopods; however, there is little doubt that micromorphic lingulate brachiopod assemblages were affected severely by the first wave of the terminal Ordovician extinction events at the end of the supernus Biozone (Harper et al. 2004).

Craniiform brachiopods
A lecithotrophic craniiform larva with its short planktonic stage had evolved in craniiforms by the Mid Ordovician Epoch (Popov et al. , 2012. This suggests that craniforms are a potentially valuable group in palaeogeographical reconstructions. However, this analysis is impeded significantly by the issue related to the current interpretation of craniide taxonomy. The generic affiliation of a number of Ordovician species referred currently to Petrocrania and Philhedra is uncertain, and both genera are a 'waste basket' for numerous poorly defined craniide taxa of uncertain affinity.

Early to Mid Ordovician
The oldest known craniides are Petrocrania? sp. from the upper Tremadocian deposits of Bavaria (Sdzuy et al. 2001) and Petrocrania? caputium Mergl 2002 from the Milina Formation (Tremadocian) of Bohemia. As yet undescribed craniides are also reported from Lower Ordovician olistoliths in the Silurian Pulgon Formation of southern Kyrgyzstan (Harper et al. 2004). The Mid Ordovician record of peri-Gondwanan craniides is sparse. It includes Celidocrania from the Dapingian to lower Darriwilian [Arenig] deposits of North China (Liu et al. 1985) and Pseudocrania from the uppermost Darriwilian to lower Sandbian deposits of the Chu -Ili peri-Gondwanan terrane (Nazarov & Popov 1980;Popov et al. 1999a), and from the upper Darriwilian deposits of the Alborz terrane (Ghobadi Pour 2008;Bassett et al. 2013) (Fig. 10.5).
The oldest known Baltoscandian craniide is Pseudocrania, first documented from the late Dapingian (mid Volkhovian) and ranging up to the top of the early Darriwilian (Kundian) (Gorjansky 1969). From the Aserian (late Darriwilian), Pseudocrania was replaced in the Baltic Ordovician Fauna by Orthisocrania. There is no record of craniides in the Early to Mid Ordovician of Laurentia and Siberia (Popov et al. 1999a).
Kazakhstanian peri-Gondwanan island arcs preserved the earliest known record of trimerellides. In the Chingiz terrane they occur in the lower Sandbian Bestamak Formation (Nikitin & Popov 1984). This assemblage includes Ussunia (which appears to retain some craniopsid-like plesiomorphic characters), together with such genera as Palaeotrimerella and Ovidiella.

Late Ordovician
The craniides went through a significant diversification during the Late Ordovician Epoch, and they became a distinctive minor component of benthic assemblages of shallow and mid-shelf settings in temperate zones and low latitudes. By that time they had spread widely to Laurentia, Baltica, Avalonia and high-latitude peri-Gondwana, but they are as yet unknown from Siberia, North and South China, and the Australian sector of Gondwana (Popov et al. 1999a). Remarkably, Late Ordovician craniides are extremely rare in Kazakhstanian peri-Gondwanan terranes, where just a single Late Ordovician species (Orthisocrania shidertensis Gorjansky 1972) has been described from the Katian (Tauken Formation) of northeastern Central Kazakhstan. A number of Ordovician craniides (e.g. Acanthocrania and Petrocrania) are known in Laurentia, Avalonia and in Baltica, on both sides of the Iapetus Ocean. Orthisocrania was endemic to Baltica in the late Darriwilian, but spread into Avalonia by the beginning of the Katian. Philhedra is probably the only Late Ordovician craniide genus that is endemic to Baltica. Its type species, P. baltica (Koken 1889), has a unique morphology, including hollow spines -a characteristic otherwise unknown in other craniides.
The earliest known Ordovician craniopsides are from the Sandbian deposits of Baltoscandia (Pseudopholidops) and the Avalon microcontinent (Paracraniops), which occupied a similar geographical position. During the Katian, craniopsides (mostly Paracraniops) became widely distributed in low latitudes. In particular, they are known from Baltoscandia (Popov &   Relative position of the main palaeocontinents is mainly after Cocks & Torsvik (2002) with modifications for peri-Gondwanan terranes after Popov et al. (2009b). 1986), Laurentia (including North America (Cooper 1956) and South Scotland (Williams 1962)), Kazakhstan (Gorjansky 1972) and Taimyr (Nikiforova 1982). Pseudopholidops was confined mostly to Baltica, but it has been found recently in the mid Katian deposits of the Zagros Mountains (M. Ghavidel Syooki, pers. comm. 2011) where it is associated with brachiopod and trilobite faunas characteristic of high-latitude Gondwana (e.g. Tafilatia, Svobodaina, Neseuretinus, Dalmanitina). During the Hirnantian, Pseudopholidops was a relatively rare, but widely distributed component of the Hirnantia brachiopod assemblage, and is reported from Baltoscandia, Wales, Poland and South China (Popov et al. 1999a).
