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J G Thewissen

Publications and source records attributed to J G Thewissen.

12 recordsLinked to original sources
PubMed2001

Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls.

Modern members of the mammalian order Cetacea (whales, dolphins and porpoises) are obligate aquatic swimmers that are highly distinctive in morphology, lacking hair and hind limbs, and having flippers, flukes, and a streamlined body. Eocene fossils document much of cetaceans' land-to-water transition, but, until now, the most primitive representative for which a skeleton was known was clearly amphibious and lived in coastal environments. Here we report on the skeletons of two early Eocene pakicetid cetaceans, the fox-sized Ichthyolestes pinfoldi, and the wolf-sized Pakicetus attocki. Their skeletons also elucidate the relationships of cetaceans to other mammals. Morphological cladistic analyses have shown cetaceans to be most closely related to one or more mesonychians, a group of extinct, archaic ungulates, but molecular analyses have indicated that they are the sister group to hippopotamids. Our cladistic analysis indicates that cetaceans are more closely related to artiodactyls than to any mesonychian. Cetaceans are not the sister group to (any) mesonychians, nor to hippopotamids. Our analysis stops short of identifying any particular artiodactyl family as the cetacean sister group and supports monophyly of artiodactyls.

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PubMed2001

Rotation of middle ear ossicles during cetacean development.

Cetacean middle ears are unique among mammals in that they have an elongated tympanic membrane, a greatly reduced manubrium mallei, and an incudal crus longum that is shorter than the crus breve. Elongation of the tympanic membrane and reduction of the manubrium is thought to be related to an evolutionary rotation of the incus and malleus out of the plane of the tympanic membrane. We examined if rotation also occurs during ontogeny by comparing the middle ears of two species of dolphins (Delphinus delphis, Stenella attenuata) at different stages of development. We observed that: the incus has the body and crural proportions as in terrestrial mammals early in development; the incudomallear complex rotates approximately 90 degrees following ossification; the tympanic membrane is not elongated until relatively late in development. Therefore, some of the unique characteristics of the cetacean middle ear develop as modifications of an initially terrestrial-like morphology.

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PubMed1999

Ankle morphology of the earliest Cetaceans and its implications for the phylogenetic relations among ungulates.

Recent molecular studies are inconsistent with ungulate phylogenetic trees that are based on morphological traits. These inconsistencies especially relate to the position of cetaceans and perissodactyls. Evaluation of the close phylogenetic ties between artiodactyls and cetaceans has been hampered by the absence of tarsal bones of primitive cetaceans, as artiodactyls are often diagnosed on the basis of their tarsus. We here describe newly discovered tarsal bones that are the oldest cetacean tarsals known. We present a character analysis for primitive ungulate tarsals and evaluate their impact on the ungulate phylogenetic tree. Tarsal data are consistent with some molecular studies in suggesting that the extant sister group of Cetacea is Artiodactyla or that Cetacea should be included within the latter order. Tarsal data do not support Cete (Mesonychia plus Cetacea) and are consistent with the exclusion of perissodactyls from paenungulates as suggested by some molecular studies.

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PubMed1998

Age-related size reduction of foramina in the cribriform plate.

Anecdotal evidence suggests that the foramina of the cribriform plate which transmit cranial nerve I decrease in size with age, but this finding has never been supported with quantitative data. It has also been observed that olfactory function declines with increasing age. It has been hypothesized that the cribriform plate foramina closure may be responsible for the olfactory performance decrease with age. We gathered quantitative data to test an age-related decline in cribriform plate foramina area. We report data for the area of patent foramina in the posterior 1 cm of 57 cribriform plates from 40 skulls of known age and sex. Analyses were performed to test for the effects of age, sex, and lateralization on foramina area. The area of patent foramina in the cribriform plate decreases with increasing age. Age is a strong covariate with foramina area (P value = 0.0025). The regression equation for the area of patent foramina is: expected area = 8.17 - (0.06) age. Adding the variable sex does not contribute significantly (P value > 0.28) to the model which utilizes age alone. Nor was there any significant lateralization in patent foramina area. The area of patent foramina in the cribriform plate decreases with increasing age, and there is no significant difference between males and females or left and right sides. Such decreases in patent foramina may be associated with impaired olfactory function in the aged.

