Dinosaurs emerged during the last 30 million years of the Triassic
Period, and quickly rose to dominate terrestrial ecosystems, being the largest,
most diverse and most abundant group of terrestrial vertebrates during the
Jurassic and Cretaceous. However the earliest Dinosaurs and their relationship
to other Archosaurs are not well understood, as the group has a poor Triassic
fossil record, being known only from a few sites in western North America,
central and southern South America and Germany.
In a paper published in the journal Palaeontology on 16 April 2014,
Grzegorz Niedźwiedzki of the Subdepartment of Evolution and Development at UppsalaUniversity, Stephen Brusatte of the School of GeoSciences at the University of Edinburgh, Tomasz Sulej of the Institute of Paleobiology and Richard Butler of
the School of Geography, Earth and Environmental Sciences at the University ofBirmingham describe a series of specimens of Dinosauriform Archosaurs
(Archosaurs closely related to Dinosaurs, but not actually classified as such)
and Theropod Dinosaurs from the Zbązszynek Beds at Poręzba in southwest Poland.
The Poręzba locality has produced a large assemblage of terrestrial
and marine vertebrate fossils, as well as marine invertebrates and trace
fossils, coprolites and plant macrofossils, pollen and spores. The vertebrate
assemblage is dominated by Turtles, with Hybodontid Sharks, Actinopterygid
Fish, Temnospondyl Amphibians and Archosaurs also present. Most of the bone
fossils are fragmentary and rounded, being found in conglomerate layers, which
almost certainly implies they were reworked, though some well-preserved
isolated bones have been found in mudstone layers.
The first specimen reported is a proximal fragment (fragment from
the end closest to the torso) of the left femur of a Silesaurid Dinosauromorph Archosaur.
This specimen preserves the femoral head and part of the shaft of the bone. The
head is separated from the shaft by a notch, the head is triangular in proximal
view (i.e. looked at from the position of the joint that connected it to the
body), the groove traversing the proximal surface of the bone is straight, and
the bone has distinct anterior and dorsolateral trochanters (expand) which are
not connected by a shelf; all of which indicate Silesaurid affinities for the
bone.
Silesauridae indet. Proximal portion of a left femur. (A) Cranial
view. (B) Caudal view. (C) Lateral view. (D) Proximal view. Abbreviations: amt,
aneromedial tuberosity; at, anterior trochanter; dt, dorsolateral trochanter; h,
head; lt, lateral tuberosity; mg, medial groove; no, notch. Scale bars
represent 10 mm. Niedźwiedzki et al (2014).
The second specimen reported is a partial right scapulocoracoid of a
Herrerasaurid, a group previously thought to lie outside the Dinosauria, but
now thought to be early members of the group of uncertain affinities, possibly
within the Theropoda or the Saurischia. The scapulocoracoid is formed of a
fused scapula and coracoid without a visible suture, a feature seen in Herrerasaurids,
some early Neotheropods (Ceratosaurs plus Tetanurans) and some non-Dinosaurian
Dinosauromorphs, but not in early non-Neotheropod Theropods, basal Sauropodomorphs
(unless Herrerasaurids are basal Sauropodomorphs) most early Ornithischians and
most nonavian Tetanuran Theropods. The fusion in this specimen is very strong,
making it impossible to see how much of each bone is present.
The acromion region of the scapula extends dorsally at nearly 90˚ to
the blade of the scapula, a feature found in Herrerasaurids and some early Sauropodomorphs
and Theropods, but different from the condition in most Dinosaurs, where this
angle tends to be very obtuse (i.e. much larger than 90˚). The dorsal margin of
the scapula is concave, though this is seen in a wide variety of Archosaurs, including
most early Dinosaur groups. The bone has a thick, elongate ridge in the centre
of its medial surface, a feature seen in other Herrerasaurids; a less prominent
ridge has also been reported in some early Theropods, but possibly also in some
Sauropodomorphs.
Herrerasauridae indet. Right scapulocoracoid. (A) Lateral
view. (B) Medial view. Abbreviations: ac, acromion process; eri, epoxy resin
infill; fo, fossa; lc, medial longitudinal crest; r, ridge; s, scapula blade.
Scale bar represents 10 mm. Niedźwiedzki et
al (2014).
The third specimen reported is a proximal fragment of the femur of a
Herrerasaurid, with the tip of the femur head missing and the bone extending to
the centre of the fourth trochanter. The head is offset from the shaft, a feature
typical of Dinosaurs. A dorsolateral trochanter is present on the lateral
surface of the femur head, a feature seen in most Dinosauromorphs (Dinosaurs
and Dinosauriform Archosaurs), this trochanter being flat with a low ridge at
its anterior margin, a feature seen in Herrerasaurids and some early Theropods
and Sauropodomorphs, but not in non-Dinosaurian Dinosauromorphs or early Ornithischians.
There is a faint ligament sulcus on the underside of the femoral head, which
has been recorded in some other early Dinosaurs. A spike-like trochanter is
present on the anterior surface of the proximal end of the specimen, which is
typical of Dinosaurs. However this trochanter lacks the distinctive dorsal
process seen in most Theropods, as does that of other Herrerasaurids. The
specimen also has a crescent-shaped trochanteric shelf on the anterolateral
edge of the proximal femur, a feature recorded previously in the Herrerasaurid Chindesaurus, from the Late Triassic of
North America, but otherwise not known in any other Dinosaur or close Dinosaur
relative. The fourth trochanter forms a prominent flange-like ridge, typical of
Dinosaurs but distinct from the mound-like fourth trochanter of Non-dinosaurian
Dinosauromorphs and Crocodile-line Archosaurs (i.e. Archosaurs more closely
related to Crocodiles than to Birds).
