Hadrosaurs were large,
herbivorous Ornithischian Dinosaurs, commonly referred to as
'Duck-billed Dinosaurs', which dominated many Dinosaur faunas in the
Late Cretaceous of North America. They were split into two groups,
the solid-crested Hadrosaurines and the hollow-crested
Lambeosaurines, with the Hadrosaurines appearing slightly earlier and
dominating earlier faunas and the Lambeosaurines appearing slightly
later then slowly rising to prominence. The Brachylophosaurins were a
small, but locally abundant, group of Hadrosaurine Dinosaurs known
from the Campanian (83.6 to 72.1 million years ago) of Montana and Alberta in North America.
There are three described species, Acristavus gagslarsoni,
which lacked a crest and which is known only from the lower part of
the Two Medicine Formation of Montana, making it about 81.4 million
years old, and two later crested forms, Brachylophosaurus
canadensis, known from the Oldman Formation of Albeta and the
Judith River Formation of Montana with all specimens thought to be at
least 77.76 million years old, and Maiasaura
peeblesorum, known from the upper part of the Two
Medicine Formation, and thought to be about 76.66 million years old.
In a paper published in
the journal PLoS One on 11 November 2015, Elizabeth Freedman Fowler
and John Horner of the Museum of the Rockies and the Department of Earth Sciences at Montana State University describe a new species of
Brachylophosaurin Hadrosaur from the Kennedy Coulee exposure of the
Judith River Formation in Hill County in northern Montana.
The new species is
described from a single disarticulated but almost complete specimen
from Kennedy Coulee, though a second fragmentary subadult specimen is
also referred to. It is named Probrachylophosaurus bergei,
where 'Probrachylophosaurus' means 'before Brachylophosaurus'
(Brachylophosaurus itself means 'short-crested Lizard'), as
was found in deposits earlier than any of those which have produced
specimes of Brachylophosaurus,
and may be ancestral to that species, and 'bergei'
honours the late Sam Berge, co-cowner of the land where the specimen
was found. The post-cranial skeleton was discovered in 1981 by Kyoko
Kishi and subsequently excavated by a University of California Museumof Paleontology team led by Mark Goodwin between 1981 and 1984. The
skull was discovered in 2007 and subsequently excavated by a Museum
of the Rockies team, in 2007-8, with the California Museum of
Paleontology material subsequently being donated to the Museum of
the Rockies.
Probrachylophosaurus
bergei skull
reconstruction. (A) Preserved skull elements, left lateral view.
Predentary not included due to its poor preservation and diagenetic
compression. (B) Outline of skull reconstruction, left lateral view.
The outline accounts for diagenetic distortion of the posterior
braincase, but otherwise does not correct for distortion of skull
elements. Outlined regions where left skull material was not
preserved are based on right bones when available. Regions with
neither left nor right material preserved are hypothesized
reconstructions based on Brachylophosaurus canadensis
skulls. (C) Braincase with left
nasal crest, dorsal view. (D) Outline of braincase reconstruction
with nasal crest, dorsal view. Reconstruction accounts for diagenetic
lateral compression and distortion of posterior braincase.
Abbreviations: d, dentary; ex, exoccipital; f, frontal; j, jugal; l,
lacrimal; m, maxilla; n, nasal; p, parietal; pd, predentary; pf,
prefrontal; pm, premaxilla; po, postorbital; q, quadrate; qj,
quadratojugal; sa, surangular; sq, squamosal. Freedman Fowler &
Horner (2015).
Brachylophosaurus
canadensis has a
large, backward-pointing, paddle-like crest, flattened and triangular
in cross-sectiom, derived entirely from the fused nasal bones, which
completely covers the supratemporal fenestrae in mature adults.
