Free Cheeks Research Articles

Multiple origins of dorsal ecdysial sutures in trilobites and their relatives

Euarthropods are an extremely diverse phylum in the modern, and have been since their origination in the early Palaeozoic. They grow through moulting the exoskeleton (ecdysis) facilitated by breaking along lines of weakness (sutures). Artiopodans, a group that includes trilobites and their non-biomineralizing relatives, dominated arthropod diversity in benthic communities during the Palaeozoic. Most trilobites – a hyperdiverse group of tens of thousands of species - moult by breaking the exoskeleton along cephalic sutures, a strategy that has contributed to their high diversity during the Palaeozoic. However, the recent description of similar sutures in early diverging non-trilobite artiopodans means that it is unclear whether these sutures evolved deep within Artiopoda, or convergently appeared multiple times within the group. Here, we describe new well-preserved material of Acanthomeridion, a putative early diverging artiopodan, including hitherto unknown details of its ventral anatomy and appendages revealed through CT scanning, highlighting additional possible homologous features between the ventral plates of this taxon and trilobite free cheeks. We used three coding strategies treating ventral plates as homologous to trilobite-free cheeks, to trilobite cephalic doublure, or independently derived. If ventral plates are considered homologous to free cheeks, Acanthomeridion is recovered sister to trilobites, however, dorsal ecdysial sutures are still recovered at many places within Artiopoda. If ventral plates are considered homologous to doublure or non-homologous, then Acanthomeridion is not recovered as sister to trilobites, and thus the ventral plates represent a distinct feature to trilobite doublure/free cheeks.

Multiple origins of dorsal ecdysial sutures in trilobites and their relatives.

Euarthropods are an extremely diverse phylum in the modern, and have been since their origination in the early Palaeozoic. They grow through moulting the exoskeleton (ecdysis) facilitated by breaking along lines of weakness (sutures). Artiopodans, a group that includes trilobites and their non-biomineralizing relatives, dominated arthropod diversity in benthic communities during the Palaeozoic. Most trilobites - a hyperdiverse group of tens of thousands of species - moult by breaking the exoskeleton along cephalic sutures, a strategy that has contributed to their high diversity during the Palaeozoic. However, the recent description of similar sutures in early diverging non-trilobite artiopodans means that it is unclear whether these sutures evolved deep within Artiopoda, or convergently appeared multiple times within the group. Here, we describe new well-preserved material of Acanthomeridion, a putative early diverging artiopodan, including hitherto unknown details of its ventral anatomy and appendages revealed through CT scanning, highlighting additional possible homologous features between the ventral plates of this taxon and trilobite free cheeks. We used three coding strategies treating ventral plates as homologous to trilobite-free cheeks, to trilobite cephalic doublure, or independently derived. If ventral plates are considered homologous to free cheeks, Acanthomeridion is recovered sister to trilobites, however, dorsal ecdysial sutures are still recovered at many places within Artiopoda. If ventral plates are considered homologous to doublure or non-homologous, then Acanthomeridion is not recovered as sister to trilobites, and thus the ventral plates represent a distinct feature to trilobite doublure/free cheeks.

A CAMBRIAN MERASPID CLUSTER: EVIDENCE OF TRILOBITE EGG DEPOSITION IN A NEST SITE

Recent evidence confirms that trilobites were oviparous; however, their subsequent embryonic development has not been determined. A ∼ 6 cm2 claystone specimen from the upper Cambrian (Paibian) Conasauga Formation in western Georgia contains a cluster of >100 meraspid trilobites, many complete with librigenae. The juvenile trilobites, identified as Aphelaspis sp., are mostly 1.5 to 2.0 mm total length and co-occur in multiple axial orientations on a single bedding plane. This observation, together with the attached free cheeks, indicates that the association is not a result of current sorting. The majority of juveniles with determinable thoracic segment counts are of meraspid degree 5, suggesting that they hatched penecontemporaneously following a single egg deposition event. Additionally, they are tightly assembled, with a few strays, suggesting that the larvae either remained on the egg deposition site or selectively reassembled as affiliative, feeding, or protective behavior.Gregarious behavior by trilobites (“trilobite clusters”) has been reported frequently, but previously encompassed only holaspid adults or mixed-age assemblages. This is the first report of juvenile trilobite clustering and one of the few reported clusters involving Cambrian trilobites. Numerous explanations for trilobite clustering behavior have been posited; here it is proposed that larval clustering follows egg deposition at a nest site, and that larval aggregation may be a homing response to their nest.

