The Macroscelideans (also termed Sengis, or Elephant Shrews) are an enigmatic group of small insectivorous Mammals, known only from continental Africa. They are noted for their rapid movement, driven by powerful hind limbs, and elongate, flexible snouts, with 20 species known today, grouped into six genera. The history and relationships of this group were difficult to unravel. They have been linked to the Mixodectids (an extinct group of insectivorous Mammals related to the living Colougus of Southeast Asia), the Glires (Rodents and Lagomorphs), the Menotyphla (now rejected theoretical group of Mammals which included Treeshrews, Colougus, Primates, and some extinct groups), and the Condylarths (another rejected group, which comprised species now considered to be early Perissodactyls and Artiodactyls). To make matters more confusing, some fossils now attributed to the Macroscelidea were previously considered to be Hyraxes or Marsupials. More recently genetic studies of Mammal classification has led to Macroscelideans being placed within the Afrotherians, a group which also includes Aardvarks, Tenrecs, Hyraxes, Sirenians, and Elephants. Subsequent re-evaluations of the morphology of African Mammals has led to evidence supporting the Afrotherian hypothesis, which is now generally accepted by zoologists.
A lack of samples from the Palaeogene-Neogene Transition zone obscures our understanding of the evolutionary history of Sengis as a group. The Palaeogene fossil record of the group comprises two subfamilies from North Africa, both of which are considered to be extinct. The Subfamily Metoldobotinae comprises a single genus, Metoldobotes, known only from the Early Oligocene Fayum Depression of Egypt, while the Subfamily Herodotiinae includes four genera, from the Eocene of Algeris, Libya and Tunisia, and the Oligocene of Egypt. Some possible Sengi material is known from Namibia, but this has not been confidently assigned to the group, and there is some dispute as to the age of the deposits from which these specimens were obtained, which may have been of Eocene, Oligocene, or Miocene origin.
The earliest known Neogene Sengi fossils date from the Miocene of Kenya, South Africa, and Namibia, and are assigned to the Myohyracinae, another extinct subfamily with dentition interpretted as indicative of a herbivorous diet. This leaves a nine million year gap in the fossil record of the group, with members of modern groups first appearing about 18 million years ago.
The deposits of the Rukwa Rift Basin of southwestern Tanzania provide a rare insight into the Vertebrate faunas of the Middle Cainozoic of Africa. The Nsungwe Formation is a series of roughly 25 million-year-old fossiliferous fluvial deposits forming part of the Red Sandstone Group. This formation has produced numerous fossil Invertebrates, Fish, Anurans, Lepidosaurs, and a wide range of Mammals, including Sengis.
In a paper published in the journal Historical Biology on 23 July 2021, Nancy Stevens and Patrick O’Connor of the Department of Biomedical Sciences and Center for Ecological and Evolutionary Studies at Ohio University, Cassy Mtelela of the Department of Geosciences at the University of Dar Es Salaam, and Eric Roberts of Earth and Environmental Sciences at James Cook University, describe Oligocene Sengi specimens from the the Late Oligocene Nsungwe Formation of southwestern Tanzania, which they ascribe to the subfamilies Myohyracinae (previously known only from the Early Oligocene of Egypt) and Rhynchocyoninae (the subfamily in which all extant Sengis are placed.
The Oligocene sedimentary deposits of the Rukwa Rift Basin are thought to represent the earliest known Cainozoic sedimentary record of rifting in the Western Branch of the East African Rift System. Fossil exposures are found along the Songwe River Valley, forming part of a laterally continuous series of interbedded fluvial sandstones, floodplain and channel fill palaeosols, lacustrine siltstones, and devitrified airfall tuffs, known as the Songwe Member of the Nsungwe Formation. Fossils tend to be discovered as isolated small specimens, including numerous teeth, jaws, and postcranial elements, from layers within this member interpreted as flash flood deposits. These beds are bracketed between volcanic ash layers from which radiogenic dates have been obtained, and are confidently dated to about 25 million years, making them Late Miocene in age.
