Pronolagus humpatensis

Pronolagus humpatensis is a small-sized fossil species of lagomorph from the early Pleistocene^[3] of Humpata Plateau, Huíla, Angola^[4] and Namibia.^[5] Its remains have been identified in limestone breccias and tufas from Plio-Pleistocene^[6] infills in the karsts of Huíla Province,^[7] and is known from fragmented leg, skull and jaw bones from multiple individuals. It was first described in a 2022 survey of extant and fossil Pronolagus species by Sevket Sen and Martin Pickford,^[8] and was so assigned to the genus mainly based on dental characteristics. It is one of only a few African fossil lagomorphs described.^[5]

Pronolagus humpatensis Temporal range: Early Pliocene or Middle Pleistocene,[1] 5.3–0.1 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Lagomorpha
Family:	Leporidae
Genus:	Pronolagus
Species:	†P. humpatensis
Binomial name
†Pronolagus humpatensis Sen & Pickford 2022[2]

History of discovery

A canyon in the Huíla Plateau, the larger geographic region wherein Pronolagus humpatensis fossils were first identified

Across the African continent, lagomorph fossils are poorly documented. In the family Leporidae specifically, many fossils are known from southern Africa, but detailed studies are lacking.^[5] Winkler and Avery, in a 2010 work on the mammals of Africa, noted that no fossil species of Pronolagus were known at the time.^[9] The fossil species Pronolagus humpatensis is the first of its genus to be described, besides the nomen oblitum P. intermedius^[10] named by Henry Lyster Jameson in 1909.^[11] It was described in 2022 by Sevket Sen and Martin Pickford in their review of Pronolagus specimens and fossils, where it was differentiated from the extant members of the genus by its overall smaller size. Particular attention was paid towards the diminished presence of the incisive foramen and narrowness of the anteroconid (a cusp at the front of the tooth) in the third lower premolar, which differ from all extant red rock hares. Other features, including relative bone sizes, were used to differentiate the species from other known leporids.^[12] The identifying fossils from this study were prepared by the dissolution of breccia by an acetic acid solution and calcium triphosphate buffer, followed by consolidation in a solution of insulating varnish.^[13]

Prior excavations in southern Africa have found evidence of Pronolagus. A 2019 excavation of breccias in Leba Cave, a location in the Humpata Plateau, discovered a single Pronolagus specimen near the surface.^[14] A 2017 excavation from the Okongwe locality of what is now Erongo Region, Namibia discovered several lagomorph teeth that bore similarities to those of P. humpatensis, but were labeled as Pronolagus sp.^[15]

Taxonomy

Pronolagus humpatensis was described based on three hemimandibles from the right side of the skull, a skull fragment, a palate fragment with some cheek teeth; several postcranial bones, including five femoral fragments, ulna, tibia, and humerus fragments; astragalus, calcaneum, metacarpal, and phalanx bone fragments. The femoral fragments belonged to juvenile individuals, based on evidence of increased bone porosity and missing head and greater trochanter pieces.^[16] These fossil fragments were excavated from limestone breccias in the Humpata Plateau from the Tchiua and Cangalongue localities and belong to multiple individuals.^[17] The holotype of the species (Tc 2'90) is a hemimandible that bears the mandibular symphysis, diastema, and corpus, which is kept at the Regional Museum of Huila. The specific name is taken from the type locality, Humpata Plateau.^[12]

Description

Pronolagus humpatensis is a fossil rabbit.

This fossil species is differentiated from sympatric species, both extant (such as those in Lepus and Pronolagus) and extinct (such as those in Trischizolagus)^[16] based on the presence or absence of dental features, as well as the comparison of cranial and postcranial bone measurements from literature.

Paleoecology

Pronolagus humpatensis is known only from the late Pliocene-early Pleistocene^[18] of Humpata Plateau in Angola, though it may also be associated with Plio-Pleistocene limestone tufas at Okongwe, Namibia^[19] due to the presence of very similar Pronolagus fossils there.^[15]

Humpata Plateau is a geological feature that rises 1,750 metres (5,740 ft) above sea level, on average. Various quarries exist across the plateau, many of which were used to mine materials for quicklime. These quarries and the cave systems of the plateau contain coarse pink and red breccias, as well as calcified bat guano deposits, that are liable to contain fossil remains.^[20] The limestone breccias P. humpatensis is associated with were formed both by accumulation of sediment in cave infills and calcification of bat guano. Some of the vertebrate fossils in similarly dated sedimentary infills in caves across the plateau are speculated to be remains from the prey of the crowned eagle (Stephanaoetus coronatus),^[21] including several small-bodied (<20 kg (44 lb)) cercopithecids.^[22]