The fossil record of ecdysis, and trends in the moulting behaviour of trilobites

Ecdysis, the process of moulting an exoskeleton, is one of the key characters uniting arthropods, nematodes and a number of smaller phyla into Ecdysozoa. The arthropod fossil record, particularly trilobites, eurypterids and decapod crustaceans, yields information on moulting, although the current focus is predominantly descriptive and lacks a broader evolutionary perspective. We here review literature on the fossil record of ecdysis, synthesising research on the behaviour, evolutionary trends, and phylogenetic significance of moulting throughout the Phanerozoic. Approaches vary widely between taxonomic groups, but an overall theme uniting these works suggests that identifying moults in the palaeontological record must take into account the morphology, taphonomy and depositional environment of fossils. We also quantitatively analyse trends in trilobite ecdysis based on a newly generated database of published incidences of moulting behaviour. This preliminary work reveals significant taxonomic and temporal signal in the trilobite moulting fossil record, with free cheek moulting being prevalent across all Orders and throughout the Phanerozoic, and peaks of cephalic moulting in Phacopida during the Ordovician and rostral plate moulting in Redlichiida during the Cambrian. This study and a review of the literature suggest that it is feasible to extract large-scale evolutionary information from the fossil record of moulting.

Ontogeny of the Ptychaspidid TrilobiteQuadraticephalus elongatusKobayashi, 1935 from the Furongian (Late Cambrian) Hwajeol Formation, Korea

The development of the trilobite pygidium involves both an articulation process at the frontal part and the formation of new segments at the rear end, and hence the development of the meraspid pygidium entails complicated morphological changes. This study deals with the ontogeny of the Furongian (late Cambrian) ptychaspidid trilobite,Quadraticephalus elongatus(Kobayashi, 1935), from the Hwajeol Formation of the Taebaek Group, Taebaeksan Basin, Korea, with a special focus on the segmentation process during the meraspid pygidial development. Compared to the ontogeny of a ptychaspidid trilobite,Asioptychaspis subglobosa(Sun, 1924), which is assumed to be an ancestral species ofQ. elongatus, the convexity of the cranidium ofQ. elongatusincreased in a slower rate; the yoked free cheek ofQ. elongatussplits to form a ventral median suture in a later developmental stage; and, a rim-like ridge, which disappeared in the early holaspid pygidium ofA. subglobosa, was maintained in the late holaspid period ofQ. elongatus. These morphological changes with growth imply that paedomorphosis was involved in the evolution ofQ. elongatus. Eleven stages are recognized for the meraspid pygidial development, which began with an accumulation phase during which the number of segments increased from three to seven, followed by an equilibrium phase with seven segments, and ended up with a depletion phase during which the number of segments within the pygidium decreased to four. During the depletion phase, the pygidial length did not increase or even slightly decreased. The onset of the epimorphic phase, in which the total number of trunk segments does not increase anymore, precedes the onset of the holaspid period, demonstrating that the developmental mode ofQ. elongatusis protomeric.

