新疆准噶尔盆地北缘铁尔斯哈巴合晚渐新世两种啮形类耳区的形态研究

我们曾描述过一块与本文描述的标本产自同一地点和层位、可能为Ａｍｐｈｅｃｈｉｎｕｓ的猬类岩骨标本（孟津等,１９９９）。后来新发现的与牙齿属同一个体的Ａｍｐｈｅｃｈｉｎｕｓ岩骨标本证明我们根据单体岩骨的分类鉴定无误。此例说明,在一定的条件下,耳区标本在某些哺乳动物类群中可以鉴定到属,甚至种。因此在形态学、生物地层学上都有一定的意义,而且会因有关标本的不断积累而越来越重要。本文记述了另外两块产自新疆准噶尔盆地北缘铁尔斯哈巴合晚渐新世地层中的岩骨。有关地层、地点资料及所用术语见孟津等（１９９９）以及其中…     

Ontogeny and cranial morphology of the tympanic region of the Tupaiidae, with special reference to Ptilocercus

The structure of the tympanic region of the skull of Ptilocercus lowii was studied in an embryo of 30 mm crown-rump length and in 5 osteocrania. As in Tupaia, the anterior wall of the bulla of Ptilocercus is not completed by a tympanic process of the alisphenoid, contrary to earlier reports. Ptilocercus resembles Tupaia in the following derived characters. The ventral wall of the tympanic cavity is formed by a rostral entotympanic and by a caudal tympanic process of the petrosal. The entotympanic develops in primary connection with the tubal cartilage. The tympanic aperture of the auditory tube is bordered by the entotympanic. The ring-shaped tympanicum is covered by the entotympanicum and is aphaneric. The musculus tensor tympani is lacking. Among mammals, these characters can be regarded as synapomorphic for the Tupaiidae, that is, to have been present in the common ancestor of the two subfamilies. From the evidence of the tympanic region, the Tupaiidae, therefore, form a monophyletic group. Besides these synapomorphies, there are remarkable differences between Ptilocercus and Tupaia in the structure of the bulla. In Ptilocercus the bulla is smaller and less pneumatized than in Tupaia. An anterior intrabullar septum, present in Tupaia, is lacking in Ptilocercus. The epitympanic wing of the alisphenoid is smaller in Ptilocercus than in Tupaia. A lateral prefacial commissure of the tegmen tympani is present in Ptilocercus, but absent in Tupaia. The caudal tympanic process of the petrosal is larger in Ptilocercus than in Tupaia. These characters are autapomorphic for the Ptilocercinae and for the Tupaiinae, respectively. They demonstrate that the auditory bulla of Ptilocercus and that of Tupaia have evolved independently to a considerable extent. An early phylogenetic separation of their respective ancestors seems likely. The tympanic region of the skull provides no evidence for close relationships of the tree shrews to the primates or to any other eutherians. The classification of the Tupaiidae in a separate order, Scandentia, is supported.     

Morphologische Untersuchungen am Mittelohr der Marsupialial

Morphological studies on the auditory bulla of marsupials The resent study on the comparative anatomy of the marsupial auditory bulla starts with that of the didelphids; ontogenetic data are included. It becomes clear from these comparisons, that the marsupial bulla is throughout composed of the same skeletal elements. Contrary to widespread opinion, there exists no good evidence for the existence of an entotympanic in any marsupial. The tympanic process of the petrosal develops relatively late in ontogeny as ‘additional bone’ (“Zuwachsknochen” of Starck ); the same holds true for the tympanic process of the alisphe-noid. Biometric study of the tymanic ring in didelhids reveals that the tympanic membrane does only slightly increase with skull size (pronounced negative allometry). It is surprising, however, that the data of the tympanic diameter do arrane at two distinct levels of regression. Several taxa of didelphids must have jumped at this level independently. The possible functional meanings of these data are briefly discussed.     

