Mammals with a long diastema typically also have dominant masseter and pterygoid muscles

A few orders of mammals contain many individuals with dominant masseter and pterygoid muscles that pull up and forward as they close the jaw. A dominant temporalis muscle that pulls the jaw up and to the rear is the more common condition in mammals. A long toothless region (diastema) is present in almost all mammals with a large masseter/pterygoid complex. The presence of a diastema, when few teeth have been lost and their size has not changed significantly over evolutionary time, implies that the jaws have lengthened, as in horses and selenodont artiodactyls. (A long jaw with a shorter diastema will also form if very long incisors develop as in rodents.) The sum of the forces of all the jaw muscles (represented by an arrow) typically divides the jaw into a posterior, toothless region and an anterior region where the teeth are located. In most mammals, the sum of all the bite forces at the teeth is maximized when the lengths of the projections of these two regions, onto a line perpendicular to the arrow, are in the ratio of 3 : 7. If the tooth-bearing region of the jaws becomes longer over evolutionary time this ratio will obviously be disturbed. A change in the location of some basic bony features of the jaw mechanism could maintain this ratio, but this requires major disruption of the skull and jaws. Alternatively, simply changing the masses of the muscles that close the jaw (smaller temporalis, larger masseter and/or pterygoid, or some combination), so that the lower jaw is pulled up and forward, rather than backward, also maintains the ratio. According to this view, if the jaw lengthens over evolutionary time, the relative sizes of the jaw muscles will change so that the masseter/pterygoid complex will become dominant.  © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society , 2008, 153 , 625–629.     

Comparative study on the cranial morphology of Gymnallabes typus (Siluriformes: Clariidae) and their less anguilliform relatives,Clariallabes melas and Clarias gariepinus

We compare the cranial morphology of four fish species with an increasing anguilliformism in the following order: Clarias gariepinus, Clariallabes melas, Gymnallabes typus, and Channallabes apus. The main anatomical‐morphological disparities are the stepwise reduction of the skull roof along with the relative enlargement of the external jaw muscles, which occurred in each of them. Gymnallabes typus and C. apus lack a bony protection to cover the jaw muscles. The neurocranial bones of C. gariepinus, however, form a closed, broad roof, whereas the width of the neurocranium in C. melas is intermediate. Several features of the clariid heads, such as the size of the mouth and the bands of small teeth, may be regarded as adaptations for manipulating large food particles, which are even more pronounced in anguilliform clariids. The jaw musculature of G. typus is hypertrophied and attached on a higher coronoid process of the lower jaw, causing a larger adductive force. The hyomandibula interdigitates more strongly with the neurocranium and its dentition with longer teeth is posteriorly extended, closer to the lower jaw articulation. The anguilliform clariids also have their cranial muscles modified to enable a wider gape. The adductor mandibulae and the levator operculi extend more posteriorly, and the anterior attachment site of the protractor hyoidei dorsalis shifts toward the sagittal plane of the head. A phylogenetic analysis of the Clariidae, which is in progress, could check the validity of Boulenger's hypothesis that predecessors of the primitive fishes, such as Heterobranchus and most Clarias, would have evolved into progressively anguilliform clariids. J. Morphol. 240:169–194, 1999. © 1999 Wiley‐Liss, Inc.     

Aspects of masticatory form and function in common tree shrews,Tupaia glis

Tree shrews have relatively primitive tribosphenic molars that are apparently similar to those of basal eutherians; thus, these animals have been used as a model to describe mastication in early mammals. In this study the gross morphology of the bony skull, joints, dentition, and muscles of mastication are related to potential jaw movements and cuspal relationships. Potential for complex mandibular movements is indicated by a mobile mandibular symphysis, shallow mandibular fossa that is large compared to its resident condyle, and relatively loose temporomandibular joint ligaments. Abrasive tooth wear is noticeable, and is most marked at the first molars and buccal aspects of the upper cheek teeth distal to P². Muscle morphology is basically similar to that previously described for Tupaia minor and Ptilocercus lowii. However, in T. glis, an intraorbital part of deep temporalis has the potential for inducing lingual translation of its dentary, and the large medial pterygoid has extended its origin anteriorly to the floor of the orbit, which would enhance protrusion. The importance of the tongue and hyoid muscles during mastication is suggested by broadly expanded anterior bellies of digastrics, which may assist mylohyoids in tensing the floor of the mouth during forceful tongue actions, and by preliminary electromyography, which suggests that masticatory muscles alone cannot fully account for jaw movements in this species.     