Ontogeny of the Furongian (late Cambrian) remopleuridioid trilobiteHaniwa quadrataKobayashi, 1933 from Korea: implications for trilobite taxonomy

AbstractThe monophyly of the trilobite Order Asaphida has been challenged. The Superfamily Remopleuridioidea was included in the Order Asaphida, based on the ventral median suture and highly bulbous protaspis of the late Furongian–Tremadocian representatives of the group. The remopleuridioid,Haniwa quadrataKobayashi, 1933 from the Furongian (late Cambrian) Hwajeol Formation of Korea, represents a primitive morphology of the Remopleuridioidea. This trilobite does not have a typical globular morphology of asaphoid protaspis, and the free cheeks remained yoked together during the whole of the development. This supports the previous proposition that the Superfamily Remopleuridioidea should be excluded from the Order Asaphida. In addition, the evolution of a highly globular protaspis of the Ordovician remopleuridioid trilobites from the less bulbous protaspis corroborates the possibility of multiple evolutions of a highly globular protaspis. It can be argued that the possession of a highly globular protaspis does not guarantee the membership of the Order Asaphida, and thus the concept of the Order Asaphida should be emended.

Ontogeny and ventral median suture of the ptychaspidid trilobite Asioptychaspis subglobosa (Sun, 1924) from the Furongian (Upper Cambrian) Hwajeol Formation, Korea

The order Asaphida has been characterized by the possession of a globular protaspis and a ventral median suture. the superfamily Dikelocephaloidea was included in the Asaphida, although there has been no reliable ontogenetic information for this superfamily, and thus whether dikelocephaloids had a protaspis of globular shape has remained unclear. the ontogenetic study of the dikelocephaloid trilobite, Asioptychaspis subglobosa, from Korea reveals that the protaspis of A. subglobosa is not of globular morphology but of general benthic adult-like morphology, heralding that the Dikelocephaloidea may not be closely related to other trilobite groups within the Order Asaphida. This protaspid morphology is clearly distinguished from the highly globular protaspis of the Remopleuridioidea, which has been considered the sister group to the Dikelocephaloidea. in addition, the ontogenetic development of the free cheeks shows that A. subglobosa possessed anteriorly yoked free cheeks during the early phase of development but formed a ventral median suture by splitting the yoked cheeks in the later phase of ontogeny. This contrasts with the previously suggested mode of ventral median suture formation of the Order Asaphida. This alternative mode of ventral median suture formation of A. subglobosa, along with non-asaphoid protaspid morphology, warrants removal of the Dikelocephaloidea from the current Order Asaphida. This study further demonstrates that the ventral median suture did not evolve only once in the evolutionary history of trilobites. With the addition of the mode here documented for Asioptychaspis, the ventral median suture is now known to have developed independently and in a different fashion at least three times in the history of Cambrian trilobites.

Reassessment of the mode of life ofPagetiaWalcott, 1916 (Trilobita: Eodiscidae) based on a cluster of intact exuviae from the Kaili Formation (Cambrian) of Guizhou, China

Pagetia is the most abundant trilobite in the Kaili Formation (lower to middle Cambrian). During the course of studying the museum collections of Kaili trilobites (n > 1000), a cluster containing 22 pagetiid individuals at various growth stages is noted. Specimens record the growth range from the degree 0 meraspid to late holaspid phases. Based on the relative completeness of the moults except for missing free cheeks, these specimens are interpreted as intact exuviae that had undergone minimal transport prior to burial. If Kaili pagetiids had pelagic or planktonic living habits as previously suggested, it would be difficult to explain the presence of intact exuviae in clusters. Therefore, Kaili pagetiids are interpreted here as having a benthic mode of life after the onset of the meraspid phase.

