Friday, March 09, 2007
Dinosaurs, Birds, and Evolution
Two interesting articles from the world of paleontology:
"Origin of Avian Genome Size and Structure in Non-Avian Dinosaurs"
Nature, 446, 180-184, 8 March 2007. Chris L. Organ, Andrew M. Shedlock, Andrew Meade, Mark Page, and Scott V. Edwards
Abstract:
Avian genomes are small and streamlined compared with those of other amniotes by virtue of having fewer repetitive elements and less non-coding DNA. This condition has been suggested to represent a key adaptation for flight in birds, by reducing the metabolic costs associated with having large genome and cell sizes. However, the evolution of genome architecture in birds, or any other lineage, is difficult to study because genomic information is often absent for long-extinct relatives. Here we use a novel bayesian comparative method to show that bone-cell size correlates well with genome size in extant vertebrates, and hence use this relationship to estimate the genome sizes of 31 species of extinct dinosaur, including several species of extinct birds. Our results indicate that the small genomes typically associated with avian flight evolved in the saurischian dinosaur lineage between 230 and 250 million years ago, long before this lineage gave rise to the first birds. By comparison, ornithischian dinosaurs are inferred to have had much larger genomes, which were probably typical for ancestral Dinosauria. Using comparative genomic data, we estimate that genome-wide interspersed mobile elements, a class of repetitive DNA, comprised 5–12% of the total genome size in the saurischian dinosaur lineage, but was 7–19% of total genome size in ornithischian dinosaurs, suggesting that repetitive elements became less active in the saurischian lineage. These genomic characteristics should be added to the list of attributes previously considered avian but now thought to have arisen in non-avian dinosaurs, such as feathers, pulmonary innovations, and parental care and nesting.
"What Pneumaticity Tells Us About 'Prosauropods', and Vice Versa." Mathew Wedel. Special Papers in Palaeontology 77, 2007, pp. 207–222.
Abstract:
Pneumatic (air-filled) bones are an important feature of the postcranial skeleton in pterosaurs, theropods and sauropods. However, there is no unambiguous evidence for postcranial pneumaticity in basal sauropodomorphs and even the ambiguous evidence is scant. Patterns of skeletal pneumatization in early sauropods and theropods suggest that basal saurischians had cervical air sacs like those of birds. Furthermore, patterns of pneumaticity in most pterosaurs, theropods and sauropods are diagnostic for abdominal air sacs. The air sacs necessary for flow-through lung ventilation like that of birds may have evolved once (at the base of Ornithodira), twice (independently in pterosaurs and saurischians) or three times (independently in pterosaurs, theropods and sauropods). Skeletal pneumaticity appears to be more evolutionarily malleable than the air sacs and diverticula that produce it. The evolution of air sacs probably pre-dated the appearance of skeletal pneumaticity in ornithodirans.
Why do I think these papers are important? Well, aside from being good science (Wedel's paper has been called "an exemplary paleobiological study"), they both indicate (once again) that structures can evolve for one purpose and then be used for another. Small genomes, for instance, evolved in non-avian dinosaurs (Saurischians, about 230-250 MYA), but were later crucial for birds (the Saurischian lineage is ancestral to Ornithodira). "Pneumaticity" (air-filled bones) also evolved in dinosaurs - some very large dinosaurs - but was equally useful to birds. Interestingly enough, air-filled bones may have independently evolved two or three times!
The genome article also points out that a number of avian traits are now associated with non-avian dinosaurs:
"These genomic characteristics should be added to the list of attributes previously considered avian but now thought to have arisen in non-avian dinosaurs, such as feathers, pulmonary innovations, and parental care and nesting."
"Pulmonary innovations" are, of course, what pneumaticity is all about.
The articles are interesting for another reason: dinosaurs are a very big deal in our house.
The news item in Nature announcing the genome article contains a slip of the tongue (pointed out to me by my 12-year-old son):
"An interesting next step would be to look at the genome sizes of the flying dinosaurs, the pterosaurs, which evolved flight independently of birds."
