Mystacinid bats (Microchiroptera) from the Australian Tertiary

Journal of Paleontology, May 1998 by Hand, S J, Murray, P, Megirian, D, Archer, M, Godthelp, H

Icarops species compare with Australia's nine living molossid species and two fossil taxa as follows. Petramops creaseri, from the middle Miocene of Riversleigh (Hand, 1990), differs from Icarops spp. in its unfused dentary symphysis, two pairs of lower incisors, nyctalodont molars with anteroposteriorly compressed trigonids, and two-rooted lower premolars both obliquely oriented in the toothrow. Living species of Micronomus and Miocene Hydromops lack the fused symphysis, broad ventral mandibular shelf, have at least two pairs of lower incisors, and P4 with roots transversely oriented. Living species of Tadarida and Chaerephon differ in their long, shallow, unfused dentaries with low ascending rami, nyctalodont lower molars, and transversely to slightly obliquely oriented lower premolars.

Other vespertilionoids (s.l.), including vespertilionids, natalids, furipterids, thyropterids, myzopodids, and the extinct philisids, lack a fused dentary symphysis, retain two or more pairs of lower incisors, and tend to have lower molars with talonid wider than trigonid. Most groups are dominated by taxa with nyctalodont lower molars (generally interpreted to be the plesiomorphic condition in bats; e.g., Sige, 1985, p. 181) but philisids, some natalids (i.e., Kerivoula and Phoniscus spp.), and vespertilionids (e.g., Myotis, Plecotus, Scoteanax, Chalinolobus) have myotodont molars like Icarops species. Few, if any, species in this very large bat group exhibit the short, robust dentary and crowded anterior toothrow seen in Icarops species.

Icarops species share with South America's noctilionids (i.e., species of Noctilio) a deep and robust dentary with tall ascending ramus and fused dentary symphysis, a single pair of lower incisors, two anterior premolars, and myotodont lower molars. However, noctilonids differ strikingly from Icarops (and Mystacina) species in having M2 with extremely wide talonid, walllike pre-entocristid and cristid obliqua meeting the metaconid, obliquely oriented P4, and small, single-rooted, lingually-displaced P2. In Noctilio species, cheek pouches are formed from posterolateral extensions of the buccinator muscle, the resulting sacculation lying over the buccal side of the mandible and occupying the posterior one-third of the horizontal ramus (Murray and Strickler, 1975). In N. leporinus, these pouches serve as temporary storage reservoirs for fish plucked from the water by the bat's long talons. In Icarops breviceps, the subalveolar crest is strong (but not nearly as well-developed as in Noctilio species) and there is a boss not far behind the mental foramen that corresponds to the fibrous origins of the lower part of the buccinator in N. leporinus. However, without a maxilla of L breviceps to examine it is impossible to gauge the extent and development of the buccinator, as an indicator of the presence of cheek pouches, and the possible fishing habits of this species.

DISCUSSION

The three new mystacinids described here are the first bats identified from among numerous vertebrates recovered from Bullock Creek (Murray and Megirian, 1992), and two of more than 35 Tertiary bats recovered from Riversleigh (Archer et al., 1994). This brings to six the number of Australian fossil localities that have now produced bats of Tertiary age. The oldest of these is an early Eocene archaeonycteridid from lacustrine sediments near Murgon, southeastern Queensland (Hand et al., 1994), and the youngest an early-middle Pliocene megadermatid from limestone breccias at Wellington Caves, New South Wales (Hand et al., 1988). Hipposiderids and megadermatids dominate the Riversleigh freshwater limestone deposits, and hence represent more than half of Australia's Tertiary bats, with emballonurids, molossids, vespertilionids, and a possible rhinolophid comprising the rest of Australia's Tertiary bats. Australian Pliocene bats are generally closely related to, or represent early populations of, extant Australian bats (e.g., Taphozous spp., Macroderma gigas). Oligo-Miocene bats include ancestors or close relatives of extant Australian taxa (e.g., Macroderma and Brachipposideros spp.), relatives of extinct and extant non-Australian taxa (e.g., Xenorhinos and Riversleigha spp.), and taxa representing archaic cosmopolitan groups with no living descendants (e.g., Petramops and Hydromops spp.). The Australian Miocene mystacinids described here provide the only Tertiary record of a very distinctive southern bat lineage of which a single, threatened species survives in New Zealand.