Settling strategy of stalked echinoderms from the Kaili Biota (middle Cambrian), Guizhou Province, South China

A total of 124 articulated echinoderms, including Sinoeocrinus lui ( n = 30), “ S. globus” ( n = 92), one indeterminate gogiid, and one indeterminate edrioasteroid, from the Kaili Formation (Middle Cambrian), eastern Guizhou, China was examined. Among them, 73% of gogiid echinoderms are preserved attached to skeletal substrates. Suitable skeletal substrates for gogiid holdfasts in the Kaili Biota are organophosphatic brachiopods; large trilobite fragments, including cranidia, free cheeks, thoracic segments, and pygidia; hyoliths; and Scenella shells. The high frequency of “ S. globus” attached to organophosphatic brachiopods is due to the high richness of organophosphatic brachiopods rather than a host-specific association in the Kaili Biota. However, the possible discrimination against attachment to small trilobites, such as Pagetia, requires further investigation. Based on 11 hand samples, echinoderm population density (EPD) varies from 0.3 to 4.5 echinoderms per 50 grind points (0.06 to 0.96 echinoderms per cm 2), depending on the percent coverage of skeletal debris. Crowding effect is evident when EPD is approximately 0.32 echinoderms per cm 2 and the skeletal coverage is less than 20%.

THEODENISIA AND PERACHEILUS: ENIGMATIC TRILOBITES FROM LAURENTIA

B oulders of late Cambrian age in conglomerates in Quebec and Newfoundland have been intensively collected and have yielded some 18 species which were referred by Ludvigsen et al. (1989) to either Theodenisia Clark, 1948, or to Peracheilus Ludvigsen, 1986. Most specimens are cranidia, rarely having the free cheek in place, and all 3 mm or less in length (sag.). Similar cranidia from strata in Vermont (Clark and Shaw, 1968), Maryland (Rasetti, 1959), Oklahoma (Stitt, 1971), Texas (Winston and Nicholls, 1967) and Alberta (Westrop, 1986) appear to be few and are in need of further study. No even partially complete exoskeleton has been found, so that isolated pygidia cannot confidently be referred to a particular species; no hypostome is known. The cranidium provides a limited basis on which to diagnose a genus, and while each of the species displays distinctive characters, it is difficult to discern one or more related groups among them. The present investigation of the holotypes of type species and material of four additional species shows how problematic is the use of current generic categories. I begin my account by pointing out differences in appearance between individual specimens. Such contrasts are greatest between the external surface of the exoskeleton and a mold of the inner, visceral surface, lesser differences resulting from exfoliation of the laminated outer layer of the exoskeleton (Fig. 1) at different levels. The holotype cranidium of Theodenisia, T. eminens (Clark, 1924) is a unique specimen, none other like it having been recovered, despite the intensive collecting. It has distinctive features, but because of its uniqueness I advocate that the name Theodenisia should only be used for this holotype (Fig. 2.1–2.3, 2.5, 2.6). The holotype and additional specimens of Peracheilus marcoui (Raymond, 1924), the type species of the genus Peracheilus , are described (Fig. 2.4, 2.7–2.15), together with four other species. These show (Figs. 3, 4) part of the range of morphology that exists, but there is little in common between them. For example, not all have S1 and S2 prominent, nor is the glabella widest anteriorly in all. Renewed intensive collecting, including from areas other than Quebec and Newfoundland, is needed in the hope of obtaining more nearly complete knowledge of the exoskeleton. Such knowledge may show whether or not a group, or groups, of related species can be recognized. As a temporary expedient I use Peracheilus ? as a genus in which to include, with question, three of the species, and adopt the more recently proposed name Calculites Fortey, 1983 for the type species Acheilus microps Rasetti, 1944. In recording the material examined, I include isolated pygidia associated with cranidia in Rasetti's collection to show that they are present, but not to confirm that I consider they belong to the same species as the cranidium.

A reconstruction of Telephina bicuspis , a pelagic trilobite from the Middle Ordovician of the Oslo Region, Norway

Well preserved parts of Telephina bicuspis described from dark limestone concretions of Llanvirn age, in the Oslo Region, Norway, exhibit many features associated with a pelagic life style for this trilobite. The free cheek has a huge holochroal eye with perfectly square lenses and a long genal spine which is unique in being directed vertically upwards rather than ventrally. Additional spines include an occipital spine, the macro spine on the rachis of the 6th thoracic segment and paired spines from each of the two rachial rings of the pygidium. These spines may have had a hydrodynamic and stabilizing function. The genal spine could have had a sensory function or been a dimorphic feature. The hypostoma is figured for the first time and a reconstruction is given for the complete exoskeleton.

