MOLAR OCCLUSION IN LATE TRIASSIC MAMMALS

1. A new genus and species of late Triassic mammal, Megazostrodon rudnerae, from Lesotho in southern Africa is described. The molars are similar to those of the British Eozostrodon parvus except that they are slightly larger and the upper molars have a large external cingulum supporting well-developed cusps. 2. Molar occlusion is discussed in two groups of late Triassic mammals: Eoxostrodon and the closely related Megazostrodon on one the hand and the unnamed primitive symmetrodonts on the other. It is shown that in Eoxostrodon the upper and lower molars did not have matching occlusal surfaces upon eruption but that wear produced matching occlusal surfaces. These surfaces are confined to the internal surface of the upper molars and the external surface of the lower molars and form a series of wide-angled triangles. The main cusp of an upper molar occluded between the main and posterior subsidiary cusp of the lower molar and the main cusp of the lower molar occluded between the main and anterior subsidiary cusp of the upper molar, 3. It is shown that the molars of Docodon and HaIdanodon were possibly derived from those of a primitive mammal such as Eozostrodon. The transition involved the development on the upper molars of an internal extension which, as it increased in size, established contact with the dorsal surfaces of two adjacent lower molars. The process involved is fundamentally different from that leading to tribosphenic molars. 4. In Megaxostrodon the main cusp of the upper molars occluded between the posterior and anterior subsidiary cusps of two adjacent lower molars, i.e. more posteriorly than in Eozostrodon. Primitive Rhaetic symmetrodonts were derived from mammals which had this type of occlusion and which were also closely related to Eoxostrodon and Megaxostrodon. The transition involved a rotation of the subsidiary cusps of the upper molars externally and those of the lower molars internally. This rotation increased the shearing surfaces between occluding upper and lower molars. Cusp rotation was carried further in the acute-angled symmetrodonts (Peralestes and Spalacotherium) and pantotheres. It appears that marked cusp rotation was coupled with the acquisition of transverse movements of the lower jaw during mastication. Transverse movement was apparently not possible in cynodonts, in Eoxostrodon (and related forms) and in Docodon. 5. The evolution of therian molars involves cusp rotation as originally proposed by the Cope—Osborn theory. Criticisms of the Cope—Osborn theory are re-evaluated in light of the new late Triassic material. 6. In Rhaetic symmetrodonts, molar wear produces matching occlusal facets, but the amount of attrition necessary to produce these facets was considerably less than in Eoxostrodon. In acute-angled symmetrodonts and in pantotheres, the molars erupt with more precise occlusal surfaces and attrition was not necessary to produce matching surfaces. 7. On the basis of the structure of the molar teeth it was concluded that Eozostrodon, Megazostrodon and Erythrotherium were closely related to the Rhaetic symmetrodonts. Slightly different occlusal relationships between upper and lower molars indicated that in these early mammals constant occlusal relations were being established. 8. Primitive cynodonts, such as Thrinaxodon, are characterized by alternate tooth replacement; there is a total lack of a constant occlusal relationship between upper and lower postcanine teeth. In Thrinaxodon individual postcanines were replaced several times. The crown structures of successive generations of postcanines were different so that a freshly erupted postcanine tooth had a crown structure quite distinct from the tooth which it replaced. It has been shown that the crown structure of one of the generations of postcanine teeth of Thrinaxodon is almost identical to that of Eozostrodon except that Thrinaxodon postcanines have a single root, On the basis of this similarity and the over-all structure of the primitive cynodont skull, it was concluded that Rhaetic mammals (excluding ictidosaurs and haramyids) could be derived from primitive cynodonts. 9. All the orders of Jurassic mammals (with the possible exception of multituber-culates) were probably derived from late Triassic mammals. The apparent close relationship of late Triassic mammals is evidence of a monophyletic origin of this class.     

早期哺乳动物三尖齿兽类牙齿的超微结构

本文用扫描电镜研究了中国云南禄丰盆地下禄丰组发现的以及英国威尔士晚三迭世的三尖齿兽类牙齿的超微结构.其釉质具有准釉柱结构,它们呈并行排列从釉牙质界延伸到釉质外表面.这种准釉柱结构由很多羽支状排列的磷灰石微晶组成.微晶的C轴与准釉柱的长轴形成交叉的角度大约为10-20度.早期哺乳动物牙齿釉质的准釉柱结构,很可能是从爬行动物无釉柱结构向哺乳动物真釉柱结构进化过程中的标志.     

