Herrerasaurus
Name:
Herrerasaurus
(Herrera’s lizard).
Phonetic: Her-rare-rah-sore-us.
Named By: Osvaldo Reig - 1963.
Synonyms: Frenguellisaurus
ischigualastensis, Ischisaurus cattoi.
Classification: Chordata, Reptilia, Dinosauria,
Theropoda, Eusaurischia, Herrerasauridae.
Species: H. ischigualastensis (type).
Diet: Carnivore.
Size: Largest known skull is up to 56
centimetres long, suggesting largest known individuals approached 5 to
6 meters long.
Known locations: Argentina, San Juan Province -
Ischigualasto Formation.
Time period: Ladinian to early Carnian of the
Triassic.
Fossil representation: Remains of several
individuals represented by partial remains, and a single complete
skull.
Herrerasaurus
was first described in 1963 yet many sources still cite the year
1988 as the year the genus was discovered. In actually however
this was the year that the first complete skull of Herrerasaurus
was
found, and because this is the discovery that really made headlines
in the scientific press, this is the date that some who don’t bother
to do more complete research put forward as discovery of the genus.
The
actual phylogenetic placement as well as wider implications of
Herrerasaurus has been the subject of a lot of
debate amongst
palaeontologists. With its bipedal stance and skull features
Herrerasaurus is considered by some to be a basal
theropod (the line
that would go on to produce giants like Allosaurus
and
Tyrannosaurus),
while other palaeontologists think that
Herrerasaurus is a basal saurischian (lizard
hipped) dinosaur that
is similar and related to the theropods, but is still separate from
them. Some palaeontologists also think that Herrerasaurus
is an even
more primitive dinosaur that predates the first major split into
saurischian and ornithscian (bird hipped) groups of dinosaurs.
Clear
identification is not helped by the fact that Herrerasaurus
has a
number of features that are spread out across all groups of dinosaurs,
although the individual features are not found in each group. As
always the placement of a prehistoric animal depends upon the
interpretation of the researchers in question, but the main problem
is that the early dinosaurs of the Triassic are not as well-known as
the later Jurassic and Cretaceous dinosaurs, and are often known from
even more partial and incomplete remains. As time goes on however it
is inevitable that more fossils of Triassic dinosaurs will be found,
and with luck they will allow for a much more complete picture of
dinosaur evolution.
As
a dinosaur Herrerasaurus was actually one of the
larger dinosaurs
currently known with most other forms such as Eoraptor
being towards
the one meter length mark. Despite its size over other primitive
dinosaurs Herrerasaurus was probably not the top
predator of its
ecosystem as the large rauisuchian
Suchosaurus
was also active in the
same area.
The
basic theropod skull form can already be seen in Herrerasaurus
along
with a similar dental arrangement. As a predator Herrerasaurus
probably focused its hunting upon small reptiles as well as the smaller
dinosaurs. The long legs of Herrerasaurus would
have seen it easily
able to keep pace with the smaller dinosaurs, and it was probably one
of the few predators that were able to catch them. Larger reptiles
like the rhynchosaur Hyperodapedon
were more heavily built and may have
also been attacked, although its stocky build may have meant that
smaller juveniles were more likely to be taken.
A
skull feature of Herrerasaurus that is usually not
seen in other
dinosaurs is that of a sliding lower jaw. This sliding jaw can be
seen in some other reptiles though so this feature is more probably a
case of individual specialisation rather than an evolutionary
characteristic. This sliding action may have allowed Herrerasaurus
to
rake its teeth through prey as well as possibly to more easily hold on
to small prey as it struggled in between its jaws.
Speed
and agility seem to have been key to the hunting strategy of
Herrerasaurus which probably would have been
seen exhibiting such
behaviour as moving amongst the undergrowth and quickly snatching up
animals that were scared out of hiding. The immediate clues to this
are the legs which have a proportionately short femur (upper leg
bone) while the lower leg bones are very long. Additionally
Herrerasaurus had a digitigrade stance which meant
that it walked on
its toes so that the metatarsal foot bones actually served to extend
the length of the leg, increasing the stride which in turn increased
the top speed of Herrerasaurus. The feet also
show a development
towards a three toed foot as while the first and fifth digits are still
present only one had a (reduced) claw, something which indicates
that they were not used to gain extra traction on the ground. In
later dinosaurs these toes would become reduced to the point that
they were vestigial (present but no longer serving any purpose).
Also the pubis on Herrerasaurus points backwards,
something that is
seen in dromaeosaurid
dinosaurs like Deinonychus
as well as birds,
and further suggests reliance upon agility. The arms are also quite
long and may have been used for holding onto prey.
A
similar dinosaur to Herrerasaurus that is also from
the same location
is Sanjuansaurus.
Further reading
- La presencia de dinosaurios saurisquios en los "Estratos de
Ischigualasto" (Mesotriasico Superior) de las provincias de San Juan y
La Rioja (Rep�blica Argentina) [The presence of saurischian dinosaurs
in the "Ischigualasto beds" (upper Middle Triassic) of San Juan and La
Rioja Provinces (Argentine Republic)] - Ameghiniana 3(1):3-20 - O. A.
Reig - 1963.
- The tibia and tarsus in Herrerasauridae (Dinosauria, incertae sedis)
and the evolution and origin of the dinosaurian tarsus - Journal of
Paleontology 63: 677–690 - F. E. Novas - 1989.
- The complete skull and skeleton of an early dinosaur - Science 258
(5085): 1137–1140 - P. C. Sereno - F. E. Novas - 1992.
- Phylogenetic relationships of the basal dinosaurs, the
Herrerasauridae - Palaeontology 35: 51–62 - F. E. Novas - 1992.
- The skull and neck of the basal theropod Herrerasaurus
ischigualastensis - Journal of Vertebrate Paleontology 13
(4):
451–476
- P. C. Sereno - F. E. Novas - 1993.
- The pectoral girdle and forelimb of the basal theropod Herrerasaurus
ischigualastensis - Journal of Vertebrate Paleontology 13
(4): 425–450.
- P. C. Sereno - 1993.
- New information on the systematics and postcranial skeleton of
Herrerasaurus ischigualastensis (Theropoda:
Herrerasauridae) from the
Ischigualasto Formation (Upper Triassic) of Argentina - Journal of
Vertebrate Paleontology 13 (4): 400–423. - F. E. Novas - 1994.
-
A new herrerasaurid (Dinosauria, Saurischia) from the Upper Triassic
Ischigualasto Formation of northwestern Argentina. - ZooKeys (63):
55–81. - Oscar A. Alcober & Ricardo N. Martinez - 2010.
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