Patterns of variation across primates in jaw-muscle electromyography during mastication

Biologists that study mammals continue to discuss the evolutionof and functional variation in jaw-muscle activity during chewing.A major barrier to addressing these issues is collecting sufficientin vivo data to adequately capture neuromuscular variation ina clade. We combine data on jaw-muscle electromyography (EMG)collected during mastication from 14 species of primates andone of treeshrews to assess patterns of neuromuscular variationin primates. All data were collected and analyzed using thesame methods. We examine the variance components for EMG parametersusing a nested ANOVA design across successive hierarchical factorsfrom chewing cycle through species for eight locations in themasseter and temporalis muscles. Variation in jaw-muscle EMGswas not distributed equally across hierarchical levels. Thetiming of peak EMG activity showed the largest variance componentsamong chewing cycles. Relative levels of recruitment of jawmuscles showed the largest variance components among chewingsequences and cycles. We attribute variation among chewing cyclesto (1) changes in food properties throughout the chewing sequence,(2) variation in bite location, and (3) the multiple ways jawmuscles can produce submaximal bite forces. We hypothesize thatvariation among chewing sequences is primarily related to variationin properties of food. The significant proportion of variationin EMGs potentially linked to food properties suggests thatexperimental biologists must pay close attention to foods givento research subjects in laboratory-based studies of feeding.The jaw muscles exhibit markedly different variance componentsamong species suggesting that primate jaw muscles have evolvedas distinct functional units. The balancing-side deep masseter(BDM) exhibits the most variation among species. This observationsupports previous hypotheses linking variation in the timingand activation of the BDM to symphyseal fusion in anthropoidprimates and in strepsirrhines with robust symphyses. The working-sideanterior temporalis shows a contrasting pattern with littlevariation in timing and relative activation across primates.The consistent recruitment of this muscle suggests that primateshave maintained their ability to produce vertical jaw movementsand force in contrast to the evolutionary changes in transverseocclusal forces driven by the varying patterns of activationin the BDM.     

A functional analysis of carnassial biting

The jaw mechanism of carnivores is studied using an idealized model (Greaves, 1978). The model assumes: (i) muscle activity on both sides of the head, and (ii) that the jaw joints and the carnassial teeth are single points of contact between the skull and the lower jaw during carnassial biting. The model makes the following predictions: (i) in carnivores with carnassial teeth the resultant force of the jaw muscles will be positioned approximately 60% of the way from the jaw joint to the tooth—this arrangement delivers the maximum bite force possible together with a reasonably wide gape (remembering that bite force and gape cannot both be maximized); (ii) in an evolutionary sense, if greater bite force is required at the carnassial tooth, either the animal will get larger so as to deliver an absolutely larger bite force or the architecture of the muscles may change, becoming more pinnate, for example, but jaw geometry (i.e. the relative positions of the jaw joints, the carnassial tooth, and the muscle resultant force) will not change; (iii) if greater gape is required, the animal will get larger so as to have longer jaws and therefore an absolutely wider gape or change its muscle architecture allowing for greater stretch while the geometry remains unchanged; and (iv) in animals with a longer shearing region (e.g. the extinct hyaenodonts) the shearing region will be approximately 20% of jaw length and the muscle resultant force will be positioned approximately 60% of the way from the jaw joint to the most anterior shearing tooth.     