Patterns of evolutionary transformation in the petrosal bone and some basicranial features in marsupial mammals, with special reference to didelphids

Twelve petrosal and four nonpetrosal characters were coded for representatives of all 15 extant genera of Didelphidae and for 16 additional genera of marsupials representing all extant orders. Three basal metatherians were used as outgroup comparison. Histological sections of a subset of the data were examined. An intermediate position of the hiatus Fallopii supports the monophyly of Didelphidae. Several basicranial regions support different clades within the Didelphidae that recent molecular work has identified, including a sister group relationship of Caluromys and Caluromysiops , the monophyly of large opossums, a Lestodelphys-Thylamys clade, and a Lestodelphys-Thylamys-Gracilinanus-Marmosops clade. Glironia lacks petrosal and jaw synapomorphies of Caluromys and Caluromysiops. The transverse canal, a synapomorphy of the crown-group Marsupialia, opens as a single foramen anterior to the carotid foramen in most marsupials or as numerous foramina in the pterygoid fossa in diprotodontians. It is either intramural (most marsupials) or simply endocranial (most diprotodontians excluding koalas and wombats). Loss of a deep sulcus in the anterior pole of the promontorium for the internal carotid artery and a rostral tympanic process of the petrosal also characterize the groundplan of the crown group Marsupialia. Pouch-young wombats show a groove in the anterior pole of the petrosal for the internal carotid artery. The absence of a prootic canal foramen in the tympanic side of the petrosal of adults supports the monophyly of Australidelphia. Some pouch-young marsupials possess a prootic canal that is later lost in ontogeny. A rather flat promontorium and a crest running medio-distally in the middle of the promontorium characterize Macropodidae.     

The angular process of Monodelphis domestica (Didelphidae, Marsupialia) and its relation to the middle ear: an ontogenetic and evolutionary morphologic study

In most marsupials, the angular process is inflected medially. By using an ontogenetic series of Monodelphis domestica, the development of this characteristic structure has been described. In contrast with the eutherian mammals, in marsupials there is retained a close connection between the dentale and the tympanicum and goniale; it is well known that these 2 elements of the middle ear are derived from the angulare and prearticulare of the reptilian lower jaw. At the neonatal stage, the dentale and tympanicum are both relatively vertically orientated; during the following 2 weeks, they take an increasingly oblique position, which is primarily caused by the rapid growth of the braincase. Only after the eruption of the first teeth, the ascending ramus of the dentale takes a more and more vertical position, whereas the angular process remains with its tip near the medioventral floor of the tympanic bulla. The bulla shows at this place a rectangular fenestra which is covered by a membrane of loose connective tissue; the tip of the angular process, which is always free of muscular insertions, maintains contacts with this fenestra throughout life. During juvenile and adult life stages, the process becomes somewhat removed from the fenestra for obvious reasons, but at a gape of about 40 to 50 degrees it inevitably must touch the "inferior tympanic membrane" and possibly also the tympanic ring. It is speculated that the relationship between the angular process and the tympanic bulla represents a specific form-function complex for sound transmission, which may be a modified retention from archaic mammalian conditions. Further details of the ontogenetic development of the tympanic region have been described which may be of some relevance for the evolutionary morphology of mammals: The tympanic process of the petrosal, which fixes the posterior end of the tympanic ring, is formed by 'Zuwachsknochen' (additional bone) but not by cartilage. The styloid process remains cartilaginous throughout life: its free tip ends in the lateral wall of the tympanic cavity and it is closely connected with the collum mallei and the posterior end of the tympanicum; it guides the chorda tympani and may therefore be homologous with the cartilage of Spence. The cartilage of Paauw is interpreted in terms of functional morphology. A model of evolutionary transformation of the dentale-tympanicum complex in mesozoic mammals in outlined on the basis of the ontogenetic findings in Monodelphis and other didelphid and dasyurid marsupials.     

Cranial circulation of the pen-tailed tree shrewPtilocercus lowii and relationships of Scandentia

The major cranial arteries and veins are described for a 30-mm crown-rump length fetus of the pen-tailed tree shrewPtilocercus lowii, and comparisons are made with cranial vessels reported in the tree shrewTupaia and with the vascular pattern reconstructed for primitive eutherians.Ptilocercus shares a number of derived features of the cranial circulation withTupaia, which, therefore, represent synapomorphies of tree shrews (Tupaiidae, Scandentia). Included are (1) the enclosure of the intratympanic portion of the internal carotid artery in a bony canal that is floored proximally and distally by the entotympanic and by the petrosal in between, (2) the enclosure of the intratympanic portion of the stapedial artery by the petrosal in a canal on the promontorium and within the epitympanic crest beneath the tympanic roof, (3) the absence of an exit for the arteria diploëtica magna, (4) an alisphenoid canal, (5) a maxillary artery that passes medial to the mandibular nerve beneath foramen ovale, and (6) a laryngeopharyngeal artery. Some of these derived features, however, are also found in certain other eutherians (e.g., numbers 2, 3, and 6 in Euprimates) and, therefore, may be used in future studies to assess the higher-level affinities of Scandentia.     