Herrerasauridae indet. Proximal portion of a right
femur. (A) Lateral view. (B) Posterior view. (C) Anterior view. (D) Close up of
area with trochanteric shelf and anterior trochanter in oblique anterolateral
view. Abbreviations: at, anterior trochanter; dt, dorsolateral trochanter; ft,
fourth trochanter; ts, trochanteric shelf; pi, pit; sr, scar. Scale bars
represent 10 mm. Niedźwiedzki et al (2014).
The fourth specimen reported is a proximal portion of the left
fibula of a Herrerasaurid. This is transversely compressed, with a shallow
depression for the joint with the fibular condyle of the tibia. The proximal
end of the bone tapers to a point in proximal view, a feature seen in all
Avian-line Archosaurs (i.e. Archsaurs more closely related to Birds than to
Crocodiles). The medial surface is shallowly concave, typical of early
Dinosaurs, but lacks the deep fossa seen in more derived Theropods. There is a
distinct notch on the anterior edge of the bone, a feature seen in other Herrerasaurids,
but not in other Dinosaurs or close Dinosaur relatives.
Herrerasauridae indet. Proximal portion of a left
fibula. (A)Lateral view. (B) Medial view. (C) Proximal view. Abbreviations: no,
notch; tu, tuberosity; de, depression. Scale bar represents 10 mm. Niedźwiedzki et al (2014).
The fifth specimen recorded is an articulated left pelvis of an
indeterminate Theropod. This has an open acetabulum (gap), which is the usual
state in Dinosaurs, though this is partially closed in some early members of
the group. The ilium, pubis and ischium are fused into one bone, but with the
sutures visible as convex bulges. This is common in early Neotheropods and seen
in some Herrerasaurids, but otherwise very unusual in Archosaurs. The
supraacetabularcrest of the ilium is clearly separated from the postacetabular
process, a common state in many non-Dinosaurian Dinosauromorphs, and early
Dinosaurs of several groups, including Saurischians, Theropods, Herrerasaurids,
Sauropodomophs and Ornithischians; however these features are fused in some
early Neotheropods and in most Ceratosaurs. The specimen has a concave notch on
the posterior margin of the ischial peduncle of the ilium, a feature seen in
some other early Theropods, but absent in non-Dinosaurian Dinosauromorphs, Herrerasaurids,
Sauropodomorphs, Ornithischians and most derived Neotheropodssuch as
Allosauroids and Coelurosaurs. The suture between the ischium and the ilium
extends horizontally at the same level as the suture between the ilium and the
pubis, a condition seen in some early Theropods and other early Dinosaurs, but
absent from more derived TetanuranTheropods. The ventral margin of the acetabulum
of the specimen is largely formed from the ischium, forming a finger-like
elongate process. This is also seen in various Coelophysoids and some Tetanurans,
but is absent in Herrerasaurids and early Sauropodomorphs and close Dinosaur
outgroups. It is present in some early Ornithischians, but the structure is
quite distinct.
Theropoda indet. An articulated left pelvis. (A) Lateral
view. (B) Medial view. Abbreviations: aop, acetabulum opening; il, ilium; ilat,
ilium antitrochanter; isat, ischium antitrochanter; is, ischium; isp, ischial
peduncle; ispp, ischial pubic peduncle; no, notch; eri, epoxy resin infill; pu,
pubis; pup, pubic peduncle; puip, pubic ischial peduncle; sac, supraacetabular
crest; sat, sacral articulation surface. Scale bar represents 10 mm. Niedźwiedzki et al (2014).
The final specimen recorded is the distal end of a right tibia,
thought to be from a Neotheropod Dinosaur. This has a distinct oblique ridge on
its anterior surfacewith a lateral malleolus, which probably contacted the
fibula of the living animal during articulation, which is seen in other
Dinosaurs and close Dinosaur relatives, but not Crocodile-line Archosaurs. A
large posterolateral process projecting laterally is present, something seen in
Neotheropods but not in other early Saurischian Dinosaurs. The posterior margin
of the tibia is divided into two surfaces divided by a distinct ridge, which is
the case in most Dinosaurs but not close Dinosaur relatives or Herrerasaurids.
The posterolateral surface is distinctly concave, as is seen in Neotheropods,
Sauropodomorphs and Ornithischians but not in non-Dinosaurian Dinosauromorphs
or Herrerasaurids.
?Neotheropoda indet. Distal portion of a right tibia. (A) Anterior
view. (B) Posterior view. (C) Distal view. Abbreviations: adt, anterior diagonal
tuberosity; af, articular facet for the ascending process of the astragalus;cf,
concave facet; lc, lateral condyle; mc, medial condyle; plp, posterolateral
process; r, ridge. Scale bar represents 10 mm. Niedźwiedzki et al (2014).
See also…
The earliest known Dinosaurs appear in the fossil record appear in the
Late Carnian (beginning of the Late Triassic, 230 million years ago) of
Argentina. By the end of the Carnian Theropods, Sauropods and
Ornithischians are all known. The Silesaurids, considered the closest
relatives of the Dinosaurs, appear in the fossil record in the Late
Anisian, about 245 million years ago, in South America and southern
Africa, suggesting that Dinosaurs originated in southern Pangea in the...
Archosaurs are the group of Vertebrates that include...
Archosaurs are the group of Vertebrates that include...
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