Evolution of asynchronous motor activity in paired muscles: effects of ecology, morphology, and phylogeny

Many studies of feeding behavior have implanted electrodes unilaterally(in muscles on only one side of the head) to determine the basicmotor patterns of muscles controlling the jaws. However, bilateralimplantation has the potential to achieve a more comprehensiveunderstanding of modification of the motor activity that maybe occurring between the left and right sides of the head. Inparticular, complex processing of prey is often characterizedby bilaterally asynchronous and even unilateral activation ofthe jaw musculature. In this study, we bilaterally implant feedingmuscles in species from four orders of elasmobranchs (Squaliformes,Orectolobiformes, Carcharhiniformes, Rajoidea) in order to characterizethe effects of type of prey, feeding behavior, and phylogenyon the degree of asynchronous muscle activation. Electrodeswere implanted in three of the jaw adductors, two divisionsof the quadratomandibularis and the preorbitalis, as well asin a cranial elevator in sharks, the epaxialis. The asynchronyof feeding events (measured as the degree to which activityof members of a muscle pair is out of phase) was compared acrossspecies for capture versus processing and simple versus complexprey, then interpreted in the contexts of phylogeny, morphology,and ecology to clarify determinants of asynchronous activity.Whereas capture and processing of prey were characterized bystatistically similar degrees of asynchrony for data pooledacross species, events involving complex prey were more asynchronousthan were those involving simple prey. The two trophic generalists,Squalus acanthias and Leucoraja erinacea, modulated the degreeof asynchrony according to type of prey, whereas the two behavioralspecialists, Chiloscyllium plagiosum and Mustelus canis, activatedthe cranial muscles synchronously regardless of type of prey.These differences in jaw muscle activity would not have beendetected with unilateral implantation. Therefore, we advocatebilateral implantation in studies of cranial muscle functionin fishes, particularly when investigating behaviors associatedwith processing complex prey. Incorporating this methodologywill provide a more detailed understanding of the coordinationand evolution of paired-muscle function in the feeding apparatusrelative to behavioral and ecological performance.     

COMPARATIVE MORPHOLOGY OF RADULAR TEETH IN CONUS: OBSERVATIONS WITH SCANNING ELECTRON MICROSCOPY

Radular teeth of 22 Indo-Pacific species of the genus Conus(Neogastropoda: Toxoglossa) were compared. On morphologicalfeatures all can be related to one of three known feeding modes:piscivorous, vermivorous and molluscivorous. Observations arereported on the radular teeth of six piscivores, thirteen vermivoresand three molluscivores. The radular teeth of piscivores areof two general types. In the first, two barbs and a posteriorly-directedprocess with a recurved tip are found at the anterior end. Inthe second, two barbs are located at the anterior end and theshaft is serrated for most of its length. An enlarged posteriorregion (terminal knob) is present in the first and absent inthe second. Molluscivores possess radular teeth with two anteriorbarbs and in some species a serrated shaft or terminal knob.The radular teeth of vermivores, which show much greater interspecificvariation than those of piscivores or molluscivores, are characterizedby one or two anterior barbs and in most species a serratedregion near the apex. A forwardly-projecting cone (basal spur)is usually located on the terminal knob. Piscivores and molluscivoreslack such basal spurs. The radular teeth of Conus are used toconvey a potent venom and hold prey firmly during feeding. Previouslyundescribed morphological features are noted on the teeth ofC. obscurus and C. lividus. Figured here for the first timeare the radular teeth of C. abbreviatus, C. aureus, C. catus,C. litoglyphus, C. pennaceus, C. rattus and C. sponsalis. ^*Present address: Department of Paleontology, University ofCalifornia, Berkeley, California 94720, U.S.A. (Received 2 April 1979;     

The anatomy and function of the feeding apparatus in two armadillos (Dasypoda): anatomy is not destiny

