Friday, November 13, 2015

[Paleontology • 2015] Probrachylophosaurus bergei • A New Brachylophosaurin Hadrosaur (Dinosauria: Ornithischia) with an Intermediate Nasal Crest from the Campanian Judith River Formation of Northcentral Montana

Probrachylophosaurus bergei 
Fowler & Horner, 2015
  Illustration: John Conway || DOI: 10.1371/journal.pone.0141304

Brachylophosaurini is a clade of hadrosaurine dinosaurs currently known from the Campanian (Late Cretaceous) of North America. Its members include: Acristavus gagslarsoni, which lacks a nasal crest; Brachylophosaurus canadensis, which possesses a flat paddle-shaped nasal crest projecting posteriorly over the dorsal skull roof; and Maiasaura peeblesorum, which possesses a dorsally-projecting nasofrontal crest. Acristavus, from the lower Two Medicine Formation of Montana (~81–80 Ma), is hypothesized to be the ancestral member of the clade. Brachylophosaurus specimens are from the middle Oldman Formation of Alberta and equivalent beds in the Judith River Formation of Montana; the upper Oldman Formation is dated 77.8 Ma.

Methodology/Principal Findings
A new brachylophosaurin hadrosaur, Probrachylophosaurus bergei (gen. et sp. nov.) is described and phylogenetically analyzed based on the skull and postcranium of a large individual from the Judith River Formation of northcentral Montana (79.8–79.5 Ma); the horizon is equivalent to the lower Oldman Formation of Alberta. Cranial morphology of Probrachylophosaurus, most notably the nasal crest, is intermediate between Acristavus and Brachylophosaurus. In Brachylophosaurus, the nasal crest lengthens and flattens ontogenetically, covering the supratemporal fenestrae in large adults. The smaller nasal crest of Probrachylophosaurus is strongly triangular in cross section and only minimally overhangs the supratemporal fenestrae, similar to an ontogenetically earlier stage of Brachylophosaurus. Sutural fusion and tibial osteohistology reveal that the holotype of Probrachylophosaurus was relatively more mature than a similarly large Brachylophosaurus specimen; thus, Probrachylophosaurus is not simply an immature Brachylophosaurus.

The small triangular posteriorly oriented nasal crest of Probrachylophosaurus is proposed to represent a transitional nasal morphology between that of a non-crested ancestor such as Acristavus and the large flat posteriorly oriented nasal crest of adult Brachylophosaurus. Because Probrachylophosaurus is stratigraphically and morphologically intermediate between these taxa, Probrachylophosaurus is hypothesized to be an intermediate member of the Acristavus-Brachylophosaurus evolutionary lineage.

Systematic Paleontology

Dinosauria Owen, 1842

Ornithischia Seeley, 1888
Ornithopoda Marsh, 1881

Hadrosauridae Cope, 1869
Hadrosaurinae Cope, 1869
Brachylophosaurini Gates et al., 2011
Definition: Modified from Gates et al. [3]: Hadrosaurine ornithopods more closely related to Brachylophosaurus, Probrachylophosaurus, Maiasaura, or Acristavus than to Gryposaurus or Saurolophus.

Referred material: UCMP 130139, a partial skull and skeleton originally described as the holotype of Brachylophosaurus goodwini [9], and later assigned to Brachylophosaurus canadensis [1, 23]. Due to the lack of a preserved nasal, and the presence of deep frontal depressions, the specimen cannot be confidently assigned to any current genus of Brachylophosaurini.

Horizon and locality: UCMP 130139 was collected from the Judith River Formation of Kennedy Coulee, Hill County, northcentral Montana, in beds equivalent to the lower Oldman Formation, with a published height of approximately 15 m above the Marker A Coal of the Taber Coal Zone of the Foremost Formation [9]. However, a remeasured section shows that the site was actually only a few meters above the Marker A Coal, and lies within the Herronton Sandstone Zone (Mark Goodwin and David Evans personal communication, 2014).

Brachylophosaurus canadensis Sternberg, 1953

Holotype: CMN 8893

Referred Material: FMNH PR 862 (partial skull); MOR 720 (braincase); MOR 794 (nearly complete articulated skeleton); MOR 940 (braincase); MOR 1071 (monodominant bonebed); TMP 90.104.01 (complete skull and articulated partial skeleton).

Horizons and localities: CMN 8893, FMNH PR 862, and TMP 90.104.01 were collected from the Oldman Formation of southeastern Alberta. CMN 8893 and FMNH PR 862 were collected in Dinosaur Provincial Park; TMP 90.104.01 was collected near Onefour and the Milk River. Of these Albertan specimens, the exact stratigraphic position is known only for CMN 8893: the Comrey Sandstone Zone (Unit 2) of the Oldman Formation. MOR 720 was collected from the upper Judith River Formation in badlands surrounding the Missouri River north of Winifred, Fergus County, central Montana. MOR 794, MOR 940, and MOR 1071 were collected from the Judith River Formation of Malta, northern Montana, in beds equivalent to the Comrey Sandstone Zone of the Oldman Formation.

Probrachylophosaurus gen. nov.

Etymology: Pro- (Latin) before, -brachylophosaurus (Greek) short-crested lizard, in reference to the new taxon’s stratigraphic position below that of Brachylophosaurus canadensis.

Probrachylophosaurus bergei sp. nov.

Etymology: Species name bergei in memory of Sam Berge, co-owner of the land where the specimen was discovered, and friend and relative of many members of the Rudyard, Montana community, who have supported paleontologic research for decades. Pronunciation: berg-ee-i

In the early years of dinosaur paleontology, specimens were collected as isolated points, each so morphologically unique that their evolutionary relationships were difficult to determine. As more fossil specimens are collected, the gaps in morphology that previously separated species are being filled. With larger sample sizes resulting in more continuous series of fossils with good stratigraphic resolution, variations in morphology can be analyzed in a more complete context, and attributed to ontogeny, evolution, taphonomic alteration, biogeography, or individual variation. A better understanding of stratigraphy and advancements in radiometric dating enable precise temporal correlation of geographically separated localities. Taxa can then be placed in temporal sequence, allowing tests of evolutionary hypotheses.

A precise stratigraphic framework is critical for determining whether morphological variations of adult specimens are due to evolution or are variations within a roughly contemporaneous population. If closely related taxa do not overlap stratigraphically, the pattern is more parsimonious with anagenesis than cladogenesis. Recent research has greatly increased the sample size and stratigraphic resolution of specimens from the Judith River and Hell Creek Formations of Montana and their Canadian equivalents, revealing several potential anagenetic lineages in Campanian and Maastrichtian ornithischians.

Because Probrachylophosaurus bergei is stratigraphically older than all Brachylophosaurus canadensis specimens, it is hypothesized to represent a basal brachylophosaur morphology, early in the evolution of this lineage from a non-crested ancestor. Thus, the small crest of Probrachylophosaurus would represent a transitional nasal morphology between a non-crested ancestor such as Acristavus and the larger crests of adult Brachylophosaurus. The fourth member of Brachylophosaurini, Maiasaura, would represent a cladogenic event, diverging from the lineage that led to Brachylophosaurus at a currently unknown point.

Elizabeth A. Freedman Fowler and John R. Horner. 2015. A New Brachylophosaurin Hadrosaur (Dinosauria: Ornithischia) with an Intermediate Nasal Crest from the Campanian Judith River Formation of Northcentral Montana. PLoS ONE. DOI: 10.1371/journal.pone.0141304  @PLOSPaleo @theJohnConway

MSU News - MSU team finds new dinosaur species, reveals evolutionary link:
New 'short-crested lizard' found in Montana via @physorg_com