Cranial morphology of the early tertiary Phenacolemur and its bearing on primate phylogeny

An early Eocene skull of the paromomyid Phenacolemur, a plesiadapoid primate, is described with particular emphasis on the ear region. The auditory bulla is composed of the petrosal and of a large ectotympanic plate which is outside of the bulla. The preserved morphology of the middle ear is distinctly more primitive than that of the older Plesiadapis. It cannot be determined with certainty whether Phenacolemur had the carotid circulation enclosed in bony tubes or not. The auditory bulla of early primates and relevant living ones is discussed and it is suggested that an extrabullar ectotympanic, as seen in all non-lemuriform fossil and extant primates, was probably the primitive ordinal condition, rather than the intrabullar ring-like ectotympanic in the Lemuriformes. Aspects of the carotid circulation are discussed as they pertain to the relationship of early Tertiary primates, living Tarsiiformes, Lemuriformes and Lorisiformes.     

New discoveries on the middle ear anatomy of Ignacius graybullianus (Paromomyidae,Primates) from ultra high resolution X-ray computed tomography

A skull of Ignacius graybullianus (USNM 421608) was studied using ultra high resolution X-ray computed tomography (uhrCT). The anatomy of the middle ear in this specimen was previously studied through partial removal of the auditory bulla on one side. The data now available allow for examination of the others unprepared ear, which is more completely preserved, as well as adding to the information available about the previously studied ear. Analysis of the relationships between the bones making up the auditory bulla confirms previous assertions that it is formed from the entotympanic, and not from the petrosal, basioccipital, or basisphenoid. Contrary to previous reconstructions of the middle ear anatomy in all known plesiadapiforms, this specimen exhibits a bony canal for the promontorial artery and/or internal carotid nerves running across the lateral extreme of the promontorium. The identification of this structure is confirmed by the clear presence of a lumen, and its origination at a posterior carotid foramen (pcf) in a position that corresponds to that identified in previous studies of the paromomyid basicranium (Am. J. Phys. Anthropol., 36 (1972) 59, Am. J. Phys. Anthropol., 89 (1992) 477). Remnants of this canal are present bilaterally in USNM 421608, which additionally supports its identification. The presence of bony canals for branches of the internal carotid artery and the internal carotid nerves is a feature seen in scandentians and euprimates that is missing in dermopterans. The unusual lateral route followed by the internal carotid nerves is a primitive euprimate feature missing in all other archontans. As such, this evidence is consistent with a close euprimate-paromomyid relationship, and the inclusion of the latter in the order Primates. The discovery of this feature in paromomyids after almost 30 years of study of the ear region of this family acts as a cautionary note to the interpretation of the middle ear in damaged specimens.     

New insights into the ear region anatomy and cranial blood supply of advanced stem Strepsirhini: Evidence from three primate petrosals from the Eocene of Chambi,Tunisia

We report the discovery of three isolated primate petrosal fragments from the fossiliferous locality of Chambi (Tunisia), a primate-bearing locality dating from the late early to the early middle Eocene. These fossils display a suite of anatomical characteristics otherwise found only in strepsirhines, and as such might be attributed either to Djebelemur or/and cf. Algeripithecus, the two diminutive stem strepsirhine primates recorded from this locality. Although damaged, the petrosals provide substantial information regarding the ear anatomy of these advanced stem strepsirhines (or pre-tooth-combed primates), notably the patterns of the pathway of the arterial blood supply. Using μCT-scanning techniques and digital segmentation of the structures, we show that the transpromontorial and stapedial branches of the internal carotid artery (ICA) were present (presence of bony tubes), but seemingly too small to supply enough blood to the cranium alone. This suggests that the ICA was not the main cranial blood supply in stem strepsirhines, but that the pharyngeal or vertebral artery primitively ensured a great part of this role instead, an arterial pattern that is reminiscent of modern cheirogaleid, lepilemurid lemuriforms and lorisiforms. This could explain parallel loss of the ICA functionality among these families. Specific measurements made on the cochlea indicate that the small strepsirhine primate(s) from Chambi was (were) highly sensitive to high frequencies and poorly sensitive to low frequencies. Finally, variance from orthogonality of the plane of the semicircular canals (SCs) calculated on one petrosal (CBI-1-569) suggests that Djebelemur or cf. Algeripithecus likely moved (at least its head) in a way similar to that of modern mouse lemurs.     

