In recent decades paleontologists have been able to document in ever-greater detail the evolution of birds from small predatory dinosaurs. After establishing the broader outlines of this transition, researchers are now focusing on specific adaptations.
It is widely believed that birds have no or at most a poorly developed sense of smell. A new study by a team of Canadian and American researchers led by Darla Zelenitsky (University of Calgary), published in the online version of Proceedings of the Royal Society B earlier this month, has shown that, in fact, early birds inherited a rather well developed sense of smell from dinosaurs.
Previous research had suggested that, as birds evolved flight, their senses of sight and balance became increasingly better developed but their sense of smell diminished. The new study by Zelenitsky and her colleagues tested this hypothesis by examining the change in size of the olfactory bulbs, the portion of the brain that receives information on odors, across the evolutionary transition from predatory dinosaurs to birds. It drew on the results of an ongoing long-term research effort by Larry Witmer and his colleagues at Ohio University to reconstruct in detail the structure of the brain and associated structures in dinosaurs and their relatives through high-tech imaging and 3D visualization of their skulls.
The new study has demonstrated that maniraptoran dinosaurs (the group including Velociraptor, the “raptor” of Jurassic Park) had fairly well developed senses of smell based on the relative sizes of their olfactory bulbs. The relative size of the olfactory bulbs appears to have remained unchanged across the evolutionary transition from these dinosaurs to birds and even increased during the early history of birds.
Early anatomically modern birds (Neornithes, which includes all present-day species of birds) still had moderately large olfactory bulbs. Lithornis, a 58-million-year-old relative of ratites (ostriches and their kin) and tinamous, is an excellent example. The relative size of its olfactory bulbs indicates that smell was important for locating food.
Figure 1. Skulls (with area occupied by the brain in blue) of a maniraptoran dinosaur (Bambiraptor), the extinct bird Presbyornis, and a pigeon and a digital reconstruction of the brain of the extinct bird Lithornis.
Among present-day birds, kiwis, ducks, flamingos, vultures, and certain seabirds have relatively large olfactory bulbs. However, the most species-rich group of present-day birds, the passerines (such as crows and sparrows), has very small olfactory bulbs. This may have led to the assumption that birds generally have a poor sense of smell and that a well-developed sense of smell represents an evolutionary specialization. The work by Zelenitsky and her colleagues has now established that quite the opposite is the case: a well-developed sense of smell was the ancestral condition for birds, retained from dinosaurs.
In the same week when the work by Zelenitsky and her team appeared, Lars Schmitz and Ryosuke Motani (University of California at Davis) published a study on eye size and daily activity patterns in dinosaurs and their relatives in Sciencexpress.
Schmitz and Motani used the size of the bony eye socket (orbit) and the outer and inner diameters of the scleral ring to assess eye size. A feature found in many non-mammalian vertebrates, the scleral ring comprises a circle of small, overlapping plates of bone that reinforce the corneal portion of the outer layer of the eyeball (sclera) and are thought to assist in changing the curvature of the cornea during focusing. The ring is sometimes still intact in exceptionally preserved fossils.
Studies on present-day vertebrates have established a correlation between eye shape and image formation by the eye. The latter relates to the brightness of the image projected on the retina. Day-active (photopic) animals have a fairly small aperture and a small internal diameter of the scleral ring for a given eye size. This prevents excessive light stimulation of the retina and improves visual acuity. By contrast, night-active (scotopic) animals have large aperture to let more light reach the retina and the scleral ring has a proportionately large internal diameter.
Schmitz and Motani found that dinosaurs, early birds, and pterosaurs (flying reptiles) included day-active and night-active forms. In one occurrence renowned for the exquisite preservation of its vertebrate fossils, the Upper Jurassic Solnhofen limestones of Bavaria (about 150 million years old), they found that some species of pterosaurs (such as Scaphognathus) were day-active whereas other species were nocturnal. Given generally similar modes of feeding, this difference in activity patterns would have allowed these different species of flying reptiles to co-exist. The oldest known bird, Archaeopteryx, also from the same geological formation, was day-active based on the dimensions of its scleral ring.
Many present-day land predators are active at dusk or at night, and thus it is not entirely unexpected that the new study found Velociraptor and other small predatory dinosaurs to be night-active (scotopic). (These predators also had relatively large olfactory bulbs based on the work by Zelenitsky and her team.) A number of other dinosaurs, including both carnivores and herbivores (such as the horned dinosaur Protoceratops), apparently were active in low but not quite dark conditions (mesopic).
Figure 2. Scleral rings in the eye sockets of the skulls of the pterosaur Scaphognathus (top, enhanced with blue outline) and the early horned dinosaur Protoceratops (below). Scaphognathus was day-active (photopic) whereas Protoceratops was active in low light conditions (mesopic).
This new research dispels the traditional notion that dinosaurs were day-active whereas mammals were nocturnal. In fact, dinosaurs (and pterosaurs) were active during the day and at night, and the picture of ecological interactions among Mesozoic land vertebrates is more complex than previously assumed.
Hans-Dieter (Hans) Sues is a vertebrate paleontologist based at the National Museum of Natural History in Washington, D.C. He is interested in the evolutionary history and paleobiology of vertebrates, especially dinosaurs and their relatives, and the history of ecosystems through time. A former member of the National Geographic Committee for Research and Exploration, Hans has traveled widely in his quest for fossils and loves to share his passion for ancient life through lectures, writings, and blogging.