Darwinius

Darwinius is a genus within the infraorder Adapiformes, a group of basal strepsirrhine primates from the Eocene epoch. Its only known species is Darwinius masillae, approximately 47 million years ago (Lutetian stage) based on dating of the fossil site.[1]

The only known fossil, called Ida, was discovered in 1983[2] at the Messel pit, a disused quarry near the village of Messel, (Landkreis Darmstadt-Dieburg, Hesse) about 35 km (22 mi) southeast of Frankfurt am Main, Germany. The fossil, divided into a slab and partial counterslab after the amateur excavation and sold separately, was not reassembled until 2007. The fossil is of a juvenile female, approximately 58 cm (23 in) overall length, with the head and body length excluding the tail being about 24 cm (9.4 in). It is estimated that Ida died at about 80–85% of her projected adult body and limb length.[3]

The genus Darwinius was named in commemoration of the bicentenary of the birth of Charles Darwin, and the species name masillae honors Messel where the specimen was found. The creature appeared superficially similar to a modern lemur.[3][4]

The authors of the paper describing Darwinius classified it as a member of the primate family Notharctidae, subfamily Cercamoniinae,[3] suggesting that it has the status of a significant transitional form (a "link") between the prosimian and simian ("anthropoid") primate lineages.[5] Others have disagreed with this placement.[6][7][8]

Concerns have been raised about the claims made about the fossil's relative importance and the publicising of the fossil before adequate information was available for scrutiny by the academic community.

Taxonomy
Franzen et al. (2009) place the genus Darwinius in the subfamily Cercamoniinae of the family Notharctidae within the extinct suborder Adapiformes of early primates.[3]

Darwinius masillae is the third primate species to be discovered at the Messel locality that belongs to the cercamoniine adapiforms, in addition to Europolemur koenigswaldi and Europolemur kelleri. Darwinius masillae is similar but not directly related to Godinotia neglecta from Geiseltal.

The adapiforms are early primates which are known only from the fossil record, and it is unclear whether they form a suborder proper or a paraphyletic grouping. They are usually grouped under Strepsirrhini—including lemurs, aye-ayes and lorisoids—and as such would not be ancestral to Haplorrhini, which includes tarsiers and simians.[9] Simians are usually called anthropoids, and while this name can be confusing, the paper uses the term anthropoids, as does associated publicity material. Simians (anthropoids) include monkeys and apes, which in turn includes humans

Franzen et al. in their 2009 paper place Darwinius in the "Adapoidea group of early primates representative of early haplorhine diversification". This means that according to these authors the adapiforms would not be entirely within the Strepsirrhini lineage as hitherto assumed but would qualify as a transitional fossil (a "missing link") between Strepsirrhini and Haplorrhini and so could be ancestral to humans. They also suggest that tarsiers have been misplaced in the Haplorrhini and should be considered Strepsirrhini. To support this view they show that as many as six morphological traits found in "Darwinius" are derived characters present only in the Haplorrhini lineage, but absent in the Strepsirrhini lineage, which they interpret as synapomorphies. These include, among others, a cranium with a short rostrum, deep mandibular ramus, loss of all grooming claws. They note "that Darwinius masillae and adapoids contemporary with early tarsioids could represent a stem group from which later anthropoid primates evolved, but we are not advocating this here, nor do we consider either Darwinius or adapoids to be anthropoids."

Concerns over cladistic analysis
Paleontologists have expressed concern that the cladistic analysis compared only 30 traits when standard practice is to analyze 200 to 400 traits and to include fossils such as anthropoids from Egypt and the primate genus Eosimias which were not included in the analysis. This contrasts with the motive openly stated by the authors, which was to list 30 anatomical and morphological characteristics "commonly used" to distinguish extant strepsirrhine and haplorrhine primates.[3] Paleontologist Richard Kay of Duke University thought the data could have been cherry-picked. Paleontologist Callum Ross of the University of Chicago considered the claim that Darwinius should be classified as haplorhine was "unsupportable in light of modern methods of classification."[11] The opinion of Chris Beard, curator of vertebrate paleontology at the Carnegie Museum of Natural History, was that Darwinius was not a "missing link" between anthropoids and more primitive primates, but that further study of this remarkably complete specimen would be very informative and could reveal relationships amongst "the earliest and least human-like of all known primates, the Eocene adapiforms."[12] In an interview published on 27 May 2009, Jørn Hurum stated that he had an open mind about the possibility that the fossil might turn out to be a lemur and that a paper on systematics to be published within about a year would mainly focus on the partial counterslab containing the inner ear and the foot bones.[13]