Maiasaura peeblesorum
has an upward pointing crest,
derived from the frontal and nasal bones, while Acristavus
gagslarsoni lacks any form of
crest. The crest of Probrachylophosaurus bergei is
also backward-pointing, flattened and derived from the nasal bones
only, though it is smaller than in Brachylophosaurus
canadensis and does
not cover the supratemporal fenestrae. This is also the state in
sub-adult specimens of Brachylophosaurus
canadensis (younger
juveniles lacked a crest at-all), though the Probrachylophosaurus
bergei specimen is larger than
most adult Brachylophosaurus
canadensis specimens,
supporting the idea that it is a separate species rather than a
sub-adult Brachylophosaurus
canadensis specimen.
The
location where the Probrachylophosaurus bergei
specimen was located has been dated to between 78.2 and 78.5 million
years old, making it intermediate in age between Acristavus
gagslarsoni and
Brachylophosaurus
canadensis. The state
of the crest in Probrachylophosaurus bergei
is also intermediate between that seen in Acristavus
gagslarsoni and that seen in
Brachylophosaurus
canadensis, supporting
the idea that the three species form an evolutionary lineage, with
Probrachylophosaurus bergei
descended from Acristavus gagslarsoni and
ancestral to Brachylophosaurus
canadensis.
Ontogenetic
and anagenetic hypothesis of Brachylophosaurin evolution.
Probrachylophosaurus bergei
is proposed as an intermediate member of the lineage leading from
Acristavus gagslarsoni to
Brachylophosaurus canadensis.
Shaded blue areas indicate known elements of Probrachylophosaurus.
The reconstruction of the MOR 1071 Brachylophosaurus
skull is a composite of an articulated skull roof with a scaled-down
copy of the MOR 794 skull outline. All skulls are scaled to the same
10 cm scale bar. The horizontal axis is not to scale; the MOR 1071
reconstruction is much closer to MOR 794 in size and hypothesized
maturity than MOR 1097 is to MOR 2919. Freedman Fowler & Horner
(2015).
The
left tibia of the specimen was sectioned to determine the age of the
specimen. Many Dinosaurs show lines of arrested growth on their long
bones, similar to the rings of trees, caused by seasonality of the
climate when they were living, having a good growth season and a poor
one (probably either a cold season or a dry one, depending on where
the species lived). Unlike modern Mammals and Birds, most non-Avian
Dinosaurs continued to grow throughout their lives, with a
rapid-growth early juvenile period followed by a longer adulthood in
which growth was much slower.
The
tibia of Probrachylophosaurus bergei
shows fourteen lines of growth, indicating that it was fourteen years
old when it died. The outermost four of these growth lines are
closely spaced, indicating that the individual reached maturity at
about ten years of age. Two of the juvenile years, six and seven,
show much reduced growth periods, suggesting that these years were
probably much more stressful than other years, with limited access to
resources. The eighth year, in contrast, appears to have been
particularly mild, with intervals of growth during the non-growing
season, possibly indicating a milder than usual winter.
Tibial
histology of MOR 2919, mid-diaphyseal cross-section. Numbered green
arrows identify lines of arrested growth (LAGs). The closely spaced
outer LAGs are indicated with tick marks along a green line. Numbers
around the circumference of the cross-section indicate the number of
LAGs preserved in that radial segment of the tibia. Note that
diagenetic crushing has displaced several segments radially inward.
Thumbnail image in lower right corner colors the area between each
pair of LAGs to highlight locations of radial displacement as well as
to indicate bone deposition between later LAGs occurring primarily
along the posteromedial region. Freedman Fowler & Horner (2015).
Probrachylophosaurus
bergei also shows a number of
pathologies (old injuries dating to before the time of death). Both
dentaries (lower jaw bones) show circular depressions surrounded by
rings of raised growth. Six of the tail vertebrae signs of damage and
subsequent healing, and since these come from an incomplete section
of the skeleton other, missing vertebrae may have also been damaged.
The metatarsals (foot bones), tibia and fibia also show signs of old
injuries. These pathologies will be the subject of a future study.
See
also...
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