Cranial Morphology of a Pantolestid Eutherian Mammal from the Eocene Bridger Formation,Wyoming, USA: Implications for Relationships and Habitat

Pantolestinae is a eutherian subfamily of mammals whose members are known from the middle early Paleocene through at least the beginning of the Oligocene of North America. They are also known from Europe, and possibly Africa. A lack of information on pantolestine skulls has prevented the use of cranial anatomy in evaluation of this group’s enigmatic higher-level phylogenetic relationships. Conversely, postcranial skeletons are well known and locomotor interpretations based on them are robust. The most complete known skull of a pantolestine, Pantolestes longicaudus (YPM 13525), is described here and compared to potential close fossil relatives and extant mammals. Semicircular canal morphology is used to test locomotor hypotheses. YPM 13525 lacks an ossified bulla. It has a mediolaterally broad basioccipital, a large entoglenoid process, and a deeply incised glaserian fissure of the squamosal, caudal and rostral tympanic processes on the petrosal, a foramen for an internal carotid artery (ICA) that entered the tympanic cavity from a posteromedial position, bony tubes enclosing the main stem and transpromontorial branch of the ICA, a large anterior carotid foramen formed within the basisphenoid, evidence of a stapedial artery ramus superior, a groove on the dorsal aspect of the basisphenoid leading to the piriform fenestra possibly for drainage of the cavernous sinus to an extracranial inferior petrosal sinus, a dorsum sellae with well-developed posterior clinoid processes, a foramen rotundum within the alisphenoid, and a sphenorbital fissure between the alisphenoid and orbitosphenoid. Overall, the morphology is not strikingly similar to any potential close relative and the phylogenetic position of Pantolestinae cannot be estimated without cladistic analysis of a character matrix that includes this new morphology and broadly samples extant and extinct eutherian taxa. Semicircular canal morphology differs from that of two likely terrestrial Paleocene mammals, Aphronorus (another pantolestid) and Eoryctes (a palaeoryctid), suggesting a different, possibly semi-aquatic, lifestyle for Pantolestes.     

The ontogenesis of the tympanic region of the Procaviidae (Mammalia: Hyracoidea)

Lindahl (1948) has described the early ontogenesis of 14 fetal stages of Procavia capensis. The oldest fetus of his series has had a headlength of 20 mm. It is the series with which the present account on the development of the tympanic region in hyracoids starts. Further middle to late fetuses have been studied by means of serial sections or macroscopic preparations. First 5 different ontogenetic stages are described, and then we follow the development of different organs in their ontogenesis. Results: 1. The development of the tympanic region is almost completed at birth, except for the external meatus. The bulla tympani is fully ossified. 2. The entotympanic forms the medial and rostral part of the Bulla tympani. Its anlage is first met at a headlength of 31 mm. From the very beginning, it is fused with the tubal cartilage. Hyracoids do not have a caudal entotympanic. The cartilaginous mass-taken for a caudal entotympanic by Van der Klaauw (1926)-is actually a tympanic process of the petrosal. Extensive discussion reveals that the structures named entotympanics in some eutherians are most likely not homologous. 3. It is almost impossible to separate the entotympanic from the tympanic in neonates already. This is due to the surprising fact that the tympanic produces secondary cartilage at the suture between the 2 elements. 4. A gonial is lacking in hyracoids; instead the malleus is fixed in a peculiar way at the tympanic. 5. According to Lindahl (1948), the stapedial artery exists till a fetal headlength of 14 mm. Then the artery obliterates but the crura stapedis with the stapedial foramen persist. 6. Hyracoids possess a tympanohyal which changes its relation to the facial nerve during ontogenesis. 7. The development of the Eustachian sac (Diverticulum tubae auditivae) is described and compared with its development in the horse, based on the few data available in literature. 8. The internal carotid artery changes its course during its ontogenesis. Up to late stages it passes over the caudal part of the Eustachian sac and along the medial side of the Bulla tympani. A sulcus caroticus might still be seen in neonates. It is only in postnatal development that it looses the course along the bulla and passes along the mediocaudal side of the Eustachian sac. 9. The formation of the Recessus meatus acustici externi in exactly takes place the way described only once by Hammar (1902). The cavitation of the recessus occurs independently of the cavity of the external meatus.     