The distribution of trimerellides during the Sandbian -Katian was consistent with their dispersion from equatorial peri-Gondwanan island arc settings. By that time they are known from the Australian sector of Gondwana (Percival 1995) and they made their first appearance in Laurentia in the early Katian (Cooper 1956;Norford & Steele 1969). In South China, an abundant trimerellide fauna is known from Katian deposits (Li & Han 1980;Rong & Li 1993;Xu & Li 2002). However, the trimerellide fossil record of this area is incomplete. Large numbers of endemics and the diachronous appearance of some genera (e.g. Eodinobolus, Monomerella and Palaeotrimerella) suggest a significant geographical isolation of all these faunas (Fig. 10.5) defined by adaptation to the restricted near-shore environments. However, all of these faunas also exhibit some similarities, which can allow establishment of the time and possible direction of faunal exchange (Popov et al. 1997). For example, early species of Eodinobolus were reported from the Sandbian to lower Katian deposits of Kazakhstan and Australia, while closely related Palaeotrimerella is known from the late Darriwilian to Katian of Kazakhstan (Popov et al. 1997) and Katian of South China (Li & Han 1980). Monomerella made its first appearance in the early to middle Katian of Australia (Percival 1995), whereas in Kazakhstan it is recorded from the mid Katian (Abak Regional Stage) and appeared in South China approximately contemporaneously in the mid Katian (Percival 1995;Popov et al. 1997).
The observed pattern of geographical distribution of Ordovician trimerellides can be explained as the result of two major migrations from Kazakhstanian terranes: (1) during the late Sandbian -early Katian interval to Gondwana and North America; (2) during the mid to late Katian to Baltica and Siberia. The westerly direction of the migration of the Eodinobolus kazakhstanicus group of species in the early Katian (Popov et al. 1997;Fig. 10.5) suggests that there were some significant changes in palaeogeography during the late Late Ordovician, because it duplicates the somewhat later dispersion of atrypoids (Popov et al. 1999b) and the early virgianids (mostly so-called Holorhynchus fauna; Rong et al. 2004). It is also noticeable that the Eodinobolus kazakhstanicus group of species often occurs as a minor component of rhynchonelliform brachiopod-dominated associations of the shallow and mid shelf, which is outside the main habitat of trimerellides (in restricted near-shore environments).
The geographical distribution of the trimerellide Gasconsia in the late Katian also mirrors the distribution of the Holorhynchus Fauna (Popov et al. 1997;Rong et al. 2004). Ordovician trimerellide biogeography indicates that the microplates and island arcs corresponding to some Kazakhstanian terranes together with South China were most probably situated in relative proximity to the Australian sector of Gondwana, and by the Katian, Baltica and South China had entered low latitudes. It is also possible that some Kazakhstanian terranes drifted westwards towards Baltica some time during the Late Ordovician (Popov et al. 1997).
The Ordovician trimerellide fauna from South China is unusual in that it contains taxa with excavated and vaulted visceral platforms; this pattern of shell morphology is otherwise unknown in the majority of the Ordovician trimerellides with the exception of the Australian Belubula, but became widespread in the Silurian. Almost all local trimerellide lineages outside of South China and Australia became extinct just before the Hirnantian, and there is no documented record of trimerellides in the Hirnantian and Rhuddanian brachiopod faunas.

Discussion
The Ordovician biogeography of linguliform brachiopods is basically controlled by the radiation and dispersion of those lineages, which survived the severe crisis of that the group experienced during the late Furongian -early Tremadocian interval. The early Tremadocian biogeography of linguliform brachiopods, with low-diversity, near-shore obolid communities inhabiting shallow clastic shelves of high-latitude Gondwana and Baltica and morphologically archaic taxa such as siphonotretides that are characteristic of Laurentia, Baltica and Gondwana, has a clear Cambrian origin. On the shallow shelves of temperate-and highlatitude Gondwana and Baltica, lingulide communities were dominated by epibenthic obolids (e.g. Obolus, Schmidtites, Ungula) that were replaced rapidly by mixed associations of burrowing and epibenthic lingulides (e.g. Thysanotus -Leptembolon Association) and finally by low-diversity associations of burrowing lingulides (e.g. the Tunisiglossa Association). The distributional pattern of the Thysanotus -Leptembolon Association, which emerged rapidly in Baltica and the Alborz terrane just prior to the shift from clastic to temperate-latitude carbonate sedimentation, suggests that it was an opportunistic fauna that flourished at a time of significant environmental changes Mergl 1997;Popov et al. 2008). During the late Tremadocian -early Floian interval, the archaic siphonotretides, such as Helmersenia and Schizambon, were replaced on the shelves of all major continents by spinose siphonotretides, which evolved originally in temperate-latitude Gondwana (Popov et al. 2009a).