Pterosaurs aren't dinosaurs, they're flying reptiles.
"Origin of Avian Genome Size and Structure in Non-Avian Dinosaurs"
Nature, 446, 180-184, 8 March 2007. Chris L. Organ, Andrew M. Shedlock, Andrew Meade, Mark Page, and Scott V. Edwards
Abstract:
Avian genomes are small and streamlined compared with those of other amniotes by virtue of having fewer repetitive elements and less non-coding DNA. This condition has been suggested to represent a key adaptation for flight in birds, by reducing the metabolic costs associated with having large genome and cell sizes. However, the evolution of genome architecture in birds, or any other lineage, is difficult to study because genomic information is often absent for long-extinct relatives. Here we use a novel bayesian comparative method to show that bone-cell size correlates well with genome size in extant vertebrates, and hence use this relationship to estimate the genome sizes of 31 species of extinct dinosaur, including several species of extinct birds. Our results indicate that the small genomes typically associated with avian flight evolved in the saurischian dinosaur lineage between 230 and 250 million years ago, long before this lineage gave rise to the first birds. By comparison, ornithischian dinosaurs are inferred to have had much larger genomes, which were probably typical for ancestral Dinosauria. Using comparative genomic data, we estimate that genome-wide interspersed mobile elements, a class of repetitive DNA, comprised 5–12% of the total genome size in the saurischian dinosaur lineage, but was 7–19% of total genome size in ornithischian dinosaurs, suggesting that repetitive elements became less active in the saurischian lineage. These genomic characteristics should be added to the list of attributes previously considered avian but now thought to have arisen in non-avian dinosaurs, such as feathers, pulmonary innovations, and parental care and nesting.
"What Pneumaticity Tells Us About 'Prosauropods', and Vice Versa." Mathew Wedel. Special Papers in Palaeontology 77, 2007, pp. 207–222.
Abstract:
Pneumatic (air-filled) bones are an important feature of the postcranial skeleton in pterosaurs, theropods and sauropods. However, there is no unambiguous evidence for postcranial pneumaticity in basal sauropodomorphs and even the ambiguous evidence is scant. Patterns of skeletal pneumatization in early sauropods and theropods suggest that basal saurischians had cervical air sacs like those of birds. Furthermore, patterns of pneumaticity in most pterosaurs, theropods and sauropods are diagnostic for abdominal air sacs. The air sacs necessary for flow-through lung ventilation like that of birds may have evolved once (at the base of Ornithodira), twice (independently in pterosaurs and saurischians) or three times (independently in pterosaurs, theropods and sauropods). Skeletal pneumaticity appears to be more evolutionarily malleable than the air sacs and diverticula that produce it. The evolution of air sacs probably pre-dated the appearance of skeletal pneumaticity in ornithodirans.
Why do I think these papers are important? Well, aside from being good science (Wedel's paper has been called "an exemplary paleobiological study"), they both indicate (once again) that structures can evolve for one purpose and then be used for another. Small genomes, for instance, evolved in non-avian dinosaurs (Saurischians, about 230-250 MYA), but were later crucial for birds (the Saurischian lineage is ancestral to Ornithodira). "Pneumaticity" (air-filled bones) also evolved in dinosaurs - some very large dinosaurs - but was equally useful to birds. Interestingly enough, air-filled bones may have independently evolved two or three times!
The genome article also points out that a number of avian traits are now associated with non-avian dinosaurs:
"These genomic characteristics should be added to the list of attributes previously considered avian but now thought to have arisen in non-avian dinosaurs, such as feathers, pulmonary innovations, and parental care and nesting."
"Pulmonary innovations" are, of course, what pneumaticity is all about.
The articles are interesting for another reason: dinosaurs are a very big deal in our house.
The news item in Nature announcing the genome article contains a slip of the tongue (pointed out to me by my 12-year-old son):
"An interesting next step would be to look at the genome sizes of the flying dinosaurs, the pterosaurs, which evolved flight independently of birds."
Pterosaurs aren't dinosaurs, they're flying reptiles.