The morphology and function of the masticatory apparatus in two armadillos, Dasypus novemcinctus and Euphractus sexcinctus are compared. Euphractus sexcinctus , a species restricted to South America, is omnivorous, eating a wide range of foods, including significant amounts of plant material and carrion. Dasypus novemcinctus is geographically the most widespread of all armadillos, ranging from northern Argentina into the United States. It is insectivorous-omnivorous, apparently consuming whatever it encounters in the leaf litter. In South and Central America, this leads to a diet with a large proportion of ants and termites; in North America, the diet is considerably broadened. The teeth, jaws and jaw musculature of E. sexcinctus are large and the configuration of the jaws maximizes force production. Dasypus novemcinctus possesses derived morphology relative to the primitive condition in armadillos and exhibits many characteristics of ant and termite-eating mammals, including reduced jaw muscles, teeth and facial bones. The apparent morphological specializations for myrmecophagy in D. novemcinctus do not, however, constrain its diet to ants and termites. It is broadly omnivorous, especially in North America. Our data highlight the difficulties in predicting diet from morphological analysis and raise questions concerning the behavioural limits imposed by morphological specialization.     

Structure and direction of jaw adductor muscles as herbivorous adaptations in <Emphasis Type="Italic">Neotoma mexicana</Emphasis> (Muridae,Rodentia)

The herbivorous adaptations of the jaw adductor muscles in Neotoma mexicana were clarified by a comparative study with an unspecialized relative, Peromyscus maniculatus. In P. maniculatus, the anterior part of the deep masseter arises entirely from the lateral side of an aponeurosis, i.e., superior zygomatic plate aponeurosis, whereas N. mexicana has an additional aponeurosis for this part of the muscle, and the fibers attach on both sides of the superior zygomatic plate aponeurosis. Although the structure of the temporalis muscle is nearly identical in the two genera, a clear aponeurosis of origin occurs only in N. mexicana. These characteristics allow fibrous tissues to be processed with a large occlusal force. The deep masseter, internal pterygoid, and external pterygoid muscles of N. mexicana incline more anterodorsally than those of P. maniculatus. The transverse force component of these muscles relative to whole muscle force is smaller in N. mexicana than in P. maniculatus, with the exception of the internal pterygoid. The anterior part of the temporalis muscle of N. mexicana is specialized to produce occlusal pressure. These findings suggest that in N. mexicana a large anterior force is required to move the heavy mandible, due to the hypsodont molars, against frictional force from food, and that the posterior pull of the temporalis, which adjusts the forward force by the other jaw adductor muscles to a suitable level, need not be large for the mandibular movement.     

Jaw muscles of new world squirrels

The jaw, suprahyoid, and extrinsic tongue muscles are described for eight species of New World squirrels, spanning more than an order of magnitude in body mass. Anatomical differences are discussed in the light of body size, natural history, and phylogeny. The relative sizes of different muscles, their orientations, and the shapes and positions of their areas of attachment vary but show few trends in relation to body size. The anatomical differences are likewise not readily explained by the mechanical requirements of the animals' diets, which are similar. The most marked anatomical differences occur in Sciurillus (the pygmy tree squirrel), as well as those genera—Glaucomys (the flying squirrel) and Tamias (the chipmunk)—that are taxonomically most distinct from the tree squirrels. sciurillus is noteworthy for its unusually small temporalis and an anterior deep masseter that is oriented to assist in retraction of the jaw. Tamias has a more vertically oriented temporalis and greater inclination in the anterior masseter muscles than the other squirrels, features that may be associated with its large diastema and relatively posteriorly situated cheek teeth, which in turn may relate to its having cheek pouches. Our results form a valuable database of information to be used in further studies of functional morphology and phylogeny. © 1995 Wiley-Liss, Inc.     