The oldest evidence of a sauropod dinosaur in the western united states and other important vertebrate trackways from grand staircase‐escalante national monument,Utah

Recent discoveries of abundant fossil footprints from the new Grand Staircase‐Escalante National Monument of southern Utah, have important implications for the spatial and temporal distribution of Mesozoic vertebrates in Triassic and Jurassic time. Since the monument's creation in 1996, fossil footprints have been reported from at least seven formations in the Mesozoic (Triassic‐Cretaceous) within the monument. By far the most significant of these discoveries are sauropod and theropod tracks from the upper part of the Middle Jurassic Entrada Sandstone and a large Apatopus trackway from the Late Triassic Chinle Formation. Tracks in the Entrada Sandstone are found at the same stratigraphic level as those in the Moab megatracksite, and so considerably extend this large ichnological complex. A wide‐gauge sauropod trackway (cf. Brontopodus) from this unit represents the first reported from the Entrada Sandstone, and so is the oldest known from the western United States. This trackway also reveals a tail trace, which is the first reliable record of a sauropod tail trace.     

A new spalacolestine mammal from the Early Cretaceous Jehol Biota and implications for the morphology,phylogeny, and palaeobiology of Laurasian ‘symmetrodontans’

‘Symmetrodontans’ are extinct mammals characterized by having a reversed‐triangle molar pattern in which three main cusps define a triangular molar crown. This dental morpholgy has been regarded as being intermediate between the ‘triconodont’ tooth and the tribosphenic pattern characterizing therians; it is a key feature in taxonomy of Mesozoic mammals and one to understand mammalian evolution and palaeobiology. Here we report a new genus and species of ‘symmetrodontan’ mammal, Lactodens sheni, from the Early Cretaceous Jehol Biota, represented by a partial skeleton with dentary and upper and lower teeth with dental morphologies well‐preserved. The new species has a dental formula of three upper incisors, one canine, three premolars, and six molars/three lower incisors, one canine, five premolars and six lower molars, double‐rooted canines, extremely low‐crowned and transversely thin premolars, and acute angled molars. The dental morphologies of molars and peculiar deciduous premolars are similar to those of Spalacolestes from North America. The associated upper and lower dentitions from one individual animal helped to clarify tooth identification of some spalacotheriids represented only by fragmentary material. Phylogenetic analyses indicate a close relationship of the new species to North American spalacolestines and faunal interchanges between Eurasia and North America, thus supporting the notion that small‐bodied spalacotheriids were diverse and had a pan‐Laurasian distribution during the Early Cretaceous. Absence of the Meckelian groove suggests acquisition of the definitive mammalian middle ear in spalacolestines, and deciduous canines and premolars in the slim and extremely long dentary imply a faunivorous diet.     

New archosauromorph fragments from the Dockum Group of Texas and assessment of the earliest dinosaurs in North America

Three small and gracile jaw fragments are recovered from the Boren Quarry, one of the lowest fossil quarries of the Upper Triassic Dockum Group of Texas. The specimens are referred to archosauromorphs, where the morphology of the dentary and teeth of two specimens resemble what is observed in basal saurischians. Growing numbers of early dinosaur fossils from the lowest quarries of the Dockum Group of Texas which correspond to the lowermost Norian raises doubts concerning early dinosaur dispersal during the late Carnian-early Norian interval and the first occurrence of North American dinosaurs which might have happened earlier than previously suggested.     

On two new species of Mesochaetopterus (Polychaeta) from the Brazilian coast

Es wurden zwei neue Arten von Mesochaetopterus (Chaetopteridae) (Polychaeta) beschrieben und mit den restlichen Arten dieser Gattung verglichen.

Der 60 cm lange Mesochaetopterus xerecus zeigt eine gewisse Ähnlichkeit mit M. tay‐lori, aber unterscheidet sich durch die Länge und Pigmentierung der Tentakel, existenz von einem Paar Augen, Anzahl der Parapodien in der vorderen Körperregion, Form der dunklen starken Borsten des 4. Segments, Anzahl der "accessory feeding organs”;, Anzahl der Zähne der Haken der Neuropodia.

Mesochaetopterus xejubus ist eine kleinere Art von ungefähr 7 cm Länge und ist in Form und Größe M. minuta am ähnlichsten. Es unterscheidet sich jedoch durch die Form und Größe des Peristomium, Farbe der Tentakeln, Anzahl der Parapodia der vorderen Körperregion, Anzahl und Form der dunklen starken Borsten des 4. Para‐podiums der vorderen Region, Anzahl der "accessory feeding organs”;, Anzahl der Segmente der mittleren Körperregion, Anzahl der Zähne der Haken der Neuropodia und schleimdrüsenreiches Epithelium der mittleren Körperregion.     

