Propoecilogale

Propoecilogale Temporal range: Early Pliocene-Early Pleistocene,
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Carnivora
Family:	Mustelidae
Subfamily:	Ictonychinae
Genus:	†Propoecilogale Petter, 1987
Species:	†P. bolti
Binomial name
†Propoecilogale bolti (Cooke, 1985)
Synonyms
Ictonyx bolti Cooke, 1985 Prepoecilogale bolti Petter & Howell, 1985 (nomen nudum)

Propoecilogale is an extinct genus of mustelid that lived in Africa from the Early Pliocene to Early Pleistocene epochs, about 3.85–1.38 million years ago.

Discovery and naming

Between 1947 and 1948, American paleontologists Frank Peabody and Charles Lewis Camp collected fossils from the cave breccia deposits in Pit 10 of Bolt's Farm, a site in Gauteng, South Africa, as part of the University of California African Expedition. Among these fossils was the nearly complete skull of a mustelid, which was sent to the Transvaal Museum and given the specimen number TM/BF201. In addition, paleontologist Donald Elvin Savage prepared a cast of this specimen, which was placed in the University of California Museum of Paleontology and cataloged as UCMP 19694. Neither the skull nor the cast would be studied until 1985, when South African-Canadian paleontologist Herbert Basil Sutton Cooke published a paper in which he described this specimen. He believed it most closely resembled the skulls of modern species belonging to the genus Ictonyx, but also differed significantly enough that it could not be assigned to any living species. Cooke therefore erected a new species which he named Ictonyx bolti, with the specific name referencing the location it was discovered, and the skull was designated as the holotype of this species.

In 1987, a study by Germaine Petter was published in which she analysed two fossilized mustelid specimens collected in Laetoli, Tanzania. One of these specimens (cataloged as LAET 248) was a partial skeleton with vertebral, skull and limb elements, while the other (cataloged as LAET 1358) only included jaw elements. Petter compared these remains with specimens of both modern and fossil mustelids and concluded that they represent the same species as the skull from Bolt's Farm named as Ictonyx bolti. Being able to analyse more complete remains than Cooke was, Petter discovered that this species was actually more similar to the extant African striped weasel (Poecilogale albinucha) than any Ictonyx species, and thus does not belong in the Ictonyx genus. Due to differences in the teeth, she decided not to place it in the same genus as the African striped weasel, though she suggested it may be an ancestor of the modern form. Petter established the new genus Propoecilogale for the species, combining the prefix pro (meaning "before") with the generic name of the African striped weasel.

Additional specimens have been attributed to Propoecilogale after the genus was established. In 1997, French paleontologist Denis Geraads studied several fossilized mandible fragments found in Ahl al Oughlam, Morocco. He recognized that these remains resembled the P. bolti material from Bolt's Farm and Laetoli, and thus belonged in the same genus, but were too fragmentary and eroded to formally assign to the same species. Geraads therefore reported them under the designation Propoecilogale sp., cf. P. bolti. A 2008 paper reported that one fossil specimen of an African striped weasel was found in Cooper's Cave, South Africa. However, this specimen, a partial right mandible cataloged as CD 3896, was reassigned to P. bolti in a 2013 study. The authors of this study note that the Cooper's Cave specimen may represent a later evolutionary stage than the P. bolti specimens from Laetoli and therefore possibly belong to a separate species, but the scarcity of the material led them to make this assignment for the time being. In addition, a complete right mandible of P. bolti was reported in 2017 to have been discovered in the Kromdraai fossil site of South Africa, and was given the specimen number KW 7359.

Spelling

The spelling of this animal's name has a somewhat confusing history. In 1985, Germaine Petter coauthored a book chapter with Francis Clark Howell in which she mentions that she will establish a new mustelid genus named Prepoecilogale, based on the fossil skull named as Ictonyx bolti by Cooke and new specimens from Laetoli. However, when the full description of this material was published two years later, there was no mention of a genus with this name. Instead, the aforementioned fossils had been described under the different genus name Propoecilogale, suggesting that Petter decided to change the spelling of the name. Since then, both of these spellings have been used in other publications by various authors, who have also attributed both references as the taxonomic authority of the genus. This was acknowledged by a study published in 2013, which clarifies that according to the International Code of Zoological Nomenclature, the valid spelling should be Propoecilogale because it was used in the original description of the taxon. Meanwhile, Prepoecilogale should be considered a nomen nudum because this spelling was established without a description.

Description

Size

Propoecilogale bolti is a small mustelid, with the holotype skull from Bolt's Farm estimated to have reached reached a total length of 50 mm (2.0 in) and a width of 31 mm (1.2 in) at the zygomatic arches when intact, though damage to the snout tip and zygomatic arches of the specimen means the true exact measurements cannot be known. This suggests that the holotype individual was comparable in size to the modern African striped weasel. Another specimen attributed to P. bolti, a lower jaw fragment from Cooper's Cave, indicates that this animal could grow larger than the size suggested by the holotype, though this specimen may represent a separate species in the Propoecilogale lineage leading towards the African striped weasel rather than P. bolti itself.

