Mousebirds are an extraordinary group of creatures. The six species in the world today are found only in sub-Saharan Africa and are considered living fossils, retaining an unusually high degree of similarity to their ancient ancestors. Hence, understanding the mousebird’s evolutionary history is of special interest to scientists, and particularly to Daniel T. Ksepka, a researcher at North Carolina State University, whose recent study of a new fossil species of mousebird has provided surprising insights into the bird’s evolutionary past and ecological diversity.
Ksepka described the new species, Celericolius acriala (shown right; arrows point to feather traces), in late 2010 in a paper published in the Zoological Journal of the Linnean Society. The species was identified from a fossil specimen collected and acquired from rock formations near the Green River by Lance Grande, a curator and paleontologist at the Field Museum in Chicago. The Green River Formation cuts through parts of Wyoming, Utah, and Colorado.
Identifying the New Species
“The first clue was the arrangement of the toes,” Ksepka said about the discovery. The first few toe bones (phalanges) were short, whereas the last ones at the end of the toes were long—proportions that are identical to those of living mousebird toes.
The distinct toe arrangements of mousebirds underlie the birds’ unusual perching behavior. “If you have ever seen mousebirds in the wild or in a zoo, the first thing that pops out about them is their unique climbing style,” Ksepka said. “While typical birds perch upright by grasping branches with both feet, you will often find mousebirds supported by one foot, hanging upside down, or clinging to the side of a branch with their feet straight out in front of them.”
“Another key feature of mousebirds is their incredibly elongated tail feathers, which account for two-thirds of the bird’s total length,” Ksepka explained. “The fossil preserved not only carbonized traces of very long tail feathers but also a strongly expanded pygostyle (tail bone) for anchoring them.” After using computer-aided analysis to compare the fossil to living and extinct mousebirds, Ksepka and his team confirmed their suspicions—they had identified a new species.
“One of the interesting findings is that this mousebird provides solid evidence for another new body plan in an extinct mousebird,” Ksepka said. Indeed, the long wings of Celericolius acriala are highly unusual for mousebirds. “Most living mousebirds have short wings and don’t fly very much, unless they are startled. [They prefer instead] to creep about through the vegetation. This behavior earned them the common name mousebird.”
The long wings of Celericolius acriala suggest that this species was capable of flying continuously for long periods, possibly even feeding on insects on the wing. “A fossil species from Europe named Oligocolius hinted that some ancient mousebirds might have had longer wingshapes,” Ksepka added. “But until the new discovery we did not have the feather traces to confirm this.”
Reconstructing the Mousebird Evolutionary Tree
Ksepka explained that an important facet of studying fossils involves determining whether they belong in so-called stem or crown parts of the evolutionary tree. “Crown fossils are very close relatives of the living species, while stem fossils branched off before the common ancestor of the living species appeared,” he said.
Along with colleague Julia Clarke, a paleontologist based at the University of Texas at Austin, Ksepka studied the evolutionary characteristics of the Celericolius specimen and of specimens of another type of mousebird known as Palaeospiza. “Both are stem mousebirds, which means that they occupy ‘dead’ branches near the base of the mousebird tree,” he said. “They left no descendants.” This finding fits with the fact that there are no extant mousebird species in North America.
However, the evolutionary tree of mousebirds also suggests that fossil species from North America, rather than forming a separate group, are positioned on the same branches as those from Europe. This is surprising particularly because living mousebirds are sedentary and do not migrate long distances.
But as Ksepka described, “The pattern of evolutionary relationships we found indicates that ancient mousebirds must have traveled back and forth between the continents at least five times. It is a surprise to find fossil evidence for such ‘stay-at-home’ birds traversing large distances repeatedly in the past.”
From the evolutionary tree, the team was also able to reconstruct the sequence of skeletal adaptations that occurred along the line from primitive to modern species. “In this case we found that the specialized feet and tail evolved very early in mousebird history, millions of years prior to the evolution of specialized wings like those of Celericolius or the shortened beak seen in modern mousebirds,” Ksepka explained.
Ecological Diversity of Mousebirds
Ksepka’s newly identified mousebird species joins a fairly long list of unique fossil mousebirds. Many of these specimens have been described from Europe and North America by researchers such as Gerald Mayr, curator at the Senckenberg Natural History Museum in Frankfurt, Germany, and biologists Peter Houde, at New Mexico State University, and Storrs Olson, at the Smithsonian Institution.
Ksepka’s work also adds to the growing body of scientific literature on the ecological diversity of fossil mousebirds. “One example is Chascacocolius, which had a powerful pointed beak with incredibly long retroarticular processes—essentially levers for opening the beak,” he said. “Chascacocolius may have used this unusual beak for breaking open fruits or probing in the ground.”
Still, despite all that is known about mousebirds, Ksepka said that it remains unclear why there are so few mousebird species in existence today. “Part of the reason may be environmental change in North America and Europe. Subtropical forests favored by the extinct species were replaced by temperate forests and more arid environments over the Cenozoic [the geologic period from 65.5 million years ago and present-day]. Songbirds also became a dominant part of the Northern Hemisphere avifauna later in the Cenozoic, and this group may have out-competed mousebirds for some niches.”
Ksepka is now studying other examples of North America’s ancient avifauna. “There is a treasure trove of Green River bird fossils to study, and each one could yield new clues into how different groups of birds changed over time and why the Eocene North America avifauna is so different from that of today.” He added, “Julia Clarke, Lance Grande, and I are now working on an extinct species that may have been related to parrots.”
Photo credits (from top): Photo by Lance Grande of the Field Museum; George Wall/NHPA.
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