During the Triassic period, before the dinosaurs rose to prominence, Earth was inhabited by
a variety of strange reptiles. Among these was a clade of small, energetic reptiles named
Lagerpetidae. Their name translates to “rabbit reptiles”, in reference to how some lagerpetid
species had traits in common with rabbits and other specialized hoppers, which suggest
they may have been hoppers as well. While not dinosaurs, they were very close relatives,
belonging to the clade Ornithodira. This is the clade that contained dinosaurs, pterosaurs,
and all species descended from their last common ancestor. The lagerpetids were neither
dinosaurs nor pterosaurs themselves but instead the most basal known species of ornithodirans,
making them critical in understanding the origins of their more famous, and often larger,
relatives. The known members of Lagerpetidae consist of six named species in four genera,
three of which are species of Dromomeron: Dromomeron romeri, Dromomeron gregorii, and
Dromomeron gigas. Dromomeron gigas was from South America while the other two were from
North America. The other three species are Lagerpeton and Ixalerpeton, both from South
America, and Kongonaphon, who was found in Madagascar. Kongonaphon and Lagerpeton both
lived near the boundary of the Middle and Late Triassic while the other lagerpetids
are only known from the Late Triassic. As shown in the proceeding reconstructions, none
of the lagerpetids are known from a complete skeleton. The bones of small animals are less
likely to fossilize and remain intact in most conditions. Indeed, many elements of lagerpetid
anatomy remain unknown in any lagerpetid specimen. From what is known, their general appearance
was similar to that of the earliest dinosaurs, such as Eoraptor. But even compared to early
dinosaurs, most of the rabbit reptiles were tiny, usually less than a meter long, with
all but the largest lucky to weigh even as much as a raven. The exception was Dromomeron
gigas. While only known from part of a femur, this femur is 67% longer than that of the
next largest lagerpetid specimen. However, even this was still small compared to many
of the contemporary Triassic dinosaurs, let alone many of the larger dinosaurs and pterosaurs
that would evolve later. Their small size was not the only difference from the early
dinosaurs. Given their long hindlimbs, lagerpetids were usually reconstructed as walking or hopping
on two legs, much like the early dinosaurs. However, some recently described lagerpetid
forelimbs indicate the humorous was longer than previously assumed in at least some lagerpetids,
making it more likely they walked on four legs, at least part of the time. Despite their
close relation to dinosaurs, such a stance makes sense, as the pterosaurs were quadrupeds,
as were the aphanosaurs, the closest relatives of the ornithodirans. Regardless of whether
they used four legs or two, lagerpetids did not move like dinosaurs. As mentioned earlier,
traits lagerpetids shared with modern species like rabbits, kangaroo rats, and actual kangaroos
suggest they were also specialized for hopping. These shared traits include long hindlimbs,
a small pelvic girdle relative to the hindlimbs, and forward-facing vertebrae. Lagerpeton also
shared an additional feature with modern hoppers: its weight was supported by only two digits.
However, Ixalerpeton lacked Lagerpeton’s forward-facing vertebrae, so it was likely
a more traditional runner. Even Lagerpeton lacked some traits present in other specialized
hoppers, such as fused sacral vertebrae. Therefore, the rabbit reptiles were likely less speacizled
for hopping than actual rabbits. Instead, in many ways, a better analogue for them may
be the small theropod dinosaurs closely related to birds, who are thought to have lived in
the top of tree before developing the ability to glide and eventually fly. While claws of
most lagerpetids are unknown, the claws of Dromomeron romeri were strongly recurved.
The curvature is outside of typical range of ground birds and lizards, but within that
of species who use their limbs for climbing or hunting. This suggests they lived as arboreal
species, leaping between tree limbs rather than hopping on the ground like rabbit analogues.
The other option for what they were using their claws for, hunting, is unlikely given
their most likely prey. Lagerpetids had small, closely packed teeth that were unserrated,
conical, and peg-like. Similar teeth are found in modern insectivorous reptiles. Microwear
on Kongonaphon’s teeth seems to confirm a similar diet, as they possess pits similar
to those left by hard-shelled insect exoskeletons. Their speed, agility, and strong jumping capabilities
would have certainly helped the lagerpetids become proficient insect catchers, able to
keep up with their small, often flying quarry that would have easily evaded most other hunters.
