Agapornis longipes

Agapornis longipes is an extinct species of lovebird that lived in what is now Gauteng, South Africa during the Early Pleistocene epoch, about 2.5–1.38 million years ago. The fossilized remains of this bird were first discovered in the Kromdraai fossil site and reported in 1969 by T.N. Pocock, though the species would not be named until decades later in 2024. By the time it was named, 96 bones attributable to this species had been collected over the years from three caves, namely Kromdraai, Swartkrans and Cooper's Cave, all located within the Cradle of Humankind World Heritage Site. Such a large number of fossils is unusual for a small bird, as the fragile bones of birds do not preserve easily and thus are usually found as fragments, yet the A. longipes remains include both complete and incomplete bones.

A. longipes was a small bird, with its wing bones showing that it would have been about the same size as the smaller extant lovebird species, yet its tarsometatarsus (one of the leg bones) was comparable in size to that of the largest living lovebirds. This indicates that the legs of this species were disproportionately long compared to other lovebirds, hence it was given the specific name longipes which means 'long-footed' in Latin. Though the many preserved specimens show that some individuals were larger than others, the body proportions did not change with size. The proportionally long legs are believed to be an adaptation for feeding on the ground, where the animal would forage for grass seeds among the dense vegetation of its environment, though it would presumably still breed in tree cavities like modern lovebirds.

Analysis of the fossil assemblage in the three caves where A. longipes remains have been found suggests that its natural habitat was mostly open grassland with nearby woodlands, and in some areas would also have large rocky outcrops or major bodies of water, though the exact conditions vary between the three localities. This environment is largely similar to what is seen in South Africa today, suggesting that this lovebird did not went extinct because of general habitat change, but possibly because of changes to the microclimate or the loss of a food source. It is believed that at least some of the A. longipes specimens were brought into the caves by western barn owls which preyed on the lovebirds and regurgitated the remains as pellets, considering the similarity of the fossil deposits to the debris seen under barn owl roost sites and the presence of barn owl fossils in the same area.

Discovery and naming

All known fossilized remains of Agapornis longipes originate from the Cradle of Humankind, a World Heritage Site about 50 km (31 mi) northwest of Johannesburg, South Africa and consisting of many fossil-bearing caves. The first to be discovered were found decades before the species was named. In 1969, T.N. Pocock reported that remains of lovebirds belonging to the genus Agapornis have been discovered at the Kromdraai fossil site (one of the caves that would later be designated as part of the World Heritage Site). He believed that two species were represented by these specimens, a smaller species around the size of a budgerigar and a larger species, but did not assign any of the specimens to any named species. The presence of at least one lovebird species at Kromdraai was further confirmed in 2010, when Thomas A. Stidham reported that the fossilized humerus of a small lovebird had been discovered during excavations at the site between 1977 and 1980, and was being housed in the Transvaal Museum. Stidham also added that the lovebird remains reported by Pocock had been lost. However this turned out to be incorrect, and those remains are actually kept at the Evolutionary Study Institute at the University of the Witwatersrand. Marco Pavia further published two papers reporting fossil remains of Agapornis, one in 2020 mentioning a humerus at Kromdraai, and the other in 2022 detailing a variety of small bones from the nearby Cooper's Cave. In both papers, Pavia identified the remains only to the genus level, stating that comprehensive analysis is needed for more specific identification.

In 2024, a study on the fossil lovebird remains found in the Cradle of Humankind was published. The authors of the study analysed specimens housed in the University of the Witwatersrand and the Ditsong National Museum of Natural History, including those formerly studied by Pocock, Stidham or Pavia, and concluded that they represent a single new species. The name Agapornis longipes was given to this species, with the specific name derived from the Latin words longus (meaning "long") and pēs (meaning "foot"). Bones of this lovebird were uncovered in three caves in the Cradle of Humankind, namely Kromdraai, Cooper's Cave and Swartkrans, and the authors described a series of remains found between 1979 and 1986 by paleontologist Bob Brain in detail. Birds have fragile bones, so fossils of small birds such as Agapornis are typically found in small fragments, but A. longipes was unusually represented by 96 bones, some entirely complete and others incomplete.

Description

Agapornis longipes was a small species of parrot with relatively large legs compared to other lovebirds. Its wing bones are comparable in size to those of the extant Fischer's, Lilian's and black-cheeked lovebirds, suggesting it was about as large as these small modern forms. However, the tarsometatarsus (a bone in the lower leg) of A. longipes is closer in size to that of larger lovebird species such as the black-winged and rosy-faced lovebirds. This makes the ratio of humerus to tarsometatarsus length in A. longipes the smallest of all known Agapornis species, indicating this species had long legs for a lovebird. Intraspecific variation in size is known in A. longipes, with some individuals being larger than others, though the ratio of wing length to leg length remains the same regardless of an individual's size. When remains of the species were first studied in 1969, it was thought that larger and smaller individuals represent two separate species. However, this size variation is within the range seen in extant lovebird species. In addition, no extant lovebird species overlap in distribution with one another, making it unlikely that A. longipes lived alongside a second species of lovebird.

