The Great Ida Debate

'Missing link' Ida lacks evolutionary insights

Appearances can be deceptive. Ida, a fossil so complete that it launched 1000 headlines in May, might actually be too damaged to reveal much about primate evolution, according to a new analysis.

When Ida, also known as Darwinius masillae, was revealed to the world, many were impressed by its exquisite preservation – even its final meal could be seen, preserved in its 47-million-year-old gut.

Jørn Hurum at the University of Oslo, Norway, and colleagues analysed the fossil and suggested it could help link both major groups of extant primate: the strepsirrhines (lemurs and lorises) and the haplorrhines (tarsiers, monkeys and apes).

That conclusion was hotly contested by many, and is contradicted in a new analysis by Erik Seiffert at Stony Brook University, New York, and colleagues. The team has just discovered a new 37-million-year-old primate in Egypt, which they have named Afradapis.

Convergent evolution
Seiffert's team carried out a phylogenetic analysis of 117 extinct and extant primates that looked at 360 morphological characteristics. Their analysis places both Afradapis and Darwinius firmly on the strepsirrhines branch. The researchers suggest that the characteristics Darwinius appears to share with the haplorrhines are the result of convergent evolution.

Seiffert stresses that although Ida possesses important features rarely found in other primate fossils, such as relative limb proportions, the anatomy that matters from an evolutionary point of view is badly preserved.

"When it comes to the key anatomical features that primate palaeontologists so often depend on, Darwinius is surprisingly uninformative," says Seiffert. The skull is crushed and the ankle damaged, he says – two areas of the body crucial to understanding early primate evolution.

And even though the Afradapis specimen is no more than a fragmentary jawbone, Seiffert's team think it provides more insight into primate evolution than its famous cousin. "The dentition of Afradapis, now known from jaw fragments [and hundreds of teeth] certainly is much more informative than that preserved in Ida," he says.

Fossil wars
Marc Godinot, a primate palaeontologist at the National Natural History Museum in Paris reckons Seiffert has a point. "Because [Ida] is a juvenile and it is crushed, there are a number of characters that cannot be checked," he says.

But Hurum is "astonished" by Seiffert's statements. "How can a fragmented lower jaw have more useful information than a complete skeleton?" He adds that Seiffert's team's conclusion is "quite harsh" given that they have seen only the photographic prints in Hurum's team's original paper and a cast of the Ida specimen.

"We are in the process of high-resolution CT scanning to show clearly the morphology of the hands and foot of Darwinius, so clear illustrations of this will be forthcoming," Hurum says.

Alfred Rosenberger at Brooklyn College, City University of New York, rejects Hurum's team's conclusion about the phylogenetic position of Darwinius, but is sympathetic to Hurum's point of view concerning the quality of Ida: "Seiffert's team's complaint about Darwinius rings a bit hollow" because few have had access to the original fossil or the high-quality images that would help primate palaeontologists interpret Ida.

Rosenberger says this reflects a wider issue in vertebrate palaeontology. "[Sharing original data files] is simply not done, which is especially egregious in the US where taxpayer dollars so often subsidise the research," he says.

Journal reference: Nature, DOI: 10.1038/nature08429