Probable predation on Upper Cambrian trilobites and its relevance for the extinction of soft‐bodied Burgess Shale‐type animals

The lower Rabbitkettle Formation of northwestern Canada is a monofacial Upper Cambrian unit of variably calcareous, argillaceous siltstone and fine-grained sandstone with rare bioclastic grainstone, deposited on a gentle slope below fair-weather wave base with no discernible fluctuation in water depth. The trilobite fauna is a mixture of pandemic agnostoids and Laurentian polymeroids, including protaspides and meraspides, and individuals are disarticulated, non-abraded and mostly oriented convex-upward. Bioclasts are interpreted as in situ elements affected only by weak bottom currents and storm-induced turbulence. A major proportion of the larger (≥5 mm across) polymeroid cranidia and pygidia in the lower part (Marjuman) of the formation are broken; large thoracic segments are often broken at the axial furrow and some broken free cheeks occur, but essentially no broken agnostoids or hypostomes were observed. Trilobites are not broken in upper beds (Steptoean), above the base of the Glyptagnostus retculatus Zone. Physical breakage cannot be dismissed entirely, but most damage is interpreted to be due to size-selective predation, possibly through lethal blows similar to those delivered by some extant stomatopod crustaceans. A possible culprit may be an animal akin to Yohoia, known from the Middle Cambrian Burgess Shale. The distribution of attacked trilobites serves as a proxy for the presence and disappearance of soft-bodied carnivores. In the Rabbitkettle Formation, it suggests that Burgess Shale-type animals may have persisted into the Late Cambrian but suffered extinction at the Marjuman-Steptoean ‘biomere’ event when most trilobite species vanished.

Revision and subdivision of the polyphyletic ‘Leonaspis’ (Trilobita)

ABSTRACTA phylogenetic analysis is performed on all species previously assigned to the odontopleurid trilobite genusLeonaspis. The PAUP analysis shows this genus, as conventionally defined, to be polyphyletic, composed of four groups of equal taxonomie rank.Leonaspis(s.s.) is here restricted to one of these groups only, a monophyletic set of species characterised by ‘four-spined’ pygidia (i.e. with four spines between the major border spines) and a thorax of nine segments.Kettneraspisis recognised for the largest of these four groups, and is composed of ‘two-spined’ species with nine thoracic segments.LeonaspisandKettneraspisbelong to the Odontopleurinae. A third group of previous ‘Leonaspis’ species constitute a new genus of the Acidaspidinae. Its species are characterised by being ‘four-spined’ and having ten thoracic segments. For these the new genusExallaspisis erected, with type speciesE. bufo. A fourth group of ‘Leonaspis’ species, being ‘four-spined’ with nine thoracic segments, belongs toEoleonaspis, an Ordovician odontopleurine genus.LeonaspisandExallaspisare temporally and spatially non-overlapping,Leonaspisbeing exclusively Gondwanan post-Wenlock, whereasExallaspisis restricted to areas north of the Rheic Ocean and ranges from basal Llandovery to Ludlow.Kettneraspisis pandemic. The condition of five epiborder spines and ten border spines on the free cheek is proposed as plesiomorphic for odontopleurids, and the border spines are shown to originate as two separate rows with alternating spines. A shift in position of the facial suture is shown to transfer the genal spine from the cranidium to the free cheek inKettneraspismeraspides of degree 1 or 2, and the previously suggested co-occurrence of a fixigenal and librigenal spine in these stages is refuted. Earlier proposed dimorphism is rejected, and two mechanisms for reduction in the number of pygidial border spines are proposed.Kettneraspis reetaesp. nov. is described.

