Bats are one of the most successful Mammal groups, with roughly 25% of all known living Mammal species being Bats (or about 50% of all living non-Rodent Mammal species). They have a fossil record dating back to the Paleocene, but their fossil record is not well studied as, like Birds, Bats have extremely light and fragile skeletons, with the effect that most fossil Bats are known from extremely limited and fragmentary material.
In a paper published in the American Museum Novitates on 16 December 2015, Gregg Gunnell of the Division of Fossil Primates at the Duke Lemur Center and the Department of Mammalogy at the American Museum of Natural History, the late Percy Buttler, formerly of the Royal Holloway University of London, the late Marjorie Greenwood, formerly of the Natural History Museum and Nancy Simmons, also of the Department of Mammalogy at the American Museum of Natural History describe a series of fossil Bats from the Early Pleistocene of Olduvai Gorge of Arusha Province in Tanzania. The bones were collected in the late 1950s and early 1960s from sediment sieved during the excavations that produced the first specimens of Zinjanthropus boisei. These deposits are thought to be between 1.80 and 1.85 million years old. Four new fossil Bat species are described from the remains.
The first new species described is placed in the genus Myzopoda and given the specific name africana, meaning 'from Africa'; there are two living species of Myzopoda today, both of which are found in Madagascar only. Older members of the family Myzopodidae (of which Myzopoda is the only living species) have previously been described from Africa, but this is the first species of Myzopoda. The species is described froma single whole left humerus, longer and more robust than those of either living species at 26.5 mm, plus two partial humeri.
(A, C) Left humerus of Myzopoda africana, compared with (B, D) the living Madagascan species Myzopoda schliemanni, in anterior (A–B) and posterior (C–D) views. Gunnell et al (2015).
The second species described is placed in the genus Cardioderma and given the specific name leakeyi in honour of palaentologist Louis Leaky, for his work in East Africa. The genus Cardioderma contains a single living species, Cardioderma cor, the African Heart-nosed Bat, which is found across East Africa including Tanzania today. The species is described from a left maxilla and right dentary, both with teeth in place. The teeth of Cardioderma leakeyi are larger and more robust than those of Cardioderma cor. Fossils referred to the genus Cardioderma have previously been recorded from the Pliocene of Ethiopoa, though this material has never been formally described.
Upper dentition of Cardioderma cor (A) compared with Cardioderma leakeyi (B–D). (A) palate (photograph). Left maxilla with P4–M3 in (B, C) lateral (drawing and photograph of cast, respectively) and (D) occlusal (drawing) views. Gunnell et al (2015).
The third species described is placed in the genus Scotoecus (Lesser House Bats) and given the specific name olduvensis, meaning 'from Olduvai'. Five species of Scotoecus are alive today, four of them widespread in Africa, the other from South Asia. Scotoecus olduvensis is described from a left dentary with intact teeth and two fragments of humerus. This species also appears to have been larger and more robust than living species in the same genus. It is the first fossil species of Scotoecus described.
Left lower dentitions of Scotoecus in occlusal view. (A) Scotoecus albofuscus; (B) Scotoecus olduvensis; (C) Scotoecus hindei. Gunnell et al (2015).
The fourth specimen is a partial right humerus tentatively referred to the living species Eptesicus isabellinus, which is found today in North Africa and the Canary Islands and possibly Iberia, but which is considered by some to be a subspecies of Eptesicus bottae, a species found from North Africa across the Middle East and into Central Asia. Modern members of the genus are found across Africa, Europe, Asia, North, Central and South America and the Caribbean, with fossil specimens previously recorded from the Pliocene of Southern Africa and the Plio-Pliocen of the Northern Hemisphere.
Right distal humerus of cf. Eptesicus isabellinus in (A) anterior, (B) posterior, (C) medial, and (D) lateral views. Gunnell et al (2015).
The next specimen described is a fragment of left humerus assigned to the genus Myotis (Mouse-eared Bats). Modern members of this genus are found across Africa, Eurasia and the Americas, and is common in the fossil record of much of this area from the Pliocene onwards, though African fossil specimens have previously only been recorded in Southern Africa and Morocco.
Two more partial humeri are referred to the genus Pipistrellus, being similar to the modern species Pipistrellus nanulus and Pipistrellus rueppelli, both of which are found in Africa. Pipistrelle Bats are found across much of the globe today, and fossil specimens are known from a number of areas; they have previously been described from Olduvai, but not yet in the fossil record anywhere else in Africa.
The forth species described is placed in the genus Nycticeinops, and given the specific name serengetiensis, meaning 'from the Serengeti'. This species is described from a right and left dentary with some teeth and a second left dentary without teeth plus eleven partial humeri. The genus Nycticeinops contains only a single living species, Nycticeinops schlieffeni (Schlieffen's Twilight Bat) a widespread African species. Nycticeinops serengetiensis appears to have been slightly larger than Nycticeinops schlieffeni, though the modern species is particularly small (at about 3-4 cm), and Nycticeinops serengetiensis is still a small Bat. This is the first fossil species of Nycticeinops described.
Mandible of Nycticeinops schlieffeni, photographs in (A, lateral and (B) occlusal views compared with Nycticeinops serengetiensis, drawings. (C–E) Nycticeinops serengetiensis, left dentary with m1–2 and all anterior alveoli in (C) occlusal, (D) lateral and (E) medial views. (F–H) Nycticeinops serengetiensis, right dentary with p4–m1 with all other alveoli in (F) occlusal, (G) lateral, and (H) medial views. Gunnell et al (2015).
The next specimen is a partial humerus tentatively referred to the living Mops condylurus, the Angolan Free-tailed Bat, a species widespread in Sub-Saharan Africa today.
Right distal humerus of Mops cf. Mops condylurus in (A) anterior, (B) posterior, (C) medial, and (D) lateral views. Gunnell et al (2015).
The next specimens described are a complete left humerus plus a partial left and two partial right humeri referred to the living species Mops thersites, the Railer Bat, a widespread African species. The genus Mops is a member of the family Molossidae (Free-tailed Bats), previously only represented in Africa by an Early Miocene specimen of Tadarida from Kenya.
Left complete humerus of Mops cf. Mops thersites. (A) Photograph and drawing in posteromedial view; (B) drawing in medial view; (C) drawing in lateral view; (D) photograph and drawing in anterior view. Gunnell et al (2015).
The final group of specimens comprises a right dentary which had two teeth when first discovered but has subsequently lost one, plus eight partial humeri referred to the living Miniopterus schreibersi, or Common Bent-winged Bat, a species found today across much of Africa, Europe and Asia.
Comparison of extant Miniopterus schreibersi (A, C, E) photographs and fossiil Miniopterus cf. Miniopterus schreibersi (B, D, F) drawings in (A–B) occlusal, (C–D) lateral, and (E–F) medial views. Gunnell et al (2015).
The majority of the Bat species a either members of species or closely related to species found in dry-to-moist savannah or open woodland close to water today. This fits well with the interpreted environment of Olduvai in the Early Pleistocene, thought to have been open woodland with freshwater springs feeding into a nearby lake. However some of the Bats present, Pipistrellus, Eptesicus, and Scotoecus, are more typically found in drier or even semi-arid open woodland, which may suggest such habitats were present nearby.
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