Absence of Suction Feeding Ichthyosaurs and Its Implications for Triassic Mesopelagic Paleoecology

Mesozoic marine reptiles and modern marine mammals are often considered ecological analogs, but the extent of their similarity is largely unknown. Particularly important is the presence/absence of deep-diving suction feeders among Mesozoic marine reptiles because this would indicate the establishment of mesopelagic cephalopod and fish communities in the Mesozoic. A recent study suggested that diverse suction feeders, resembling the extant beaked whales, evolved among ichthyosaurs in the Triassic. However, this hypothesis has not been tested quantitatively. We examined four osteological features of jawed vertebrates that are closely linked to the mechanism of suction feeding, namely hyoid corpus ossification/calcification, hyobranchial apparatus robustness, mandibular bluntness, and mandibular pressure concentration index. Measurements were taken from 18 species of Triassic and Early Jurassic ichthyosaurs, including the presumed suction feeders. Statistical comparisons with extant sharks and marine mammals of known diets suggest that ichthyosaurian hyobranchial bones are significantly more slender than in suction-feeding sharks or cetaceans but similar to those of ram-feeding sharks. Most importantly, an ossified hyoid corpus to which hyoid retractor muscles attach is unknown in all but one ichthyosaur, whereas a strong integration of the ossified corpus and cornua of the hyobranchial apparatus has been identified in the literature as an important feature of suction feeders. Also, ichthyosaurian mandibles do not narrow rapidly to allow high suction pressure concentration within the oral cavity, unlike in beaked whales or sperm whales. In conclusion, it is most likely that Triassic and Early Jurassic ichthyosaurs were ‘ram-feeders’, without any beaked-whale-like suction feeder among them. When combined with the inferred inability for dim-light vision in relevant Triassic ichthyosaurs, the fossil record of ichthyosaurs does not suggest the establishment of modern-style mesopelagic animal communities in the Triassic. This new interpretation matches the fossil record of coleoids, which indicates the absence of soft-bodied deepwater species in the Triassic.     

Ricciisporites属的分布及其地层学意义

概述了Ｒｉｃｃｉｉｓｐｏｒｉｔｅｓ属的基本形态、分类和时空分布。由于形态特殊，层位稳定，一般丰度较高，故此属为北半球中生代最重要分子之一，主要出现于瑞替阶和下里阿斯阶，在中国仅见于华南上三叠统，是划分三叠系／侏罗系的标志分子。其母体植物可能生长于距渴湖、海湾岸边较近的生境。     

LOPHOSTEUS SUPERBUS PANDER: ZÄHNE, ZAHNKNOCHEN UND BESONDERE SCHUPPENFORMEN

Morphological and histological studies of numerous remains of Lophosteus superbus Pander from erratic boulders of Beyrichia Limestone prove that the peculiar teeth recently brought to notice by Gross (1969) belong to Lophosteus and are not of acanthodian origin. The structure of these teeth as well as that of some tooth-bearing bones and scales is described. The investigation was extended to cover the teeth of smaller Devonian actinopterygians and cross-opterygians to establish whether Lophosteus is an actinopterygian, a crossopterygian, or even a member of a hitherto unknown group of Osteichthyes. Results unfortunately proved negative, as the very small teeth of various species offer too few distinct and symptomatic histological characteristics. A solution of this question must await the discovery of determinable skull and lower jaw bones of the genus Lophosteus.
Die morphologische und histologische LTntersuchung zahlreicher in nordwest-deutschen Geschieben des Beyrichienkalkes gefundener Reste von Lophosteus superbus Pander beweist, daß die kürzlich von Gross (1969) erwähnten, eigenartigen und nicht von Acanthodiern stammenden Zähne zu Lophosteus gehören. Der Bau der Zähne und einiger Zahnknochen und Schuppen wird beschrieben. Um die Fragen zu klaren, ob Lophosteus zu den Actinopterygiern oder zu den Crossopterygiern gehört oder gar zu einer bisher unbekannten Gruppe der Osteichthyes, wurde die Untersuchung auch auf die Zähne kleiner devonischer Actinopterygier und Crossopterygier ausgedehnt. Das Ergebnis war leider negativ: die sehr kleinen Zähne der verschiedenen untersuchten Arten bieten zu wenig kennzeichnende und unterscheidende histologische Merkmale. Nur von der Entdeckung bestimmbarer Schädel- und Unterkiefer-Knochen der Gattung Lophosteus kann die Entscheidung dieser Frage erhofft werden.     