Another remarkable feature of linguliform biogeography is the origin and dispersion of the associations of borrowing lingulides during the Early to Middle Ordovician. The earliest associations of borrowing lingulides (e.g. Tunisiglossa Association) are documented from high-latitude Gondwana (Havlíček 1989). This group emerged in subequatorial latitudes only from the Darriwilian onwards. There is no clear Mid Ordovician record of the 'Lingula'-type associations (after Ziegler et al. 1968) in Laurentia and Baltica. However, they are relatively well documented from Siberia (Kanygin & Yadrenkina 1977) and the Kazakhstanian terranes Popov et al. 2007). Lingulides are able to survive through the stress of strong changes of salinity variations resulting from freshwater input (Emig 1986;Hammen & Lum 1977). It is possible that such adaptation evolved originally in reaction to survival pressure in near-shore environments subjected to seasonal ice and snow melting at high latitude. Their expansion to the low latitudes along the western Gondwanan coast occurred during short-term cooling events in the early to mid Darriwilian. Remarkably, these lingulide associations settled in peri-Gondwanan Kazakhstanian terranes that were affected by tropical monsoons, where they inhabited nearshore tidal flats .
Recovery and proliferation of micromorphic linguliform brachiopod associations took place in the late Tremadocian-Dapingian interval. By the Darriwilian, they formed a characteristic element of pantropical faunas. An important component of these faunas includes taxa descendant from Furongian faunas of tropical peri-Gondwanan island arcs (acrotretide families Eoconulidae, Ephippelasmatidae, Scaphelasmatidae and Torynelasmatidae; . However, initial radiation of the Ordovician acrotretide lineages related to the family Acrotretidae occurred in Laurentia (Holmer et al. 2005). The lingulide genera Elliptoglossa and Paterula and the acrotretide family Biernatiidae probably dispersed from temperate-latitude Gondwana and Baltica (Mergl 1999(Mergl , 2002Popov et al. 2007;Ghobadi Pour et al. 2011). Temperate-latitude micromorphic brachiopod associations differ mainly in their lower diversity and in the absence of such taxa as Ephippelasma, Rhysotreta, Spinilingula, Spondylotreta, Undiferina and torynelasmatids, which were confined mainly to tropical latitudes. There are also a few taxa (e.g. Aipyotreta and Myotreta) that are confined to temperate-latitude faunas.
It is probable that the positions of major continents and climatic belts were major controlling factors in the distribution of linguliforms through the Ordovician, while in the longer term, the oceans did not represent a significant barrier to their dispersal.
Latitudinal differentiation of the micromorphic lingulate brachiopod faunas is more evident during the early to mid Darriwilian, but mainly disappeared by the late Darriwilian to Sandbian, when pantropical taxa (e.g. Ephippelasma, Rhysotreta, Spinilingula, Undiferina) spread to Baltica and South China. A possible explanation is that this is related to the suggested cooling episode during the early to mid Darriwilian (Ghobadi Pour et al. 2007;Cherns & Wheeley 2009), which is most evident from the regions of temperate-latitude peri-Gondwana, within the so-called 'overlap zone' (see also Fortey & Cocks 2003). The subsequent warming, which probably occurred synchronously, as indicated by the mid-Darriwilian carbon isotope excursion (Bergström et al. 2008), resulted in a southern shift of the climatic belts and the migration of some characteristic taxa from the tropics to higher latitudes (Ghobadi Pour & Popov 2009).
In the character of their proliferation and dispersion, craniiforms share similarities with other groups of the Palaeozoic Evolutionary Fauna, including atrypide brachiopods, bryozoans and ostracods. There is no convincing Cambrian record of the Subphylum Craniiformea, but it is likely that it originally evolved and dispersed from Gondwana and associated microcontinents and island arcs. The earliest craniides are known from the late Tremadocian of temperate-to high-latitude peri-Gondwana (Mergl 2002;Sdzuy et al. 2001), and they had a peri-Iapetus geographical distribution through most of the Ordovician. They are very rare or absent in tropical Gondwana, South China and Kazakhstanian terranes, and are not yet documented from Siberia.
The trimerellides probably evolved in a tropical peri-Gondwanan island arc setting (Chingiz terrane). Their dispersion and major features of biogeography mirror those of atrypoids, which were also characterized by lecithotrophic larvae (Freeman & Lundelius 2005) and the early virgianids (Popov et al. 1997).
Craniiform biogeography reflects the approach of Avalonia towards Baltica some time in the Sandbian (e.g. migration of Orthisocrania to Avalonia). The first appearance of the trimerellide Eodinobolus in Laurentia during the early Katian suggests relative proximity to the Australian sector of Gondwana at that time. However, the direct faunal exchange between Laurentia and equatorial Gondwana became minimized by the mid Katian. The major route of trimerellide migration pathways was directed to the west, towards Baltica and Siberia, probably because of numerous island arcs and microplates (concentrated near the equator west of Gondwana), which may have served as bridges for those groups of benthos characterized by specific environmental requirements and low dispersion potential. The pantropical distribution of the trimerellides Eodinobolus and Monomerella during the Katian may be related to the ecological tolerance of these taxa, because they often occur together with various associations dominated by rhynchonelliform brachiopods, outside the main habitat of trimerellides.