BROODING AND NON-BROODING DACRYDIUM (BIVALVIA: MYTILIDAE): A REVIEW OF THE ATLANTIC SPECIES

The Atlantic species of the marine bivalve genus Dacrydium arereviewed, with particular emphasis on their hinge and protoconchcharacters. The basic groundplan of a Dacrydium comprises afunctional primary ligament, a paired series of primary teeth,and a posterior row of secondary teeth separated from the latterby a secondary ligament; this can be transformed into a singleseries either by loss of the secondary ligament and mergingprimary and secondary teeth, or by loss of secondary teeth andligament through paedomorphosis. Twelve species are recognized, of which eleven are illustrated.One abyssal species is not separable morphologically from theIndian Ocean D. speculum Poutiers, 1989 and is new to the Atlantic;four new species (D. wareni, D. dauvini, D. filiferum and D.balgimi) are described; a Caribbean form which is hardly distinctfrom the Eastern Pacific D. elegantu-lum Soot-Ryen, 1955, isdescribed as a new subspecies D. e. hendersoni. The larvae are brooded in D. hyalinum (Mon-terosato, 1875),D. viviparum Ockelmann, 1983 and D. balgimi. The brooding specieshave larger larvae (protoconch 210 to 315 µm long) thanthe non-brooding (protoconch 120 to 150 µm long), andreach a smaller adult size (1.4 to 3 mm instead of 4.5–5mm). A phylogenetic reconstruction is attempted using parsimonyanalysis of hinge and shell characters as well as the brooding/nonbrooding character. (Received 22 October 1996; accepted 28 November 1996)     

Mutations that affect flightin expression in Drosophila alter the viscoelastic properties of flight muscle fibers

Striated muscles across phyla share a highly conserved sarcomere design yet exhibit broad diversity in contractile velocity, force, power output, and efficiency. Insect asynchronous flight muscles are characterized by high-frequency contraction, endurance, and high-power output. These muscles have evolved an enhanced delayed force response to stretch that is largely responsible for their enhanced oscillatory work and power production. In this study we investigated the contribution of flightin to oscillatory work using sinusoidal analysis of fibers from three flightless mutants affecting flightin expression: 1) fln⁰, a flightin null mutant, 2) Mhc¹³, a myosin rod point mutant with reduced levels of flightin, and 3) Mhc⁶, a second myosin rod point mutant with reduced levels of phosphorylated flightin. Fibers from the three mutants show deficits in their passive and dynamic viscoelastic properties that are commensurate with their effect on flightin expression and result in a significant loss of oscillatory work and power. Passive tension and passive stiffness were significantly reduced in fln⁰ and Mhc¹³ but not in Mhc⁶. The dynamic viscous modulus was significantly reduced in the three mutants, whereas the dynamic elastic modulus was reduced in fln⁰ and Mhc¹³ but not in Mhc⁶. Tension generation under isometric conditions was not impaired in fln⁰. However, when subjected to sinusoidal length perturbations, work-absorbing processes dominated over work-producing processes, resulting in no net positive work output. We propose that flightin is a major contributor to myofilament stiffness and a key determinant of the enhanced delayed force response to stretch in Drosophila flight muscles. flight muscles; muscle mutants; myosin     

Characteristics of dentition in gekkonid lizards of the genus <Emphasis Type="Italic">Teratoscincus</Emphasis> and other Gekkota (Sauria,Reptilia)

The dentition and tooth crown microstructure of gekkonids and eublepharids are examined. Scanning electron microscopy shows that the lingual surface of teeth in these lizards has one, two, or, occasionally, several cusps separated by grooves. The teeth of geckoes usually have two (lingual and labial) cusps in the apical region. With respect to the number of teeth, the majority of Gekkota fall into two groups. The first includes a few species with many teeth (50 or more) in the dentary and maxilla, the eublepharids Goniurosaurus and Aeluroscalabotes, and the gekkonid Cyrtopodion louisiadensis. The second group, comprising most of the species, is subdivided into two subgroups, species with 20–30 or 30–40 teeth in jaw bones. Teratoscincus belongs to the first subgroup of the second group.     