With their arboreal lifestyle, they would have had largely remained out of reach of
most of their own predators while still having access to numerous insects unavailable to
their many of their competitors. As for the larger Dromomeron gigas, its larger size may
have allowed it to hunt small vertebrates, though since no teeth of Dromomeron gigas
have been found yet, this is simply speculation. As of right now, no fossil impressions have
been found of the integument, or outer covering, of lagerpetids, or any other Triassic ornithodiran.
While such early reptiles have been traditionally reconstructed as scaly, the rabbit reptiles
may have looked a lot cuter than portrayed in these early reconstructions. Feathers were
present in later theropod dinosaurs and filaments similar to the primitive feathers in early
theropods have also been found in both ornithischian dinosaurs and even pterosaurs. Therefore,
proto-feathers may have been ancestral to Ornithodira. The lagerpetids would have certainly
benefited from such filaments. The reptiles the ornithodirans were descended from already
had a high metabolism, which the early ornithodirans inherited. However, the small size of the
rabbit reptiles and other basal ornithodirans meant they had a higher surface to area ratio,
which would have resulted in them losing heat to their environment much more rapidly than
their larger ancestors. The evolution of primitive feathers would have provided enough thermal
insulation to make such a high metabolism much more viable for a creature their size.
Lagerpetidae holds an important place in Ornithodira. Ornithodira is split into two subgroups: Pterosauromorpha,
which consist of the pterosaurs and those ornithodirans who share a more recent common
ancestor with them than with dinosaurs, and Dinosauromorpha, which consist of the dinosaurs
and those ornithodirans who instead share a more recent common ancestor with them than
the pterosaurs. Dinosauromorpha contains many other species besides dinosaurs, including
Lagosuchus, Saltopus, and the diverse silesaurids. Lagerpetidae was once placed in Dinosauromorpha
as well, specifically as the earliest diverging branch. On the other hand, besides the true
pterosaurs, no other reptiles have been definitively placed in Pterosauromorpha. The only notable
exception is a small reptile named Scleromochlus, whose placement remains controversial due
to the poor preservation of its otherwise largely complete remains. The result is a
large gap between the highly specialized pterosaurs and their terrestrial ancestors, which has
left the question of how pterosaurs evolved flight a mystery, as the earliest known pterosaurs
were already capable fliers. As it turns out, the rabbit reptiles are the key to solving
this puzzle. In the past few years, new lagerpetid fossils, such as partial skulls and forelimbs,
have shown stronger similarities with pterosaurs than dinosaurs. As a result, it seems that
the lagerpetids were an early branch of Pterosauromorpha instead of Dinosauromorpha. It isn’t just
the exterior of the lagerpetids’ bones that are similar to pterosaurs. Using micro-CT
scanning, paleontologists have found additional similarities, including large semi-circular
canals. In the words of Sterling Nesbitt, one of the paleontologists behind the discovery:
"It has to do with the semicircular canals [in the ear], which orients you in 3D space.
The shape of those canals correlates with ecology and how you move your head — basically,
are you agile or not? And a lot of things that have flight have semicircular canals
with a really large and characteristic [shape] because you're flying, you're in a lot more
3D space." The flocculum, the part of the brain that aids in processing information,
was also large in both pterosaurs and lagerpetids, though to a lesser extent. The lagerpetids
were likely using these features while chasing insects through the trees. For the pterosaurs,
their large flocculum and semi-circular canals helped them keep track of their environment
while flying through the air. While lagerpetids were still incapable of flight, it seems they
already had many of the traits that would prove critical in its evolution in pterosaurs,
who would then take these traits to even further extremes. The lagerpetids provide a basis
for how pterosaur evolved flight. The evolution of skin flips, like those of modern flying
squirrels, in more advanced pterosauromorphs would have allowed them to make greater leaps
between tree limbs. These skin flaps would eventually form wings. They were later enlarged
by the massive extension of the fourth finger, which the wing membranes were attached. Similar
to the ancestors of birds, flapping would have allowed them to extend the time they
could glide between jumps until they had acquired true flight. Despite their importance in understanding
their more iconic relations, the lagerpetids were more than just a stepping stone in the
evolution of pterosaurs. The rabbit reptiles were successful in their own right, coexisting
with pterosaurs and dinosaurs for tens of millions of years across of much of Pangea.
While the lagerpetids became extinct at the end of the Triassic period, their relatives
would live on and rule the sky for the next 135 million years. Thank you for watching.
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