Skull and teeth

The skull of Propoecilogale exhibits features which have been considered intermediate between those of Poecilogale and Ictonyx, two living genera related to this prehistoric animal. The ear canal opens more laterally (facing sideways) than that of Poecilogale, though not as much so as in Ictonyx. The squamosal sutures are directed more laterally like those of Ictonyx, whereas these sutures are directed more anteriorly (facing forwards) in Poecilogale. The tympanic bullae (smooth, bulging protrusions on the tympanic part of the temporal bone) are not as inflated as those of Ictonyx, but wider than those of Poecilogale. These bullae are slightly inflated, elongated and narrow in Propoecilogale, each bearing a carotid canal midway down their length and a long ridge near the border of the alisphenoid (a protrusion of the sphenoid bone). These ridges would have served as an attachment point for muscles. The mastoid process (a bony projection behind the ear canal) is indistinct.

The upper dentition of Propoecilogale has a dental formula of 3.1.3.1, identical to that of Ictonyx, but differing from that of Poecilogale in having an extra premolar. The teeth show some ancestral characteristics such as a forward-pointing protocone on the fourth premolar and the number of premolars present, which are also seen in Ictonyx. However, there are also more derived features similar to those in the teeth of Poecilogale, such as regression of the hypocone and metaconid of the first molar, fusion of its metacone and paracone, and weakening of the posterior accessory cusp of the fourth premolar.

Postcrania

While the holotype specimen preserves only a skull, postcranial elements are known in the specimen LAET 248 from Laetoli. All the defining traits of Propoecilogale are features of the skull and teeth, with the postcrania exhibiting only characteristics typical of mustelids in general. The head of the humerus is wider when measured transversely than antero-posteriorly (front to back), and the upper part overhangs the greater tubercle. The other end of the humerus is widened, with the inner edge of the epiphysis protruding to its side. The diaphysis (bone shaft) of the humerus is curved at the upper end when viewed from the animal's side, and the crest-like attachment point for the deltoid muscle can be seen on the front of its surface. The ulna diaphysis is flattened from side to side, with the hind part compressed into a crest-like form. At the top of the ulna is a curved olecranon (elbow protrusion) with a rounded outline. Near the ankle end of the tibia, the internal malleolus (ankle protrusion) can be seen at the front inner corner of the articular surface. In the calcaneum, the front part of the articular surface is widened. The neck of the talus bone is well-developed and bends away from the inner edge of the trochlea (the pulley-like joint with the tibia).

Classification

Propoecilogale is a member of the family Mustelidae, and specifically belongs in the tribe Ictonychini. When fossils of the type species P. bolti were first described in 1985, they were attributed to the genus Ictonyx, the type genus of Ictonychini, under the name Ictonyx bolti. In addition, the tribe was assigned to the subfamily Mustelinae at the time. However, Propoecilogale has been recognized as a separate valid genus containing this species since 1987, and Ictonychini is now placed in the subfamily Ictonychinae alongside its sister tribe Lyncodontini. A 2024 study suggests that the two tribes diverged from each other during the Late Miocene epoch in Asia, and the Ictonychini tribe would have spread into Africa soon afterwards. The phylogenetic analysis conducted by Germaine Petter in 1987 recovered Propoecilogale to be more closely related to all other African members of Ictonychini than to any of the modern or prehistoric Eurasian members, and in particular was most closely related to the African striped weasel of the genus Poecilogale. The cladogram below shows the results of this analysis:

	Ictonychini †Baranogale †Pliovormela †Oxyvormela Vormela †Propoecilogale Poecilogale Ictonyx Poecilictis

The modern African striped weasel has been proposed to be a descendant of Propoecilogale

Because it is so closely related to the extant African striped weasel, it has been proposed that Propoecilogale is an ancestor of this modern-day animal, an idea first brought up in 1987 by Petter. The African striped weasel exhibits derived features in its skull and teeth compared to the more ancestral condition retained in extant members of the genus Ictonyx. Propoecilogale shows features intermediate between these two conditions, likely representing a transitional form between the ancestral Ictonychini morphology and the more derived anatomy of the African striped weasel. In addition, a specimen attributed to Propoecilogale bolti from the Early Pleistocene deposits of Cooper's Cave shows a larger first molar tooth with a more reduced metaconid compared to the Pliocene-aged specimens from Laetoli. The size of the molar in the Cooper's Cave specimen is more similar to that of an African striped weasel, but because the metaconid is absent in the modern species, this specimen cannot be attributed to it. Being geologically younger than the Laetoli P. bolti specimens and older than the extant species, it is possible that the Cooper's Cave specimen represents an intermediate stage between the two in the evolution of the Propoecilogale-Poecilogale lineage.