Effects of jaw adductor hypertrophy on buccal expansions during feeding of air breathing catfishes (Teleostei,Clariidae)

Some species of Clariidae (air breathing catfishes) have extremely well developed (hypertrophied) jaw closing muscles that increase the maximal biting force of these species. As these enlarged jaw muscles tightly cover the suspensoria, which are firmly connected to the neurocranium, we expect diminished lateral expansions during suction for species with hypertrophied jaw muscles. In turn, this could imply a reduced suction performance for these species. Compared to Clarias gariepinus, which has relatively small jaw closers, Clariallabes longicauda shows a clear hypertrophy of the jaw adductors. A kinematic analysis of prey capture in these two species is presented here. As predicted, Clariallabes longicauda shows less lateral expansion (average abduction of the hyoids of 19.0°) than Clarias gariepinus (abduction of 31.1°). However, our data indicate that the decrease in lateral expansion capacity in the species with excessive adductor development is compensated for by a larger and faster ventral expansion of the buccal cavity by depression of the hyoid.     

THYSANODONTINAE: A NEW SUBFAMILY OF THE TROCHIDAE (GASTROPODA)

New genera Thysanodonta, Carinastele and Herbertina are referredto a new subfamily of the Troch-idae, Thysanodontinae, on thebasis of extremely distinctive jaw and radula. At up to 1400times longer than broad, the teeth are the most slender of anyknown gastropod, and they are unique among archae-ogastropodsin having harpoon-like tips with back-wardly inclined barbs.While food and feeding mode are unknown, it is suggested thatthey feed suctorially, perhaps on cnidarians. New species describedare Thysanodonta aucklandica, T. wairua, Carinastele kristelleae,C. jugosa, and C. coronata from New Zealand, T. boucheti fromNew Caledonia, and Herbertina eos and H. cognata from SouthAfrica. The Tasmanian Basilissa nicetarium Hedley & Mayis tentatively referred to Carinastele (Received 23 September 1987;     

Morpho-histogenic studies on tendrils of Passiflora

The ontogenetic development of the tendril and its associatedorgans is investigated in 17 species of Passiflora. The shootapex shows a single tunica layer though the second layer simulatestunica. The cytohistological zonation is not a constant feature.In P. caerulea Linn., it is distinct at leaf initiation butin P. pruinosa Mast., P. vespertilio Linn., and P. watsonianaMast., it is indistinct. The main axillary bud differentiatesfrom the peripheral meristem of the shoot apex. The differentiationof this bud into floral and tendril menstems occurs at a nodeimmediately below the shoot apex in P. minima Blanco. and Pracemosa Brot. In other species this differentiation generallyoccurs at the lower nodes. The floral meristem is initiatedas an accessory bud from this bud, thus forming a bud complex.The residuum of the bud complex develops as a tendril. The thirdaccessory bud which does not originate from this bud complex,develops into a vegetative branch. The fundamental nature ofthe vascular relationship between the flower, tendril, accessorybud, subtending leaf, and the axis is similar in most of theinvestigated species.     

An Approach to the Biomechanics of the Masticatory Apparatus of Early Miocene (Santacrucian Age) South American Ungulates (Astrapotheria, Litopterna, and Notoungulata): Moment Arm Estimation Based on 3D Landmarks

Notoungulates, litopterns, and astrapotheres are among the most representative mammals of the early Miocene Santacrucian Age. They comprise a diversity of biological types and sizes, from small forms, comparable to rodents, to giants with no analogues in modern faunas. Traditionally, all of them have been considered herbivores; this diversity is reflected in different morphologies of the masticatory apparatus, suggesting a variety of feeding habits. The application of biomechanics to the study of fossil mammals is a good approach to test functional hypotheses. Jaws act as a lever system, with the pivot at the temporomandibular joint, with masticatory muscles providing the input force, whereas the output force is produced by the teeth on food. The moment arms of the lines of action of the muscles can be estimated to analyze relationships between bite force and bite velocity. A morphogeometric approach inspired by Vizcaíno et al. (1998) is applied to estimate muscle moment arms in a static 3D bite model based on three-dimensional landmarks and semilandmarks on crania with mandibles in occlusion. This new 3D geometric method to evaluate jaw mechanics demonstrated its reliability when applied to a control sample of extant mammals that included carnivores, herbivores, and omnivores. Our results indicate that, except for Pachyrukhos, in no Santacrucian ungulate does the masseter muscle have greater mechanical advantage than the temporalis. Among them, notoungulates have a better configuration to develop force on the molar tooth row than litopterns. This indicates a diet richer in tough plant materials for Santacrucian notoungulates (e.g., grass or even bark) than for litopterns (e.g., dicots). This is consistent with recent ecomorphological approaches applied to this fauna. Finally, the approach proposed here proves to be useful for comparing masticatory performance and it is a powerful tool to validate ecomorphological dietary hypotheses in fossil taxa.     

