While dinosaurs are incredibly popular, most people don’t know much about their ancestors.
Dinosaurs and their closest living relatives, the crocodilians, belong to the clade Archosauria.
The first archosaurs evolved during the Triassic period, the first period of the Mesozoic Era.
For the entire Triassic Period, dinosaurs and the other members of their branch of Archosauria,
Avemetatarsalia, were actually less common than the crocodilian branch, Pseudosuchia.
While most of the early avemetatarsalians were small, swift animals who lived in the
shadows of the pseudosuchians and other reptiles, the oldest and most basal of the dinosaurian
cousins was very different.
Named Teleocrater, it was a carnivore with a length between two to three meters.
This stem-dinosaur both provides a window into what the ancestors of its more famous
cousins were like and was an impressive animal in its own right.
Teleocrater’s fossils are from the Manda Formation in Tanzania, which was formed during
the beginning of the Middle Triassic.
While Teleocrater has only recently been named and properly described, its fossils were first
found in 1933.
They were then described in a PhD thesis by Alan Charig during 1956.
Unfortunately, since it was never formerly published, the name “Teleocrater” wasn’t
considered official.
While Teleocrater was mentioned in popular literature by Dr. Charig, he procrastinated
on making a proper publication about it until his death in 1997, in part due to how strange
and incomplete the Teleocrater fossils were.
A similar fate had befallen other fauna from the Manda Formation, such as Mandasuchus and
Mambawakale.
More Teleocrater fossils were finally found in 2015 at a site suspected to be the same
one where the originals had been found.
These fossils came from at least three individuals and finally gave science a good understanding
of Teleocrater’s anatomy.
In 2017, it was formerly described and the name Teleocrater became official.
Teleocrater is very different from what scientists had assumed the first ancestors of the dinosaurs
were like.
While the first dinosaurs were bipeds, Teleocrater still walked on all fours.
In 2017, it was believed the other more advanced avemetatarsalians walked on two legs like
the dinosaurs.
However, subsequent reexamination has found most of the other early avemetatarsalians
were also quadrupeds, and bipedalism may have been a dinosaurian innovation.
Still, hints of the shift to two legs can be found in Teleocrater’s hindlimbs, which
were longer than its forelimbs.
While most other early avemetatarsalians were specialized runners, two legged or not, Teleocrater
lacked these adaptions despite its light build.
Indeed, it is not thought to have even been capable of sustained running Its ankle was
also very different from those of the dinosaurs and their close relatives, the pterosaurs.
While their ankles were hinge-like, Teleocrater’s ankles instead had a more flexible arrangement
called the “crocodile-normal ankle”.
As the name implies, the crocodile-normal ankle was formerly thought to be unique to
the crocodile branch of Archosauria, Pseudosuchia, while hinge-like ankles were thought to be
ancestral to Avemetatarsalia.
Teleocrater instead proves that crocodile-normal ankle was actually ancestral to both branches
of archosaurs, with the dinosaur-like ankles only evolving in later, more derived avemetatarsalians.
While a long neck is more in line with other early avemetatarsalians, Teleocrater’s neck
was long even by their standards, which would have limited the size of its head.
Furthermore, the vertebrae which made up the front half of Teleocrater’s neck were incredibly
long and their neural spines were tall and blade-like.
Most of Teleocrater’s skull still hasn’t been found, but combined with the skull fragments
of the closely related Yarasuchus, its small head seems to have vaguely resembled that
of a theropod dinosaur.
The only known Teleocrater tooth was ziphodont, meaning it was recurved, laterally compressed,
and serrated.
Ziphodont teeth are indicative of carnivory and were also present in most of the later
predatory dinosaurs.
Given its size in relation to the other contemporary carnivores, along with its small head, Teleocrater
held the niche of a medium sized predator.
The description of Teleocrater in 2017 led to the recognition of an entirely new clade
called Aphanosauria.
The other aphanosaurs are Spondylosoma, Dongusuchus, and the previously mentioned Yarasuchus.
For years the other aphanosaurs had been shuffled across Archosauria with no clear idea where
they fit in due to their perplexing and fragmentary nature.
However, the more complete Teleocrater has served as a prehistoric Rosetta Stone.
It shares features with all of them and has enough diagnostic features to determine its
place in Archosauria.
All known aphanosaurs had a similar size and shape as Teleocrater.
They also all lived during the Middle Triassic, between 247 to 237 million years ago.
Yarasuchus lived in India, Spondylosoma lived in Brazil, and Dongusuchus lived in Russia.
Given the shape of Pangea during the Middle Triassic, this indicates the aphanosaurs had
a global distribution.
Teleocrater and the other aphanosaurs are the oldest and most basal known branch of
Avemetatarsalia.
All other known avemetatarsalians belong to the clade Ornithodira, which is the clade
formed by the dinosaurs and the flying pterosaurs.
Notably, before the description of Teleocrater, no unambiguous avemetatarsalian had been found
that was not also ornithodirans.
Until a few years ago, it was assumed that the first avemetatarsalians were small like
the first ornithodirans.
However, while Teleocrater and the other aphanosaurs were far from giants, they absolutely dwarfed
the first ornithodirans.
It is now hypothesized that the first avemetatarsalian was very much like the aphanosaurs and its
descendants only later became smaller.
Supporting this, the less specialized pseudosuchians, such as Decuriasuchus, somewhat resembled
the aphanosaurs.
Archosaurs were ancestrally endothermic, and the avemetatarsalians obtained an even higher
metabolism.
A high metabolism meant that Teleocrater was more active and grew quicker than today’s
reptiles, with the exception of birds.
Examination of its bone histology has found that its metabolism was transitional, higher
than early archosaurs and their relatives, but still lower than ornithodirans.
Teleocrater’s size and metabolism may explain why feathers first evolved.
Primitive feather-like structures have been found in both dinosaurs and pterosaurs, suggesting
they were also present in their last common ancestor.
The first feathers are thought to have been used by the ornithodirans as thermal insulation,
retaining the precious heat generated by their high metabolisms.
Notably, the smaller an animal is increases it surface area to volume ration, which also
increases how quickly its heat is lost to the environment.
Therefore, the shift from large aphanosaur-like animals to small animals with even higher
metabolisms would have significantly increased the evolutionary pressure for thermal insulation,
potentially leading to the development of the first feathers.
Of course, this assumes that Teleocrater itself didn’t already have them, which is also
possible.
Teleocrater and the other aphanosaurs haven’t yet been found in rocks from the Late Triassic
and it is doubtful they ever will.
During the Middle Triassic, the world had just experienced two major cases of faunal
turnover.
The first was the Great Dying, a devasting mass extinction between the Permian and Triassic
periods.
Shortly afterwards, the remaining synapsids, who were the previous dominant animals, were
mostly replaced by archosaurs and their relatives.
Because of this, the new species that swiftly replaced them were not always optimized for
their new niches.
While no doubt a fearsome hunter, Teleocrater had several short comings.
It lacked the power possessed by the large pseudosuchian predators but lacked the speed
of early dinosaurs like Coelophysis.
Therefore, while they were viable predators, aphanosaurs soon became obsolete in the face
of better adapted competition.
That is not to say that Teleocrater was an evolutionary failure.
It is likely that the direct ancestors of the dinosaurs and pterosaurs were almost identical
to it.
The diversity of both of these groups exploded after the Triassic Period and they went on
to become some of the most successful vertebrates the world has ever seen, and in the case of
the dinosaurs, they still are.
Therefore, Teleocrater’s legacy lives on.
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