Sensory field maps of proetide trilobites

The sensory fields of four Wenlock trilobites Proetus (Proetus) concinnus, Warburgella (Warburgella) stokesii, Warburgella (Warburgella) scutterdinensis, and Harpidella (Harpidella) maura are described and found to be species specific. Certain features, such as the small cell polygon size, and prismatic layer comprising about one tenth of the cuticle thickness are characteristic of Order Proetida. Intraspecific variation is related to ontogenetic development, most markedly in the field of the free cheek of P. (P.) concinnus. The proetid occipital pustule may have functioned as a light-sensitive organ. The tropidium W. (W.) stokesii is a ridge with a central canal or groove, rather than a tubular structure, and probably contained a sensory receptor rather than a circulatory system. Despite microstructural distribution being species specific, it is of limited practical value in taxonomy as the fine details are easily lost during diagenesis.

Articulation and exuviation in Cambrian trilobites

Articulation of the exoskeleton is reviewed in trilobites where the inner portion of the thoracic pleura was horizontal. Devices enabling articulation in the thorax of the olenelloidHolmia, and of species ofParadoxidesare described. The thoracic pleura in these taxa did not have the inner portion horizontal, but the body could be enrolled. Arching of the body, and opening of the cephalic sutures, are considered to have been essential preliminary manoeuvres in exuviation, allowing egress anteriorly of the newly moulted animal. Inversion of the free cheeks and rostral-hypostomal plate beneath the thorax in exuvial configurations ofParadoxidesandOgygopsismay be explained by such manoeuvres. There is no compelling evidence for an opening between cephalon and thorax in these specimens, and such an opening does not appear to have been necessary in freeing the vital soft parts and facilitating emergence. Exuviae consisted of the mineralized exoskeleton, held together by articulatory membranes and the unmineralized cuticle that had covered the ventral side of the body and the appendages. Consideration needs to be given to how such exuviae came to be buried and partially preserved, to the effects of progressive decay of unmineralized cuticle before burial, and of transport before or during burial, before claiming that a particular specimen is an exuvial configuration. The criteria for recognizing such a specimen are problematical, but it is considered that the symmetrical inversions described here may be such configurations.

Ontogeny of an Early Cambrian eodiscoid trilobite from Henan, China

Zhang, Xi-guang, 1989 01 15: Ontogeny of an Early Cambrian eodiscoid trilobite from Henan, China. Lethaia, Vol. 22, pp. 13–29). Oslo. ISSN 0024–1164. Numerous phosphatized specimens from Xichuan, Henan, China permit a detailed study of the morphology and ontogeny of the Eady Cambrian eodiscoid trilobite Neocobboldia chinlinica Lee. Several morphological changes of pygidia are seen to accompany growth, by means of which 13 quantitatively defined stages with a probable growth rate of 1.20, and 4 developmental periods (anaprotaspid, meta-protaspid, meraspid. and holaspid) have been recognized for this species. The anaprotaspis is the first description for codiscoids. It shows great but gradual changes in morphology with progressive maturity. Morphometric and qualitative analyses indicate that three thoracic segments were formed regularly and then released from the pygidium during the meraspid period; and more than one instar might have occurred within a stage. The exoskekton consists of two well-preserved layers, both bearing fine ornament that showed gradual change with growth as well. The free cheek with visual surface attached and the hypostoma show many of the fundamental characters of eodiscoids. Some morphological differentiation in cranidia may reflect a sexually dimorphic condition. Statistical and taphondmic analyses indicate that these specimens of N. chinlinica may represent a complete population. The larvae may have lived in swarms in a warm shallow water environment. □Trilobita, eodiscoid, morphology, ontogeny, paloeo-ecology, Early Cambrian, China, Henan.

