Leptoptilos robustus

The genus name Leptotilos is derived from the Greek word lepto meaning "thin or slender", which refers to the storks slim build and the Greek word ptilo meaning "down or soft feather", referring to the soft feather down covering the frame of the members of Leptotilos stork. The species name, "robustus" is derived from Latin word, robur meaning "hardness or strength". The species name robustus is a reference to the notably large size of the tibiotarsus and the thickness of its cortex.[1]

Debate over the relation between the Liang Bua specimen and other stork species was compared using the size measurements of the fragments found to extant species. It can be inferred from the recorded dimensions of the bones that L. robustus was substantially taller and heavier than other species of Leptoptilos, which reach a maximum weight of 9 kg. The only other known Leptoptilos species that outweighs robustus is Leptoptilos falconeri which is estimated to have reached up to 2 meters and weighed approximately 20 kg.[1] L. robustus may have evolved in situ on Liang Bua from a flying ancestor. Unfortunately, the sparse fossil record of birds from South-East Asia makes the evolutionary history of robustus difficult to trace. Only Java has yielded other Leptoptilini fossils including L. dubius and L. titan from the Pleistocene. L. falconeri remains have been found in a wide geographical range from Central Asia, Africa, and central Europe. This wide range is evidence that L. robustus may be at least descended from that species. The morphological differences between the two however, rule out conspecificity. The dimensions of the fragments found suggests that L. robustus is most closely related to L. dubius, the two sharing a common ancestor.[1]

Size of L. robustus compared in size to a human

L. robustus most likely had a slender body characteristic of extant members of Leptoptilos, but was much larger in body size and height, with individuals reaching up to at least 1.8 meters (about 6 feet) tall and weighing approximately 16 kilograms (36 lb).[1] From Liang Bua, fragments of a left ulna, left carpometacarpus, left tibiotarsus, and a nearly complete left femur were discovered and described in 2010. In 2013, the tip of a maxilla, a left and right prosimal scapula, two furculae, a humeral and ulnar, a right proximal radius, two right ossi carpi radiales, a right femur, four phalanges and long bone fragments were additionally discovered and described.[2] The bone fragments found of this species were indicative of its large size when compared to skeletal measurements from the extant species, Leptoptilos dubius.[1] Due to having heavy bones and a heavy body it was formerly assumed that it rarely, if ever, flew. The thick walled tibiotarsus, constituting the thickest of all other extant and extinct Leptoptilini species, is an unusual feature for flying birds which usually selects for minimum mass. As well as heavier than normal bone structure, measured size ratios were thought to suggest that it may have had reduced forelimbs and therefore flight capabilities.[1] The body size was similar in dimension to L. dubius, with the exception of its tibiotarsus. However, the fragmentary nature of the ulna and the carpometacarpus found did not allow an accurate estimate of wingspan length and the question as to whether or not L. robustus could fly was formerly uncertain.[1] A 2022 study, describing additional elements of the wing, found that there was no proportional reduction in the length of the wing as might be expected if it had reduced flight capabilities or was flightless, and thus it was likely fully flight-capable. The size of L. robustus is not unusual compared to many extinct members of the genus, which reached comparable sizes.[3]

Mouth of the Liang Bua cave where L. robustus was discovered

Leptoptilos robustus was discovered in the Liang Bua limestone cave on the island of Flores, Indonesia which is located about 13 kilometers northwest of Ruteng. The bone fragments were collected from Pleistocene sediments at a recorded depth of 4.25-4.70 meters.[1] The bones found were mostly likely the remains of one individual; all that were bones discovered within a single sector were of the same body size and no other large-bodied bird bone fragments were uncovered within that area. The fragments found were most likely from adult individuals indicated by the smooth surface of the bones, ossification, and the fusion of the astragulus and tibia.[1] Comparing the osteological features, large size, and thick bone wall of the tibiotarsus to extinct and extant members of Leptoptilos, the fossil remains from Liang Bua were assigned as a new species robustus. L. robustus was described by Hanneke J.M. Meijer and Rokus Awe Due in April 2009.

The island of Flores, Indonesia is distinct in its isolation, separated from the Sunda continental shelf by deep water all year round. Currently, the limited number of species and the unbalanced nature of the species present is a consequence of the extremely insular nature of the island. Liang Bua supports only a few of the clades from the mainland and lacks specific groups such as mammalian carnivores. This resulted in the distinct dwarfism of mammals and the gigantism of other vertebrate species, an effect known as the island rule. L. robustus in the Pleistocene epoch would have experienced similar, if not the same, geographical conditions as the extant species residing on the island today.