The Ear Region of the Pen-tailed Treeshrew, Ptilocercus lowii Gray, 1848 (Placentalia, Scandentia, Ptilocercidae)

The ear region of the pen-tailed treeshrew, Ptilocercus lowii Gray, 1848 (Scandentia, Ptilocercidae), is described and illustrated in detail based on five museum specimens from the National Museum of Natural History, two with the auditory bulla removed exposing the intratympanic surfaces. Soft tissues (arteries, veins, nerves, and muscles) are reconstructed onto the adult skulls based on published reports of these elements in a fetal P. lowii. Comparisons are made with four specimens of the common treeshrew, the tupaiid Tupaia glis (Diard, 1820), from the Carnegie Museum of Natural History, including one with the auditory bulla removed. The mammalian ear region widely is regarded to be a rich source of characters for phylogenetic analysis. This study supports this view by identifying numerous features that are shared between the two treeshrews as well as numerous features that distinguish them. Several features used in the past to distinguish tupaiid treeshrews from primates are found to differ between P. lowii and T. glis: the composition of the bony tubes for the internal carotid artery and the composition of the intrabullar septa and spaces. Despite the compositional differences, it seems likely that the bony carotid tubes and intrabullar septa and spaces shared by P. lowii and T. glis occurred in their common ancestor. Evaluating the utility of these and other ear region features awaits future phylogenetic analysis of treeshrews and related Euarchontoglires.     

Petrosal and inner ear anatomy and allometry amongst specimens referred to Litopterna (Placentalia)

New isolated petrosals from the Itaboraí beds of Brazil (late Palaeocene or early Eocene) are here described and referred to the early diverging litoptern Miguelsoria parayirunhor, based on phylogenetic, size, and abundance arguments. Both the external and internal anatomy of these specimens were investigated, which for the first time document many details of the auditory region of a Palaeogene litoptern. Our cladistic analysis, which included our new observations, failed to recover a monophyletic Litopterna but did not exclude it. A constrained analysis for the monophyly of this order showed that several features such as a (sub)quadrangular and anteroposteriorly elongated tensor tympani fossa and a large notch in the vicinity of the external opening of the cochlear canaliculus may constitute synapomorphies for Litopterna. The evolution of several other auditory characters amongst Litopterna is discussed and the relative dimensions of the inner ear and surrounding petrosal in the group were also investigated. This allowed detection of negative allometry of the bony labyrinth within the petrosal, which was confirmed by measurements and regression analysis across a larger sample of placental mammals. This scaling effect probably has an important influence on several characters of the bony labyrinth and petrosal, amongst which are the length of the vestibular aqueduct and cochlear canaliculus. It demonstrates that many aspects of the morphological variation of the bony labyrinth need to be thoroughly investigated before being incorporated into phylogenetic analyses. © 2015 The Linnean Society of London     

Cranial morphology of Aegyptopithecus and Tarsius and the question of the tarsier-anthropoidean clade

New crania of the Oligocene anthropoidean Aegyptopithecus provide a test of the hypothesized tarsier-anthropoidean clade. Three cranial characters shared by Tarsius and some modern anthropoideans (apical interorbital septum, postorbital septum, "perbullar" carotid pathway) were examined. 1) An apical interorbital septum is absent in Aegyptopithecus. A septum does occur in Galago senegalensis (Lorisidae) and Microcebus murinus (Cheirogaleidae), so the presence of a septum is not strong evidence favoring a tarsiiform-anthropoidean clade. 2) In Aegyptopithecus and other anthropoideans, the postorbital septum is formed mainly by a periorbital flange of the zygomatic that extends medially from the lateral orbital margin onto or near the braincase. The postorbital plate of Tarsius is formed by frontal and alisphenoid flanges that extend laterally from the braincase to the zygomatic's frontal process, which is not broader than the postorbital bars of other prosimians. Periorbital flanges evolved in Tarsius for support or protection of the enormous eyes, as suggested by the occurrence of maxillary and frontal flanges that cup portions of the eye but do not separate it from temporal muscles. 3) The internal carotid artery of Aegyptopithecus enters the bulla posteriorly and crosses the anteroventral part of the promontorium. The tympanic cavity was probably separated from the anteromedial cavity by a septum stretching from the carotid channel to the ventrolateral bullar wall. In Tarsius, the carotid pathway is prepromontorial, and a septum stretches from the carotid channel to the posteromedial bullar wall. Quantitative analyses indicate that anterior carotid position has evolved because of erect head posture. The cranium of Oligocene anthropoideans thus provides no support for the hypothesized tarsier-anthropoidean clade.     