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PubMed1993

Origin of underwater hearing in whales.

All described fossil and Recent cetaceans have relatively similar ear bones (malleus, incus and stapes) that strongly diverge from those of land mammals. Here we report that the hearing organ of the oldest whale, Pakicetus, is the only known intermediate between that of land mammals and aquatic cetaceans (whales, dolphins and porpoises). The incus of Pakicetus is intermediate with respect to inflation, crural proportions, and position of the mallear joint. The incus and mandible of Pakicetus indicate that the path of soundwaves to its ear resembled that of land mammals. These fossils suggest that the first whale was amphibious, and corroborate the hypothesis that artiodactyls (for example, pigs, camels and ruminants) are the closest extant relatives of cetaceans.

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PubMed1991

Distinctive cranial and cervical innervation of wing muscles: new evidence for bat monophyly.

The traditional view that Old World fruit bats (Megachiroptera) and insect bats (Microchiroptera) are closely related has been challenged by claims that Megachiroptera are the sister group to flying lemurs (Dermoptera) or Primates. We found that the specialized muscles of the rostral part of the wing in Microchiroptera and Megachiroptera receive double innervation by both the facial nerve and cervical spinal nerves, suggesting that bats are monophyletic. Innervation by the facial nerve also occurs in Dermoptera and suggests that bats and Dermoptera share a common ancestor that had wings.

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PubMed1990

Postcranial osteology of the most primitive artiodactyl Diacodexis pakistanensis (Dichobunidae).

Diacodexis pakistanensis is the smallest and most primitive artiodactyl. It is known from the early Eocene of the Indo-Pakistani subcontinent. It retains a clavicle, has five complete digits in the manus and four in the pes and is digitigrade. The elongated limbs, reduced ability to pronate and supinate, double pulleyed astragalus and reduced lateral digits indicate that D. pakistanensis is cursorial. However, the shape of the spinous processes of the sacrum, the size of the deltoid tubercle of the humerus, the shape of the proximal femur, the narrow distal trochlea of the astragalus and proportions of the long bones indicate that it is less cursorial than any other dichobunid artiodactyl.

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PubMed1989

Mammalian frontal diploic vein and the human foramen caecum.

The course of the frontal diploic vein in the mole Talpa and the tree shrew Tupaia is described and compared to the frontal diploic vein of other mammals. The frontal diploic vein in Talpa and Tupaia connects the dorsal sagittal sinus to the veins of the orbit and has an emissary function. In certain other mammals it has a diploic function and may drain towards the orbit (e.g., Orycteropus) or towards the dorsal sagittal sinus (e.g., Didelphis). The frontal diploic vein of these mammals is not homologous to the vein of the human foramen caecum, but to the human frontal diploic vein. The vein of the foramen caecum is a problematic structure: its incidence in embryos and children is not clear.

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PubMed1986

The descriptive and functional myology of the fore-limb of the Aardvark (Orycteropus afer, Pallas 1766).

The myology of the fore-limb of the Aardvark, Orycteropus afer (Pallas 1766) (Tubulidentata, Mammalia) is redescribed on the basis of the dissection of two specimens. The results were complemented with data derived from X-ray pictures and a motion picture. Some remarks are made on the digging techniques in the Aardvark and other fossorial mammals. In Orycteropus, the power stroke comprises synchronous bending of shoulder-, elbow and metacarpophalangeal joints. The most important muscles involved are the Mm. cutaneus trunci, latissimus dorsi, pectoralis profundus, triceps brachii, flexor digitorum superficialis and -profundus.

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