Only one scapulocoracoid (a unit of the shoulder girdle) of this species has been found, and the sternal part of it is not preserved. The clavicular facet of the scapula is large, with a protrusion on its ventral (lower) side, and overhangs a deep fossa (depression or hollow). On the dorsal (upper) side of the groove of the supracoracoideus muscles, there is a rounded tubercle. The humerus has a prominent dorsal tubercle and a deep attachment for the brachialis muscle, making it more similar to that of the extinct Agapornis attenboroughi than any extant lovebirds. The ulna of A. longipes has a curved shaft like that of all other lovebirds, but otherwise differs from them in having a less protruding, stouter olecranon (elbow protrusion) and a carpal tubercle which protrudes ventrally rather than distally (towards the underside instead of the wing tip). The carpometacarpus has an elongated, ridge-like pisiform process, which is a derived feature unique to the Agapornis genus. There is also a continuous ridge connecting the pisiform process to the alula process, a feature not seen in other lovebird species.

A single tibiotarsus of A. longipes is known, and is similar in structure to that of other lovebirds (and unlike most other African parrots). The tarsometatarsus has a canal where both the flexor hallucis longus muscle and the flexor digitorum longus muscle would have been attached, and a separate canal is present for the superficial flexor tendons. Such a canal structure of the tarsometatarsus is also known in other Agapornis species, as well as the related hanging parrots of the genus Loriculus. In A. longipes, the trochlea (pulley-like grooved structure) at the head of the third metatarsal protrudes further and is wider at the proximal end than in any living lovebird species, and the tarsometatarsus is also straighter and narrower at the proximal end.

Paleobiology

Because all but one living species of lovebirds eat mainly grass seeds and secondarily feed on fruit, it is believed that Agapornis longipes had a similar diet. While some extant lovebirds search for food in trees, the ones with comparably longer legs relative to their wings (namely the grey-headed, yellow-collared and Fischer's lovebirds) feed on the ground. A. longipes has the proportionally longest legs of all known lovebirds, suggesting that it lived primarily on the ground rather than in trees, feeding on grass seeds in the tall, dense vegetation of the South African grasslands. Even so, the breeding habits of A. longipes are still presumably similar to those of living lovebirds, which require tall trees with either natural cavities or holes made by other birds (such as African barbets and woodpeckers) to lay their eggs in. It was reported by T.N. Pocock in 1969 that one humerus found at the Kromdraai fossil site was of an immature bird, suggesting that the surrounding area was used as a nesting ground for A. longipes rather than simply being part of its non-breeding range.

Known from a large number of fossil remains, including 24 specimens of at least seven individuals from Cooper's Cave, A. longipes was likely a common animal in its natural habitat, being the fifth most common bird in terms of percentage of identified remains at this site and the third most common in terms of number of individuals represented. The species may be similarly abundant in Kromdraai, where it is known from 23 specimens, but since these remains are spread across multiple stratigraphic layers, it is difficult to verify the true abundance of this bird there. Some of the A. longipes specimens from Kromdraai and Cooper's Cave come from deposits with numerous small animal remains, which are believed to be debris from fossilized owl pellets. In particular, the remains in the Kromdraai deposits are most similar to debris under roost sites of western barn owls, and fossils attributed to Tyto cf. alba are known from both localities. This suggests that barn owls used these caves as roosting spots and were at least an occasional predator of A. longipes.

Paleoenvironment

Fossilized remains of Agapornis longipes have been discovered in three caves, namely the Kromdraai fossil site, Cooper's Cave and Swartkrans. These caves are situated within the UNESCO World Heritage Site of the Cradle of Humankind in South Africa, so named because this area bears fossils of early hominins spanning from the Late Pliocene to Late Pleistocene epochs. However, in spite of extensive searches, specimens of this lovebird have not been recorded from localities other than the aforementioned three caves. The environment that A. longipes inhabited was similar to the savanna and veld grasslands of modern South Africa. Living in the Cradle of Humankind, the species lived alongside a wide variety of other animals, including the hominins Paranthropus and Homo. Because the environment of A. longipes has not changed significantly, it is likely that the bird was driven to extinction by microclimatic changes or the extinction of a food source rather than a change in the general environment.