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The anatomy of the petrosal and associated middle ear structures are described and illustrated for the brown rat, Rattus norvegicus (Berkenhout, 1769). Although the middle ear in this iconic mammal has been treated by prior authors, there has not been a comprehensive, well-illustrated contribution using current anatomical terminology. Descriptions are based on specimens from the osteological collections of the Section of Mammals, Carnegie Museum of Natural History, and a CT scanned osteological specimen from the Texas Memorial Museum. The petrosal, ectotympanic, malleus, incus, stapes, and inner ear were segmented from the CT scans. The petrosal of the brown rat is only loosely attached to the cranium, primarily along its posterior border; it is separated from the basisphenoid, alisphenoid, and squamosal by a large piriform fenestra that transmits various neurovascular structures including the postglenoid vein. The extent of the piriform fenestra broadly exposes the tegmen tympani of the petrosal in lateral view. The floor of the middle ear is formed by the expanded ectotympanic bulla, which is tightly held to the petrosal with five points of contact. The surfaces of the petrosal affording contact with the ectotympanic bulla are the rostral tympanic process, the epitympanic wing, the tegmen tympani, two of the three parts of the caudal tympanic process, and the tympanohyal, with the ectotympanic fused to the last. The ectotympanic in turn is fused to the elongate rostral process of the malleus, which is only discoverable through the study of juvenile specimens. In addition to osteology, the major nerves, arteries, and veins of the petrosal are described and illustrated based on the literature and osteological correlates. The petrosal of the brown rat is compared with those of several Eocene rodents to put the extant form in the context of early members of the rodent lineage. Comparisons benefitted from CT scans of the middle Eocene ischromyoid Paramys delicatus Leidy, 1871, from the western United States, affording the first description of the endocranial surface of the petrosal in an Eocene rodent. The petrosals in the Eocene fossils are more tightly held in the cranium, but the ectotympanic contacts the petrosal through the same five points, with some modifications. The most unexpected discovery in Paramys delicatus was the presence of a prominent tentorial process of the parietal in contact with the reduced crista petrosa.
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The element of Paaw in marsupials and the middle ear of Philander opossum (Linnaeus, 1758) (Didelphimorphia, Didelphidae)
A small piece of cartilage or bone, the element of Paaw, occurs in the tendon of the stapedius muscle in some extant marsupial and placental mammals. It has been nearly a century since the last comprehensive treatment of the distribution of the element of Paaw in mammals. The current report updates knowledge on this structure by synthesizing the subsequent literature and providing new observations of extant marsupials from the collections of the Section of Mammals, Carnegie Museum of Natural History, and two online resources for CT scanned data: DigiMorph.org and MorphoSource.org. We found an element of Paaw in some representatives of all seven extant marsupial orders: Didelphimorphia, Microbiotheria, Notoryctemorphia, Peramelemorphia, Paucituberculata, Dasyuromorphia, and Diprotodontia. In the first four orders, the element is substantial, longer than the long axis of the fenestra vestiuli (oval window), which holds the stapedial footplate; it is smaller than the long axis of the fenestra vestibuli in Paucituberculata and we do not have measures to report for the last two orders. In most marsupials examined, the element of Paaw contacts the petrosal behind the oval window, suggesting it functions as a sesamoid bone, increasing the lever arm of the stapedius muscle. Although there is some variability in the presence of the bone both between and within individual museum specimens, we interpret this as the result of preparation techniques rather than true variation. To place the element of Paaw in its anatomical context, we describe in detail the ear region and middle-ear auditory apparatus of the gray four-eyed opossum, Philander opossum (Linnaeus, 1758), a didelphid from Central and South America, based on a CT scanned specimen from Carnegie Museum of Natural History. It has an ossified element of Paaw with a volume greater than the stapes. Comparisons are made with petrosals of Didelphis marsupialis Linnaeus, 1758, and Monodelphis domestica (Wagner, 1842), also based on CT scanned specimens.
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- Award ID(s):
- 1654949
- PAR ID:
- 10248814
- Date Published:
- Journal Name:
- Annals of Carnegie Museum
- Volume:
- 87
- Issue:
- 1
- ISSN:
- 1943-6300
- Page Range / eLocation ID:
- 1-35
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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