ULTRASTRUCTURE OF THE FILAMENTOUS HABIT IN THE DIATOM NAVICULA CONFERVACEA (KÜTZ.) GRUN.1

Frustules of the filamentous diatom Navicula confervacea (Kütz.) Grun. are united by the overlapping of marginal teeth located alongside the junction of valve face and mantle. Scanning and transmission electron microscopy show this connecting mechanism to differ from the previously reported inter-digitating spines or teeth in the genera Melosira, Stephanodisus, and Fragilaria.     

Miniaturisation and the origin of mammals

In this paper, the hypothesis of miniaturisation to explain the origin of mammals (Rowe 1993, Mammals phylogeny: mesozoic differentiation, multituberculates, monotremes, early therians, and marsupials. New York: Springer-Verlag, p. 129–145) is discussed, based on three lines of evidence resulting from new discoveries of eucynodonts in the Late Triassic of Southern Brazil (Bonaparte et al. 2003, Rev Bras Paleont 5:5–27; 2005, Rev Bras Paleont 8:25–46; 2006, New Mexico Museum Nat Hist Sci Bull 37:1–8; 2010, Rev Bras Paleont) that are: (1) the incomplete fossil record of eucynodonts known until 2003; (2) the structure of the primary palate rejects the ancestral condition of thrinaxodontids, probainognathids, chiniquodontids and cynognathids to the earliest mammals; and (3) the relatively large postdentary bones of the Middle Triassic brasilodontids that are otherwise very small in size (skull 44 mm long) suggest that small size per se did not help to improve the middle ear or other sophisticated organs present in the earliest mammals (Rowe 1993; Kemp 2005, The origin and evolution of mammals. Oxford University Press, p. 1–391). Small size possibly was not a secondary character, but a persistent primitive one. This new interpretation has resulted from comparative study of non-mammalian eucynodonts discovered in the Middle and Late Triassic of Brazil and those known previously. The general acceptance of the hypothesis of miniaturisation is thus a consequence of the poor fossil record of Middle and Late Triassic eucynodonts before 2003.     

Therian Teeth of Unusual Design from the Mid-Cretaceous (Albian–Cenomanian) Cedar Mountain Formation of Utah

Enigmatic, abundant mammalian teeth from the medial Cretaceous of Utah are shown to belong to antemolar loci, based on dentulous jaw fragments; isolated teeth representing several upper premolar loci and the reconstructed c-p4 series are identified. Three species, differing in size and morphology, can be recognized. Morphological appropriateness, relative abundance, and distributional data indicate that the teeth can be referred with some confidence to the three symmetrodonts known from the Cedar Mountain Formation: Spalacolestes cretulablatta, S. inconcinnus, and Spalacotheridium noblei. If the specimens represent replacement or successional teeth, they are strikingly atypical for Mesozoic mammals, particularly in their low crowns and high degree of molarization at posterior loci. Jaw structure, wear pattern, and aspects of tooth morphology (e.g., proportions, degree of molarization, enamel thickness) favor the alternative hypothesis that these teeth are deciduous. Diphyodonty at all antemolar loci is generally assumed to represent the primitive condition for mammals, though fossil evidence is scant; some of the earliest mammals are known to undergo replacement only at the last premolar locus, with ontogenetic loss (rather than replacement) mesially. Available evidence suggests that, like the eupantothere Dryolestes, North American spalacotheriid symmetrodonts probably underwent single replacement at most or all premolar loci and that the deciduous series became progressively more molariform distally, particularly at the p3–4 loci. Assuming that these teeth are deciduous, their great abundance in the Cedar Mountain Formation (and, apparently, elsewhere in the Cretaceous of North America) suggests that North American spalacotheriids were subject to unusually high juvenile mortality rates or, more probably, that succession at premolar loci took place late in ontogeny, compared to other Mesozoic mammals.     