Most experts hold that the higher primates (simians) evolved from Tarsiidae, branching off the Strepsirrhini before the appearance of the Adapiformes. A smaller group agrees with Franzen et al. that the higher primates descend from Adapiformes (Adapoidea). The view of paleontologist Tim White is that Darwinius is unlikely to end the argument.[14]

Philip D. Gingerich states that the seven superfamilies of primates are commonly associated in the higher taxonomic groupings of suborders Anthropoidea and Prosimii as an alternative to Haplorhini and Strepsirrhini, depending on the position of Adapoidea and Tarsioidea. He puts forward a phylogeny in which the higher primates evolved from Darwinius, which he groups with other Adapoidea. He shows the Adapoidea together with the Tarsioidea as representing early diversification of the suborder Haplorhini and shows the Strepsirrhini as having branched off directly from the earliest primates.[15] The Revealing the Link website uses this taxonomic grouping and states that Darwinii is from an early group of primates just prior to diversification into the anthropoids (monkeys, apes and humans) and the prosimians (lemurs, lorises and tarsiers).[16]

Erik Seiffert and colleagues at Stony Brook University argue that Darwinius is on the branch towards the Strepsirrhini and is not a 'missing link' in the evolution of the Anthropoidea.[17] A phylogenetic analysis of 360 morphological characters in 117 extinct and modern primates places Darwinius in a now-extinct group of strepsirrhines along with a newly discovered 37-million-year-old Egyptian primate, Afradapis. Seiffert believes that characteristics that appeared to show a relationship to haplorrhines are due to convergent evolution[18] and has said that "the PR hype surrounding the Darwinius description was very confusing.”

Type specimen
The type specimen is missing only its left rear leg. It has been named Ida[4] after the daughter of Jørn Hurum, the Norwegian vertebrate paleontologist from the Natural History Museum, University of Oslo, who secured one section of the fossil from an anonymous owner and led the research.[20] In addition to the bones, remains of Ida's soft tissue and fur outline are present along with remnants of her last meal of fruit and leaves. The animal is about 58 cm (23 in) from nose to tail, or roughly the size of a small, long-tailed cat.

The lemur-like skeleton of the fossil features primate characteristics of grasping hands with opposable thumbs and nails instead of claws. These would have provided a "precision grip" which, for Ida, was useful for climbing and gathering fruit. Ida also has flexible arms and relatively short limbs.[21] The fossil is missing two anatomical features found in modern lemurs: a grooming claw on the foot and a fused row of teeth, a toothcomb, in the bottom jaw.[22]

Digital reconstructions of Ida's teeth reveal that she has unerupted molars in her jaw, indicating by comparison with modern squirrel monkeys that she was 9–10 months old and would have reached adulthood at 36 months. The shape of Ida's teeth provides clues as to her diet; jagged molars would have allowed her to slice food, suggesting that she was a leaf and seed eater. This is confirmed by the remarkable preservation of her gut content. Furthermore the lack of a baculum (penis bone) found in all lower primates means that the fossil was from a female.[3] X-rays performed on Ida revealed that her right wrist was healing from a fracture which may have contributed to her death. The scientists speculate whether she was overcome by carbon dioxide fumes while drinking from the Messel lake. Hampered by her broken wrist, she slipped into unconsciousness, was washed into the lake and sank to the bottom, where unique fossilisation conditions preserved her for 47 million years.