Cranial anatomy of Ignacius graybullianus and the affinities of the Plesiadapiformes

A nearly complete cranium of Ignacius graybullianus provides increased understanding of the cranial anatomy of Plesiadapiformes. In nearly all details of cranial anatomy, Ignacius differs markedly from primates. USNM 421608 exhibits a long tapering snout, small widely spaced orbits, and a complete lack of postorbital process or bar. Large olfactory bulbs are inferred from the wide interorbital space. The marked flare of the zygomatic arches suggests that Ignacius possessed large and powerful temporal muscles. The basicranial region is particularly well preserved and reveals a distinct suture between the petrosal bone and an entotympanic bulla. This suture is visible on both the left and right sides of the skull and dispels the hypothesis that Ignacius and, by inference, other Plesiadapiformes share the primate synapomorphy of a petrosal bulla. To test the phylogenetic position of Ignacius, cranial characters were identified and scored for Ignacius, Plesiadapis, Cynocephalus, and a number of primates, bats, and scandentians. Two erinaceomorph insectivores were also included to allow the assessment of archontan monophyly. These characters were incorporated into a maximum-parsimony analysis to determine the phylogenetic position of Plesiadapiformes. There are several important phylogenetic conclusions that can be inferred from this analysis: 1) Ignacius and Plesiadapis make up a monophyletic clade; 2) Plesiadapiformes may be the sister group of Dermoptera; 3) Scandentia, not Plesiadapiformes, is the sister group of Primates; and 4) Primates, plesiadapiforms, bats, colugos, and scandentians may not form a monophyletic clade Archonta. Consequently, the taxon Archonta is in need of review. © 1992 Wiley-Liss, Inc.     

The anatomy and phylogenetic significance of the carotid arteries and nerves in strepsirhine primates

The internal carotid artery in lorisiform primates was identified on the basis of its close association with the internal carotid nerve (postsynaptic sympathetic trunk). An atrophic vessel in the lorisine Arctocebus resembles the internal carotid artery of Lemur in that it enters the tympanic bulla and promontory tube closely associated with the internal carotid nerve. According to this criterion of homology, the enigmatic lorisiform extrabullar carotid artery is not homologous with the internal carotid (promontory) artery of lemuriform primates or the medial internal carotid artery of the dog. Hypotheses about strepsirhine evolution must take these morphological facts into account.     

Position and bony structures of the vestibular apparatus in the guinea pig]

The study of the inner ear of the guinea pig intended to give an explanation to what extent there are differences discernible in relation to the human labyrinth. Additional histological research should clarify the question, if structural differences exist in the osseous labyrinth capsule of the same animal. It has turned out that in normal headbearing the position of the semicircular canals deviates from the human vestibular apparatus. The semicircular canals are nearly vertical to each other, but in comparison to the human labyrinth they are shifted around the longitudinal axis of the utriculus caudal by ca. 30 degrees. In general the position of the vestibulo-cochlear organ is fixed to a great extent by the inclined course of the petrosal pyramid. This different position of the semicircular canals in man and animal is supposed to be due to the phylogenetic evolution and the adjustment to upright walk. Size and extension of the single semicircular canals are very different within the same animal. These differences in size indicate causalities of form and function. The relations in the build of the osseous labyrinth are extremely complicated. Compared to the other corporal regions the static parts of the petrosal pyramid are exceptional massive and of remarkable hard consistency. In the inner capsule of the ear there are three different bone strata to be seen. The characteristic lamel structure is most solid nearest to the semicircular canals. The fetal characteristics in the maturing process of the petrosal bone were traced a long while in the postnatal life. The typical building of the labyrinthal bone structures contributes to the mechanical stability of the capsule.     

Age estimation in bowhead whales using tympanic bulla histology and baleen isotopes

Tympanic bullae and baleen plates from bowhead whales of the Western Arctic population were examined. Growth layer groups (GLGs) in the involucrum of the tympanic bone were used to estimate age of the whales, and compared to stable isotope signatures along transects of baleen plates and the involucrum. The involucrum of the tympanic bone consists of three regions that form in utero, during nursing in the first year, and during the first decades of life, respectively. Life history events, such as annual migration, are recorded in the bowhead tympanic bulla. It is likely that bone growth in the bowhead tympanic occurs during periods of high food intake, while slow or arrested growth occurs during periods of low food intake. Comparisons between numbers of GLGs in the tympanic, number of isotopic oscillations in a baleen plate, length of the baleen plate, and total whale length show correlation coefficients as high as 0.97. The tympanic GLG method is particularly useful for estimating the age of whales up to 20 yr old.