Biomechanical properties of the jaws of two species of Clevosaurus and a reanalysis of rhynchocephalian dentary morphospace

Rhynchocephalians were a successful, globally distributed group of diapsid reptiles that thrived in the Mesozoic. Multiple species of Clevosaurus existed worldwide in the Late Triassic and Early Jurassic, characterized by shearing bladelike teeth perhaps functionally analogous to the carnassial teeth of mammals. Morphometric analysis shows that the dentary morphospace of clevosaurs differs significantly from that of other rhynchocephalians. Five Clevosaurus species occupied islands in the Bristol Channel archipelago of the UK, but generally not those occupied by mammaliaforms, suggesting dietary character displacement. Identifying the diet of such ancient, small tetrapods has been difficult. To identify the nature of their feeding mechanics and ecology, we apply finite element analysis to two near complete three-dimensional skulls of the species Clevosaurus hudsoni and Clevosaurus cambrica to estimate bite force, resistance to bending and torsion and the distribution of stresses in the jaws during biting. Both species had bite forces and tooth pressures sufficient to break apart chitin indicating that, like early Mesozoic mammaliaforms, clevosaurs could feed on tough-shelled beetles and possibly small vertebrates. In addition, the mechanical advantage of the jaws falls within the range of early mammaliaforms, so though we cannot demonstrate niche partitioning between members of the two clades, it raises the prospect that they may have been functionally similar.     

A traversodont cynodont from the Middle Triassic (Ladinian) of Baden-Württemberg (Germany)

Nanogomphodon wildi n. gen., n. sp. is based on a tiny lower postcanine tooth from the lower Lettenkeuper (Lower Keuper or Erfurt Formation; Ladinian) of Michelbach an der Bilz (Baden-Württemberg). It represents the first record of a traversodont cynodont from the Middle Triassic of Europe and exhibits a distinctive combination of dental features. Along with recent discoveries of other traversodont taxa from the Upper Triassic of eastern North America,Nanogomphodon indicates the existence of a distinct lineage of these cynodonts in the Northern Hemisphere.      

Review of the monotreme fossil record and comparison of palaeontological and molecular data

Monotremes have traditionally been considered a remnant group of mammals descended from archaic Mesozoic stock, surviving to the present day on the relatively isolated Australian continent. Challenges to this orthodoxy have been spurred by discoveries of 'advanced' Cretaceous monotremes (Steropodon galmani, Archer, M., et al., 1985. First Mesozoic mammal from Australia-an Early Cretaceous monotreme, Nature. 318, 363-366) as well as by results from molecular data linking monotremes to therian mammals (specifically to marsupials in some studies). This paper reviews the monotreme fossil record and briefly discusses significant new information from additional Cretaceous Australian material. Mesozoic monotremes (including S. galmani) were a diverse group as evidenced by new material from the Early Cretaceous of New South Wales and Victoria currently under study. Although most of these new finds are edentulous jaws (limiting dental comparisons and determination of dietary niches), a range of sizes and forms has been determined. Some of these Cretaceous jaws exhibit archaic features-in particular evidence for the presence of a splenial bone in S. galmani-not seen in therian mammals or in post-Mesozoic (Tertiary and Quaternary) monotreme taxa. Tertiary monotremes were either archaic ornithorhynchids (toothed platypuses in the genera Monotrematum and Obdurodon) or tachyglossids (large echidnas in the genera Megalibgwilia and Zaglossus). Quaternary ornithorhynchid material is referable to the sole living platypus species Ornithorhynchus anatinus. Quaternary echidnas, however, were moderately diverse and several forms are known (Megalibgwilia species; 'Zaglossus' hacketti; Zaglossus species and Tachyglossus aculeatus).     

Late Jurassic Mammals from Tendaguru, Tanzania, East Africa

Records of Mesozoic mammals are extremely rare in Africa. The only previous record from the Upper Jurassic of Africa is a fragmentary mandible without teeth of Brancatherulum tendagurense. Here I report the discovery of two new mammals from the Upper Jurassic of Tendaguru, Tanzania. The fossils were recovered from the Middle Saurian Bed of the Tendaguru Series. A lower molar of a triconodontid mammal is described as Tendagurodon janenschi gen. et sp. nov., and a fragmentary dentary of a eupantothere as Tendagurutherium dietrichi gen. et sp. nov. The eupantothere in particular contributes to documenting the evolution of mammals during the Mesozoic. The posterior portion of the mandible of Tendagurutherium dietrichi gen. et sp. nov. shows that the angular (tympanic) bone was not yet completely separated from the dentary, a previously undocumented stage of eupantotherian middle ear evolution.     

