Embryological data and its phylogenetic implications

It is now more than a century since embryological studies [the Reichert-Gaupp theory, refuted by Otto (1984)] demonstrated the homology of the mammalian malleus and incus with the articular and quadrate bones which formed the ancestral jaw joint of gnathostomes. This tremendous transition can be traced in fossils by comparing basal synapsids through therapsids to early mammals. A fetal mammal shows that the angular (=tympanic), articular (=malleus), and quadrate (=incus) develop in the same positions they occupy in the cynodont skull (Allin 1975; Maier 1987). The importance of new fossils (Rich et al. 2005), however, is demonstrated by Martin and Luo (2005): the separation of the middle ear bones must have evolved independently among the therians and the mono-treme mammals. The middle ear and surrounding basicranium have played a decisive part in mammalian systematics over the past years (e.g. Sanchez-Villagra et al. 2002).

In 1979—2 years before Auditory Regions of Primates and Eutherian Insectivores was published—R.D.E. MacPhee presented a new interpretation of the disputable issue of "primate entotympanics.'' Not all mammals show these mysterious skeletal elements. As otic characters are essential for primate taxonomy, this was a potential criterion for distinguishing primates from their relatives [e.g., colugos (O Figure 3.1) and tree shrews]. Mivart (1881) defined this specific element as "entotympanic," while Wincza (1896) introduced the term "meta-tympanic'' (van Kampen 1905). A few influential morphologists (van Kampen

O Figure 3.1

Top: Fetal Cynocephalus volans (Dermoptera). Cranial length 41 mm, frontal section. Note the entotympanics. c.E = caudal entotympanic, C.t. = cavum tympani, F.r. = Fenestra rotunda, M.m. = Manubrium mallei, P = petrosal, R.m.a. = Recessus meatus acustici externi, Tu.K. = Tuba auditiva-cartilage, Ty = tympanic, x,y = cartilage. Sections have a thickness of 25 mm. Not to scale. (Modified from van der Klaauw 1922). Center: Late-fetal Cynocephalus variegatus (DUCEC 806). Crown-rump length 136 mm. It shows a frontal section through the promontorium in front of the tegmen tympani. Arrowheads point to branches of the internal carotid nerve running in a transpromontorial position. Scale bar = 0.33 mm. "Cat" = fused cartilage of the auditory tube and rostral and caudal entotympanics, bo = basioccipital, co = cochlea, ct = cavum tympani, e = ectotympanic, eam = external acoustic meatus, pe = petrosal, pr = promontorium of petrosal, t = tympanic membrane. (Modified from Wible and Martin 1993, © Plenum Press). Bottom: The next step. Frontal section of a juvenile Cynocephalus volans-specimen, of 145 mm crown-rump length, from the collection of the Institute of Systematic Zoology in Tübingen. It illustrates the ossification of the fused cartilage of the auditory tube and the rostral and caudal entotympanic (o"cat"). Hunt and Korth (1980) reported but they did not illustrate the fate of this important anatomical detail. Sections have a thickness of 40 mm. Not to scale

1905; van der Klaauw 1931) already championed the existence of primate entotympanics.

Van der Klaauw (1922) published an ingenious work (Uber die Entwickelung des Entotympanicums) in which he made a strong case for two different types of entotympanic, rostral and caudal. Developing in the anteromedial corner of the membranous tympanic floor, the rostral entotympanic grows posteriorly. Unlike the tympanic processes, entotympanics grow and develop within the fibrous membrane of the tympanic cavity (MacPhee 1979).

Starck (1975 p 143) reported for Tarsius that the existence of an entotym-panic is not yet proven. In the younger stage, the tympanic region does not show any peculiarities and an entotympanic cannot be detected. However, an older specimen showed a floor of the middle-ear cavity that is formed by endochondral bone. It was closely connected with the otic capsule, but it showed a rostral extension near the auditory tube: "The ossified bone still contains some cartilaginous tissue, and this would indicate the presence of a true entotympanic, since the petrosal components of the floor are usually formed by periosteal bone. These results are in accordance with the findings of van Kampen (1905).'' (My emphasis.) According to MacPhee (1979), primate tympanic floor elements do not develop like any known entotympanic but, rather, like other tympanic processes. Prenatal Microcebus demonstrates secondary cartilage in the rostral tympanic process of the petrosal, which is interpreted as an embryonic adaptation—the explanation of the primordial fusion of once separate entities is no longer required. The hypothesis of a suppressed entotympanic is made more unlikely by the endochondral development of the caudal tympanic process of the petrosal (contra Starck 1975). To quote MacPhee (1979 p 43): "Therefore, with regard to the constitution of the primate ventral wall, and with some necessary violence to Newton's quip, non fingo ossa - I posit no bones.'' Eureka, a new primate characteristic was identified via the careful interpretation of ontogenetic data.

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