Techniques of exuviation in Australian species of the Cambrian trilobiteRedlichia

Three forms of exuviation, involving inversion of exoskeletal elements, are described in three Australian species of the Cambrian trilobite Redlichia: R. forresti (Foord), R. idonea Whitehouse and R. micrograpta Öpik. They are: inversion of the pygidium, in four specimens of R. forresti; inversion of some thoracic segments of R. idonea; and lateral inversion of a free cheek, in single specimens of R. forresti and R. micrograpta. This is the first record of thoracic segment inversion during exuviation in trilobites. Exuviae with inverted exoskeletal elements provide information on the techniques of exuviation, in particular the direction of egression of the postecdysial trilobite from the proecdysial exuvia and also in this genus, the extent of enrolment of the thorax.

Techniques of trilobite exuviation

Examples of the trilobites Toxochasmops extensus (Boeck, 1838), Asaphiscus wheeleri Meek, 1874, Encrinurus mitchelli Foerste, 1888, Ogygopsis klotzi (Rominger, 1887), Paradoxides davidis Salter, 1863 and Oryctocephalus spp. which are interpreted as exuvial configurations, are described. Four specimens of Toxochasmops extensus arc known in which the pygidium rests either directly behind the eephalon, or with only three intervening thoracic segments. It is considered that during exuviation the old pygidium became wedged behind the cephalon. This facilitated its removal. An ecdysial configuration of Asaphiscus wheeleri is described which possesses inverted and partially rotated free cheeks. In addition, part of the thorax of the specimen is wedged obliquely behind the cephalon. This is considered to have aided withdrawal of the trilobite from its old exoskeleton. Two specimens of Encrinurus mitchelli are described which possess free cheeks inverted beneath the cranidium by lateral rotation, in a manner similar to that of A. wheeleri. Three examples of Ogygopsis klotzi are described, one a failed exuvia and two in which the free cheeks were inverted and rotated through 180° with respect to their original position and came to rest beneath the thorax. An identical exuvia of Paradoxides davidis trapezopyge is also described. Two specimens of Oryctocephalus exhibiting two different arrangements of inverted free cheeks are recorded. Possible mechanisms for each of these free cheek inversions are proposed.

Retention of complete maxillary dentures related to soft tissue function

The influence of free tongue, lip, and cheek function on the retention of complete maxillary dentures was investigated for five experienced denture wearers. Test dentures were designed with full palatal coverage and functionally determined filling in of the vestibular sulcus. The front teeth were arranged primarily to meet cosmetic demands--that is, anterior to the top of the residual alveolar ridge. Retention was measured as resistance to dislodgement-provoking loads applied vertically to the incisive edge of the central incisors, using a miniature bite force recorder. All the participants were able to load their front teeth with 35 N or more without loss of retention. None of them experienced denture dislodgement provided the tongue, lips, and cheeks wee allowed to act freely. If the peripheral soft tissues were separated from the vestibular denture flange, no obvious effect on denture retention could be detected. Physically preventing the tongue from pressing against the posterior part of the denture reduced the retention significantly. Measurements of anterior loads tolerated after stepwise reductions of the denture extension indicated that the tongue acted primarily by pressure against the tuber regions. Tongue pressure against the central parts of the palate and lip or cheek pressure against the vestibular flange seemed to be of less importance.

Two rare trilobites from the Tremadoc Leimitz Shales, Germany

Kaufmannia lauta Sdzuy 1961, and Lichakephalus erbeni Sdzuy 1955, were previously known only from a few specimens each. New material giving additional information is described and the taxonomic position of these genera in the classification of trilobites at the ordinal level is discussed. Kaufmannia is renamed Kaufmannella nom. nov. The cranidium of K. lauta has a strong glabellar spine preserved and the free cheek is described for the first time. Representatives of the Suborder Asaphina and the Superfamily Ceratopygacea are separated from the Redlichiida and placed in an independent order, the Asaphida. New specimens of cranidia of Lichakephalus erbeni reveal that an earlier interpretation of the glabellar furrows is in error. The furrows are now interpreted in agreement with Opik (1937) as having a similar arrangement to Platylichas. Lichakephalus is closely related to the Lichidae as well as to the Cambrian Eoacidaspididae, and homology of the glabellar lobes suggests that the three typical pairs of lo...