Using "Mighty Mouse" to understand masticatory plasticity: myostatin-deficient mice and musculoskeletal function

Knockout mice lacking myostatin (Mstn), a negative regulatorof the growth of skeletal muscle, develop significant increasesin the relative mass of masticatory muscles as well as the abilityto generate higher maximal muscle forces. Wild-type and Mstn-deficientmice were compared to investigate the postnatal influence ofelevated masticatory loads due to increased jaw-adductor andbite forces on the biomineralization of mandibular articularand cortical bone, the internal structure of the jaw joints,and the composition of temporomandibular joint (TMJ) articularcartilage. To provide an interspecific perspective on the long-termresponses of mammalian jaw joints to altered loading conditions,the findings on mice were compared to similar data for growingrabbits subjected to long-term dietary manipulation. Statisticallysignificant differences in joint proportions and bone mineraldensity between normal and Mstn-deficient mice, which are similarto those observed between rabbit loading cohorts, underscorethe need for a comprehensive analysis of masticatory tissueplasticity vis-à-vis altered mechanical loads, one inwhich variation in external and internal structure are considered.Differences in the expression of proteoglycans and type-II collagenin TMJ articular cartilage between the mouse and rabbit comparisonssuggest that the duration and magnitude of the loading stimuluswill significantly affect patterns of adaptive and degradativeresponses. These data on mammals subjected to long-term loadingconditions offer novel insights regarding variation in ontogeny,life history, and the ecomorphology of the feeding apparatus.     

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.     

PHYLOGENY OF THE GENUS ROSTANGA (NUDIBRANCHIA), WITH DESCRIPTIONS OF THREE NEW SPECIES FROM SOUTH AFRICA

Rostanga elandsia sp. nov., Rostanga aureamala sp. nov. andRostanga phepha sp. nov. are characterized by having the radulawith slender innermost lateral teeth, which lack denticles onthe inner side of the cusp and have a single denticle on theouter side. The outermost lateral teeth of these three speciesare elongate, but shorter than in other species of the genus.In addition, R. aureamala is the only species of the genus withrachidian teeth and R. phepha is unique within the genus Rostanga byvirtue of its white coloration with dark spots. A phylogenetic analysis shows that the three new species fromSouth Africa and Rostanga setidens (Odhner, 1939) are the sistergroup of the rest of the genus. The species from Japan and MarshallIslands (North Pacific Ocean) are basal in the sister cladecontaining the other species of Rostanga Bergh, 1879. The tropicalIndo-Pacific species of Rostanga are not monophyletic. The Atlanticand Eastern Pacific species form a monophyletic, derived clade,being the sister group of Rostanga australis Rudman & Avern,1989, which has a narrow range restricted to south eastern Australia.The widespread Indo-Pacific species Rostanga bifurcata Rudman& Avern, 1989, is the sister group of Rostanga dentacus Rudman& Avern, 1989, also widesprad in the tropical western Pacific. This phylogeny suggest s a viariant origin of the Sourth African, Atlantic-EasternPacific, and probably North Pacific species, whereas in thetropical Indo-Pacific most sister speceis are sympatric. (Received 16 May 1999; accepted 31 July 2000)