The first specimen described is placed within the Myohyracinae and given the name Rukwasengi butleri, where 'Rukwasengi' indicates a Sengi from the Rukwa Rift Basin, and 'butleri' honours Sengi expert Percy Butler, who first described the Family Macroscelidea. The specimen comprises partial right maxilla preserving teeth M2-M3 (the second and third molars).
The teeth of the specimen are both slightly worn. The M2 molar is 2.45 mm in length and 3.3 mm in width, with the broadest point near the base of the mesial cusps. The paracone is slightly taller than the metacone, and placed more bucally. The tooth also has a strong, mesially curved lingual sulcus. The protocone and hypocone are directed anteriorly and have sharp lingual margins. There are three rounded fossettes on the occusal surface, two on the buccal half 1of the tooth, and a smaller one positioned centrally. The M2 molar is triangular and has three roots. It measures 1.2 mm by 1.5 mm.
The second specimen is assigned to the Rhynchocyoninae, and named Oligorhynchocyon songwensis, where 'Oligorhynchocyon' refers to the fact that it is a member of the Rhynchocyoninae from the Oligocene, and 'songwensis' means 'from Songwe', in reference to the Songwe River. The specimen comprises a left P4 (fourth premolar) tooth.
The tooth is a molariform premolar with two roots, and is quite worn. It is 4.1 mm in length and 1.95 mm in width, with the widest point across the protoconid and metaconid. The tooth strong anterobuccal cingulid, although it has a faint rugosity in that position. The paraconid is low and wide, and only half the height of the other trigonid cusps. This paraconid is distinct, but linked to the protoconid by a small paracristid, which ascends along the posterobuccal aspect of the cusp. Another preprotocristid descends along the anterolingual aspect of the protoconid, with the two meeting in a notch just posterobuccal to the paraconid. The protoconid and metaconid are aligned transversely, subequal in size, and connected by a narrow protocristid, with the two forming an unbroken posterior trigonid wall. The hypoconid and entoconid are also aligned transversely and subequal in size. The trigonid and talonid basins are steeply sloped lingually and cusp heights are uneven across the tooth; the protoconid and metaconid approximately 50% taller than the paraconid, entoconid and hypoconid.
A third specimen, RRBP 07433, is a slightly worn three-rooted left upper molar (M2 or M3). This is also tentatively referred to Oligorhynchocyon songwensis, as they appear compatible in size, and the specimen appears to belong to a member of the Rhynchocyoninae. This specimen is 2.8 mm in length and 2.6 mm in width, with cusps largely subsumed in crests. A large paracone is connected to a well-developed anteriorly projecting parastyle by a preparacrista. The anterior cingulum is pronounced. A wide preprotocrista extends halfway to a point between the paracone and the parastyle. Only the bases of the roots remain, with these being subequal in size.
These specimens represent the oldest example of a Myohyracine from East Africa, and the oldest known, and first Palaeogene, example of a Rhynchocyonine. The oldest know Sengis are Eocene in age, but to date very few pre-Miocene specimens have been discovered, with all of these early specimens referred to extinct families, rather than the living Rhynchocyonine.
The previously oldest members of the Rhynchocyonine. were Miorhyncocyon meswae, from the early Miocene Meswa Bridge locality in Kenya, Miorhynchocyon clarki and Miorhynchocyon rusingae, both from the early Miocene Songhor locality, also in Kenya, and Brachyrhynchocyon and Hypsorhynchocyon, from the early Miocene Northern Sperrgebiet locality in Namibia. Some possible Rhynchocyonines have also been recorded from the Eocene Eocliff locality in Namibia, and the early Miocene of Uganda, but these are not well dated nor confirmed taxonomically.
Members of the Myohyracine have previously been recorded from the Miocene Chamtwara and Songhor localities in Kenya, and the Miocene of Namibia, with the youngest known specimens being from the Fort Ternan locality in Kenya and the Bosluts Pan in South Africa, both being about 13 million years old.
The presence of members of the Rhynchocyonine and Myohyracine in the Nsungwe Formation shows both were present in East Africa by about 25 million years ago, before the Palaeogene-Neogene Transition, providing an important insight into the history of the group, and underscoring the importance of these deposits for our understanding of the development of modern African Mammal faunas.
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