Basal Laurasiatherians Insectivores And Bats

The Laurasiatheria, one of the two clades within Boreoeutheria, includes insectivores and bats as the basal members to a clade Ferungulata, which includes cetartiodactyls, perissodactyls, carnivores and pholi-dotans (see Box 10.6). The group radiated presumably during the Cretaceous, and before 90 Myr ago, at least to judge from the oldest fossils, the zhelestids from Uzbekistan (see p. 312). Insectivores and bats of such great antiquity are so far unknown.

10.8.1 Lipotyphla: hedgehogs, moles and shrews

Insectivores (Stephan et al., 1991; Nikaido et al., 2003; Douady et al., 2004) have often been said to be the 'most primitive' living placental group. Certainly, living shrews and hedgehogs lack many specialized adaptations and they are ecologically close to some ofthe earli est mammals, but there is no reason to regard them as any more primitive than the primates or the carnivores. The term 'Insectivora' was used, but it included tenrecs and golden moles, now a part of Afrotheria (see p. 323), so making it polyphyletic, and the alternative term Lipotyphla is generally used.

The shrews (soricomorphs) arose in the Mid-Palaeocene. Late Cretaceous records of insectivores are debated (Archibald, 2003). The palate of the Oligocene shrew Domnina (Figure 10.30(a)) shows the W-shaped pattern of ridges on the upper molar teeth that is typical of the group. The moles, closely related to the shrews, arose in the Eocene. The forelimbs, which are used in burrowing, are broad and paddle-like, and the mole humerus (Figure 10.30(b)) is a very characteristic broad bone with large processes for the attachment of powerful muscles.

The hedgehogs (erinaceomorphs) arose in the Eocene. The most spectacular hedgehog was Deinoga-lerix,a long-limbed dog-sized animal (Figure 10.30(c)) from the late Miocene, which was probably covered

Fig. 10.30 Basal laurasiatherians: insectivores (a-d) and bats (e): (a) palate of Domnina, an Oligocene shrew; (b) the broad humerus of the living mole Cryptoproctes; (c) the giant Miocene hedgehog Deinogalerix drawn in proportion to the living Erinaceus; (d) skull of Erinaceus; (e) the Eocene bat Icaronycteris. [Figure (a) modified from McDowell, 1958; (b, d) redrawn from various sources; (c) after Butler, 1981; (e) after Jepsen, 1970.]

Fig. 10.30 Basal laurasiatherians: insectivores (a-d) and bats (e): (a) palate of Domnina, an Oligocene shrew; (b) the broad humerus of the living mole Cryptoproctes; (c) the giant Miocene hedgehog Deinogalerix drawn in proportion to the living Erinaceus; (d) skull of Erinaceus; (e) the Eocene bat Icaronycteris. [Figure (a) modified from McDowell, 1958; (b, d) redrawn from various sources; (c) after Butler, 1981; (e) after Jepsen, 1970.]

with stiff hair rather than spines (modified hairs). Deinogalerix was five times as long as the European hedgehog Erinaceus and it must have been a dramatic sight as it charged about the hot grasslands of southern Italy. The skull of Erinaceus (Figure 10.30(d)) shows some derived characters of the Insectivora (Butler, 1988), such as the loss of the jugal and the absence of a postorbital process (present in most placentals).

10.8.3 Chiroptera: bats

The bats include about 1000 species today and the reason for their success is their advanced flying capabilities that make them effectively 'birds of the night' (Jepsen, 1970; Altringham, 1996). There are two groups of bats, the megachiropterans or fruit bats and the more abundant microchiropterans, the small insect-eaters.

Bat remains have been found in the latest Palaeocene, but the oldest well-known form is the early Eocene Icaronycteris (Figure 10.30(e)). Already all the key microchiropteran features are there: the humerus, radius (and fused ulna) and digits are all elongated, and the flight membrane is supported by the spread fingers 2-5 (digit 1, the thumb, is much shorter). The shoulder girdle is modified to take the large flight muscles on the expanded scapula on the back and the broad ribs and sternum on the front. The hindlimbs are strong, and the feet are turned backwards so that Icaronycteris could hang upside down as modern bats do. The eyes are large and the ear region shows specializations for echolocation. The modern bat groups arose mainly in the late Eocene and Oligocene, but remains are often scrappy.

Only very rare conditions of preservation can preserve the dramatic detail seen in Icaronycteris. It was found in the Green River Formation of Wyoming, a deposit better known for its extensive fish faunas (see p. 183). Other excellent specimens of bats, complete with skin impressions, have been found in the renowned oil shale deposits of Messel in Germany (see Box 10.8).

The phylogeny of bats has been disputed. For years, most people had assumed that the fruit bats and microchiropterans formed a single clade. Pettigrew (1991) suggested, however, that the fruit bats are more closely related to dermopterans and primates than to microbats, and that the flying adaptations of the two 'bat' groups had arisen independently. The case was disputed vigorously by Baker et al. (1991), who listed 27 synapomorphies of the head, postcranial musculature, nervous system and placenta found only in fruit bats and microbats. Further morphological (Simmons and Geisler, 1998) and molecular (Madsen et al., 2001; Murphy et al., 2001; Nikaido et al., 2003; Springer et al., 2003) studies have strongly confirmed the monophyly of Chiroptera.

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