Paleobiology

Because Propoecilogale is so similar and closely related to the modern African striped weasel, differing mainly in the details of its teeth, it can be inferred to have comparable habits with its extant counterpart. All species in the Ictonychini tribe are predatory animals, including the African striped weasel, though this weasel has a very specialized diet and feeds almost exclusively on small rodents, with birds being eaten occasionally and all other potential prey being rejected. This differs from the more generalist diets of other members of Ictonychini such as the striped polecat, which will eat a wide range of small animals and eggs. Propoecilogale was likely a predator as well, and its teeth show a mix of features similar to those of the African striped weasel and characteristics closer to those of the striped polecat, though the level of specialization in its diet has not been studied. In its natural habitat, Propoecilogale would have lived alongside various species of mongooses, which would have already been present in Africa at the time that this mustelid's ancestors first entered the continent. Mongooses occupy a similar ecological niche of "small predatory carnivore" as mustelids, and thus may have competed with them to some extent. However, mustelids like Propoecilogale may have had an advantage in that they can take on larger prey (including animals larger than themselves) and excavate burrows to breed in. In addition, African striped weasels are nocturnal, allowing them to minimize competition with the diurnal mongooses, and Propoecilogale may have had similar habits.

Paleoenvironment

Fossils of Propoecilogale are rare, often only found as fragmentary specimens, and have been discovered in a wide yet sparse range of sites across Africa. These localities extend as far north as Ahl al Oughlam, Morocco, to the Tanzanian site of Laetoli in the east, and the South African caves of Bolt's Farm, Kromdraai and Cooper's Cave in the south. This indicates that Propoecilogale was very widespread across the African continent, and the rarity of its fossils may be because its small bones are easily overlooked and difficult to preserve rather than a sign of the scarcity of the animal itself.

Propoecilogale is believed to have first evolved during the Zanclean stage of the Early Pliocene epoch, with the geologically oldest known fossils of this animal originating from Laetoli, Tanzania. Specifically, the remains of two individuals have been discovered in the Upper Laetolil Beds, which the use of argon-argon dating has shown were deposited between 3.85 and 3.63 million years ago. The environment of the Upper Laetolil Beds during this time is believed to have been dominated by a mosaic of grassland, shrubland and open woodland habitats, and was mostly stable, although drier and more open habitats started to become more dominant towards the end of this stretch of time. This is supported by the fossil assemblage of the beds, which is largely made up of animals preferring open habitat such as bovids of the Alcelaphini and Neotragini tribes, rodents of the genera Pedetes, Saccostomus and Heterocephalus, grassland birds like francolins and guineafowl, and brood balls of dung beetles. The presence of woodland is evidenced by fossils of forest gastropods and tree-climbing monkeys of the genera Rhinocolobus and Cercopithecoides. Unlike many other Pliocene-aged fossil sites in East Africa, the absence of fossils of aquatic animals (such as crocodiles, hippos, fish or turtles) at Laetoli suggests it represents a drier environment with no large permanent bodies of water, although ephemeral rivers and ponds would have been present during the rainy season, and perennial springs would have been an important water source for animals during the dry season. The fact that most of the sediments of the Upper Laetilil Beds are volcanic tuffs suggests that ash falls would have occured regularly at Laetoli, with the (currently extinct) volcano Satiman being the likely source of the ash.

Fragmentary jaw elements attributed to Propoecilogale have been reported from the cave deposits of Ahl al Oughlam near Casablanca, Morocco. The deposits of this locality were most likely formed 2.5 million years ago, though could potentially have been laid down at any point between 2.8 and 2.4 million years ago, ranging from the latest part of the Pliocene epoch across the Pliocene-Pleistocene boundary into the earliest Pleistocene. The fossil assemblage of Ahl al Oughlam is believed to have originated from an open landscape with only sparse wood cover and no large permanent sources of freshwater, as indicated by the absence of tree-dwelling monkeys and near-absence of any forest or woodland bovids, whereas remains of desert-adapted gerbilline rodents are by far the most numerous fossils from this site. The mixture of species associated with cooler temperatures (such as the walrus Ontocetus emmonsi) and warmer climates (such as the parrot Agapornis atlanticus) may suggest that the site had high seasonality, and these species may have been seasonal visitors to the area rather than permanent residents. Most of the large animal bones were found beneath a vertical chimney, suggesting at least some animals fell into the cave from above, though many were likely kills brought in by carnivores using the cave as shelter. The carcasses gathered in the pit may have attracted more carnivores to scavenge on them, only to end up trapped themselves.