An emergent field of research in dinosaur paleobiology investigates the relative importance of linear, non-branching evolution (anagenesis) compared with branching evolution (cladogenesis). Increasingly, paleontologists are discovering that many dinosaur species are arranged into anagenetic lineages of rapidly evolving "transitional" species which do not overlap in time. These transitional species usually differ only slightly from their forebears, typically in the shape and size of display structures such as horns or crests.
At the SVP 2015 annual meeting, Dr Elizabeth Freedman Fowler of the Museum of the Rockies, and Great Plains Dinosaur Museum, Montana, presented on two new transitional species of duckbilled (hadrosaurid) dinosaurs from the Upper Cretaceous Judith River Formation of northern Montana.
"This particular part of the Judith River Formation is important because it yields a dinosaur fauna that is intermediate in age between an older fauna from the Two Medicine Formation of western Montana, and a younger fauna from the famous Oldman and Dinosaur Park Formations of Alberta". Dr Freedman Fowler continued, "because the formation is intermediate in age, then it is exactly where we might expect to find new, intermediate kinds of dinosaur".
The first new dinosaur described by Dr. Freedman Fowler is a transitional form of Brachylophosaurus, a large hadrosaurid with a broad paddle shaped crest over the back of its head. The second, a transitional Gryposaurus hadrosaurid, has an arched nasal crest on its snout, and is known from a bonebed of at least 10 individuals, ranging from juvenile to adult.
"The Gryposaurus bonebed is a fantastic site because the different growth stages show us that as Gryposaurus grew, the arch on its nose gets larger and moves backward, so that the nose of a one year old looks very different from the nose of a three year old", Dr. Freedman Fowler explained, "the most interesting thing is that we see the same morphological trend through time as Gryposaurus evolves. The preceding species from the Two Medicine Formation has a low crest over the middle of the nose, whereas in the succeeding species the crest is taller and more retracted towards the eyes"
This suggests that successive generations of Gryposaurus grew larger crests by changing the timing or pace of crest development during growth. Fragmentary juveniles of the transitional Brachylophosaurus species suggest that this same process is also occurring in that lineage. Changing of timing or rate of development is called heterochrony, a process which is being increasingly recognized as a major driving force in evolution.
"Heterochrony is key to understanding how evolution actually occurs in these dinosaurs, but to study heterochrony we need large collections of dinosaurs with multiple growth stages, and a really precise time framework for the rock formations that we collect them from" said Dr. Freedman Fowler.
This kind of research has only really become possible with recent technical advances in the radiometric dating of rocks, coupled with increased intensity of fossil collecting in North America.
"The Late Cretaceous of western North America is the only place in the world where we can do these kinds of intense paleobiological studies on dinosaurs. Nowhere else combines the precise dating of rocks coupled with an exceptional fossil record that has been so extensively collected."
Dr. Freedman Fowler predicts that many more transitional species remain to be discovered; "We've been collecting dinosaurs in this region for over a century, yet there are still exciting discoveries being made every year".
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AUTHOR CONTACT INFORMATION
ELIZABETH FREEDMAN FOWLER
Montana State University and Museum of the Rockies
Bozeman, MT, USA
eafreedman@gmail.com
ANAGENESIS AND ONTOGENY OF HADROSAURINE DINOSAURS IN THE CAMPANIAN (LATE CRETACEOUS) WESTERN INTERIOR OF NORTH AMERICA: TWO NEW TRANSITIONAL TAXA FROM THE JUDITH RIVER FORMATION OF MONTANA
FREEDMAN FOWLER, Elizabeth A., Montana State University and Museum of the Rockies, Bozeman, MT, USA
The Hadrosaurinae are a diverse clade of dinosaurs that were abundant across the Campanian Western Interior of North America, and are thus an ideal group for studying high-resolution evolutionary trends. Here I present two new hadrosaurine taxa from the Judith River Formation, Kennedy Coulee, Montana (equivalent to the lower Oldman Formation, Alberta). Phylogenetic and geometric morphometric analyses, combined with recalibrated radiometric dates, demonstrate that the new taxa form morphologic and stratigraphic intermediates within the lineages of Gryposaurus and Acristavus-Brachylophosaurus. The new species of Gryposaurus is from a monodominant bonebed of at least ten individuals and three size classes: juvenile, subadult, and adult. Its dentary tooth width and the shape and position of its nasal crest are morphologically and stratigraphically intermediate between G. latidens (lower Two Medicine Formation) and G. notabilis (lower Dinosaur Park Formation). In the stratigraphically lowest Gryposaurus species, G. latidens, the nasal crest is low and anterodorsal to the posterior narial fenestra. The nasal crest becomes progressively higher and more posteriorly located in stratigraphically younger species. A similar trend occurs ontogenetically; small specimens have relatively anteriorly located low nasal crests that grow dorsally higher and migrate posteriorly in larger specimens of the same taxon. The new genus of brachylophosaurin has a short posteriorly-oriented nasal crest hypothesized as an intermediate evolutionary state between the stratigraphically lower (lower Two Medicine Formation) crestless Acristavus and the stratigraphically higher (middle Oldman Formation) Brachylophosaurus, with its wide posteriorly elongated crest. The nasal crest of Brachylophosaurus elongates posteriorly ontogenetically. Histologic analysis demonstrates that the holotype of the new genus is relatively more mature than the largest Brachylophosaurus specimen, so its smaller crest size is not due to the ontogenetic status of the holotype. Thus, in Gryposaurus and Acristavus-Brachylophosaurus lineages, directional trends in nasal crest morphology are observed both through ontogeny and between stratigraphically separated non-overlapping taxa, suggesting that the new taxa are transitional members of anagenetic lineages, and that the evolution of cranial display structures in hadrosaurines proceeds by heterochrony. Grant Information Geological Society of America, Ameya Preserve, N. Myhrvold, D. Sands, E. Short, D. Wagoner, Royal Ontario Museum, University of California Museum of Paleontology