As a totally extinct group, stegosaurs
are only known from fossils. Clear stegosaur fossils are known
from North America, Europe, Asia, and Africa, while questionable
specimens have also been reported from South America, Australia, and
India. Stegosaur fossils have not been reported from Antarctica,
which is likely a reflection of sampling bias (little palaeontological
work has been carried out on the continent) rather than actual absence.
The earliest known stegosaur body fossils come from the Bajocian,
a stage of the Middle Jurassic. However, putative stegosaur
footprints have been reported from the Early Jurassic of France and
Australia. The youngest stegosaur fossils are Early Cretaceous in
age, and the absence of stegosaur remains from well-sampled middle-Late
Cretaceous units worldwide strongly indicate this group went extinct
during the beginning stages of the Cretaceous. Below are brief
descriptions of the stegosaur record on particular continents.
North America
The characteristic stegosaur
Stegosaurus, as well as
Hesperosaurus,
are known from the Upper Jurassic Morrison Formation, a series of
channel and floodplain sandstones and mudstones that crops out in 13
different states in the western United States. Taphonomic studies
indicate that
Stegosaurus
fossils overwhelmingly occur in channel sandstones, which some
scientists argue indicates that these dinosaurs likely inhabited
floodplains and were separated ecologically from sauropods. Most
Stegosaurus
fossils are disarticulated elements preserved with disarticulated
elements of other dinosaur species, but a spectacular assemblage of
nearly complete
Stegosaurus
fossils at one site in Colorado may reflect drought-induced mass
mortality. In the Morrison, Stegosaurus fossils are found
alongside those of several other dinosaurs, including the predatory
Allosaurus, as well as
Camptosaurus,
Apatosaurus,
Camarasaurus, and
Diplodocus. The photo shows a team of American scientists working in the Morrison Formation.
Asia
The highest diversity of stegosaurs comes from Asia, with at least five
valid genera (and several fragmentary specimens that questionably have
been named) known from the People's Republic of China. In
general, many Chinese stegosaurs are known from more complete specimens
than those genera from North America and Europe. The likely basal
stegosaur
Huayangosaurus is
known from the Middle Jurassic of Sichuan, while most other Chinese
stegosaurs are known from the Late Jurassic of this province. The
large number of named China stegosaurs prompted some authors to argue
that this group must have originated in Asia, but a recent study by
Susannah Maidment finds many of these genera to be based on remains
that are too fragmentary to be reliably named. Thus, this high
diversity is somewhat of a mirage, although even with this reassessment
there are still more stegosaurs known from Asia than from any other
continent. The photo shows a small village in rural Tibet, which
was the site of a stegosaur discovery in the 1970s.
Africa
The stegosaur
Kentrosaurus
(left, photo courtesy S.C.R. Maidment) is represented by a plethora of
specimens from the Upper Jurassic Tendaguru Formation of Tanzania, yet
when compared with other Tendaguru dinosaurs this stegosaur is a
relatively minor element of the fossil fauna. Remains of this
stegosaur are known from nearshore marine sediments, and some
particular quarries have yielded the remains of multiple individuals of
several size classes. Stegosaur material has also been reported
from Middle Cretaceous rocks in Morocco, but this referral is disputed.
If substantiated, this specimen would extend the range of
stegosaurs and push their known extinction date into the Middle
Cretaceous.
Europe
The earliest descriptions of stegosaurs were based on specimens from
the Early Cretaceous of England, described by Gideon Mantell in the
1840s. As with Mantell's
Megalosaurus
(one of the first dinosaurs to be discovered, studied scientifically,
and named), this stegosaur material is fragmentary, and has led to much
taxonomic confusion. At least three valid stegosaur genera are
known from England, however:
Dacentrurus,
Lexovisaurus, and
Craterosaurus.
Dacentrurus and
Lexovisaurus have also been reported from France, as have many isolated and
fragmentary stegosaur fossils.
Dacentrurus has also been reported from Spain and Portugal. The photo at left was taken of a
generalized stegosaur model on display outside the National Museum of
Natural History in Paris.
Author: Stephen Brusatte
Last updated: 01/11/2006
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