On the teeth of the recent cassiduloidEchinolampas depressa gray,and on some liassic fossil teeth nearly identical in structure (Echinodermata,echinoidea)

Summary The young recent cassiduloidEchinolampas depressa has a lantern and teeth which are fully developed, but which never function because they disappear completely after metamorphosis. The lantern resembles morphologically that of the clypeastroid family Fibulariidae. The teeth are built-up of tooth elements, which have small primary plates, large lateral plates, and a cluster of prisms, and they are nearly identical to the teeth of fibulariids. In addition, the fine structure of teeth from liassic fossils was analyzed. This was the first investigation ever of the microstructure of fossil echinoid teeth. The examined teeth were well preserved, and proved to have nearly the same structure as the teeth ofEchinolampas. The results of this investigation prove that teeth of cassiduloid-clypeastroid structure were already in existence in the Liassic.Using micro structure as a comparative basis the author distinguishes between echinoid teeth of the clypeastroid type (present in Cassiduloida, Clypeastroida, and in all probability in Oligopygoida), and those of the regular type (present in all recent regular echinoids and in all probability in Holectypoida). The well-known aulodont, stirodont, etc., types of teeth and lanterns belong to the regular type. Teeth of the regular type are highly complex in structure, and certainly cannot be the result of evolutionary convergence. From the morphological point of view the clypeastroid type is the simpler one, and there is no indication that it is the result of secondary simplification. For this reason the anatomical features of the clypeastroid type of echinoid teeth are considered to be the more primitive. Contrary to the opinion of several authors, this author maintains that echinoids possessing a masticatory apparatus of the clypeastroid type are by no means descendents of stirodont ancestors.

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Zusammenfassung Der rezente CassiduloideEchinolampas depressa hat nur in der Jugend eine Laterne. Diese Laterne und ihre Zähne sind vollständig entwickelt, obwohl sie nicht in Funktion treten, sondern nach der Metamorphose vollständig verschwinden. Morphologisch gleicht die Laterne derjenigen der clypeastroiden Familie Fibulariidae. Die Zähne werden von Zahnelementen zusammengesetzt, die kleine Primärplatten, große Lateralplatten und ein Büschel von Prismen besitzen. Diese Zähne sind fast identisch mit denen der Fibulariiden. — Außerdem wurde die Feinstruktur fossiler Zähne, die aus der Lias stammten, analysiert. Dies war die erste Untersuchung über die Mikrostruktur fossiler Seeigelzähne überhaupt. Die Struktur war bei den untersuchten Zähnen gut erhalten, und es wurde nachgewiesen, daß sie fast identisch mit der Struktur derEchinolampas-Zähne ist. Die Untersuchung beweist, daß Zähne von cassiduloid-clypeastroider Struktur schon in der Lias existierten.Aufgrund der Mikrostruktur unterscheidet der Autor bei den Seeigelzähnen den clypeastroiden Typ (vorhanden bei den Cassiduloida, Clypeastroida und höchstwahrscheinlich bei den Oligopygoida), und den regulären Typ (der beiallen rezenten regulären Seeigeln und höchstwahrscheinlich bei den Holectypoida vorhanden ist). Die klassischen aulodonten, stirodonten usw. Typen von Zähnen und Laternen gehören alle zum regulären Typ. Die Zähne des regulären Types sind von sehr komplexer Struktur, und dieser Typ ist sicher nicht das Ergebnis von Konvergenzen. Vom morphologischen Standpunkt aus ist der clypeastroide Typ der einfachere, und es gibt keinen Hinweis, daß es sich um sekundäre Vereinfachung handelt. Deshalb werden die anatomischen Merkmale des clypeastroiden Types als die relativ ursprünglicheren betrachtet. Im Gegensatz zur Ansicht mehrerer Autoren sind Seeigel, die einen Kauapparat vom clypeastroiden Typ besitzen, keinesfalls von stirodonten Ahnen abzuleiten.

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Abbreviations ab abaxial tooth section - ad adaxial tooth section - H pillar-bridges - HF main fold of PP - KA chewing part - L lamellae (regulars) - LNK lamellae-needle-complex (regulars) - LP lateral plates - M median plane - mA middle part of PP (regulars) - ML median ridge - N needle - P prisms - PP primary plates - Py pyrite-crystals - S lateral ledge - sA side part of PP (regulars) - sa smooth area for attachment of interpyramidal muscles - Sch shaft - SP secondary plate (regulars) - U umbo (growing center of the tooth element) - Y adaxial end of LP - zA central part of PP (regulars)