Formed during the Early Cretaceous period, the Yixian Formation in northeastern China
is famous for containing fossils of numerous feathered dinosaurs,
including many of the first birds. But as dinosaurs were just beginning to take flight,
the freshwater lakes below them were home to a very different type of animal.
Named Hyphalosaurus, this aquatic reptile looked similar to the pachypleurosaurs from the Triassic
period, or for a less obscure comparison, like a cross between a lizard, a newt, and a plesiosaur.
Hyphalosaurus is known from thousands of fossils, more than any other reptile from
the Yixian Formation, resulting in it being the best understood species in a formation
that already contains exceptional fossils. Unlike many of the more famous marine reptiles,
Hyphalosaurus was small, only 80 centimeters long. Its neck was about the same length
as its entire abdomen while the tail was even longer. Hyphalosaurus consisted of two species,
the type species Hyphalosaurus lingyuanensis, and Hyphalosaurus baitaigouensis, which is the species
known from the most specimens. While both are from the Yixian Formation, Hyphalosaurus baitaigouensis
lived after Hyphalosaurus lingyuanensis, who may have been its direct ancestir.
They were similar to each other in most respects, though Hyphalosaurus lingyuanensis had 19 neck
vertebrae while Hyphalosaurus baitaigouensis had 26. With so many vertebrae, the necks of
both species would have been very flexible.As one would expect from a species with such long necks,
Hyphalosaurus’s head was small, though this was less extreme in juveniles. Inside its small,
flattened skull were sharp, peg-like teeth. As befitting an aquatic animal, its feet were webbed.
Like crocodilians, its tail was deep and flattened, with soft tissue further increasing
the height of the tail. This resulted in a tail very well adapted for swimming,
though it would have been somewhat awkward while on the surface, a problem intensified by its long
neck. It is therefore no surprise that all Hyphalosaurus fossils are preserved in deep,
freshwater lake environments alongside other deep-water fauna. This is not merely a case of
preservation bias as Hyphalosaurus is known from numerous specimens and is absent from shallower
waters in the Yixian Formation. Therefore, it likely didn’t come ashore that often, if at all.
Despite their resemblance to each other, Hyphalosaurus was not closely related to
the pachypleurosaurs, plesiosaurs, or any of the other famous Mesozoic marine reptiles.
Nor was it closely related to lizards or the contemporary dinosaurs. Instead,
it was part of an obscure clade of reptiles named Choristodera. Like Hyphalosaurus, other
choristoderans were aquatic, freshwater reptiles. While some choristoderans could come ashore,
in some species only the females could, and some were entirely restricted to the
water. This was far from the only difference from each other. Despite their obscurity,
the choristoderans were very diverse. Some looked similar to lizards, including Monjurosuchus,
a contemporary of Hyphalosaurus. Others, such as Shokawa, looked more like Hyphalosaurus.
Some of the other choristoderans, like Champsosaurus, instead converged upon a
crocodilian-like shape. Despite this, they were not closey related to crocodilians either. Indeed,
Choristodera was not a closely related of any known group of reptiles, living or extinct. The
ancestors of the choristoderans seem to have diverged from the other reptile linages during
the Permian period. While most choristoderans were ambush predators, Hyphalosaurus is thought to have
been an exception, as long necks are common in pursuit predators. As for what it was hunting,
prey size would have been limited by Hyphalosaurus’s proportionally small head.
While known from thousands of specimens, only one Hyphalosaurus skeleton has been verified
to have preserved gut contents, the ribs of a small fish. However, if fish were a common
component of Hyphalosaurus’s diet, it would be expected such finds would be more common.
One possibly is there has simply not been enough research to examine Hyphalosaurus fossils for such
gut contents. Another is that Hyphalosaurus may have primarily hunted soft bodied invertebrates
that wouldn’t leave identifiable remains. While its diet may remain shrouded in mystery, there
are enough Hyphalosaurus specimens to identify a birth rare defect. A very young Hyphalosaurus,
either a newborn or late-stage embryo, is preserved with two heads, each with its own neck.
While very uncommon, living reptiles have been found multiple heads. However, this Hyphalosaurus
is the only known case of such a mutation present in a fossil reptile. While impressive,
when it comes to health, possessing two heads is not better than one. Two-headed individuals
are only likely to live long in captivity. In the case of the two-headed Hyphalosaurus,
if it wasn’t outright still-born it died not long after birth. And despite being a reptile,
“birth” is at least as an appropriate word as “hatch” for a Hyphalosaurus. Like many prehistoric
marine reptiles, they also gave birth to live young as opposed to laying eggs on the shore.
Unlike placental mammals, this was achieved by retaining the soft-shelled eggs inside the mother
until they were ready to hatch. Most marine reptiles who gave birth to live young
did so to avoid having to come to shore at all, which allowed them to become fully aquatic.
Given how Hyphalosaurus lived in deep lakes, it likely evolved live birth for similar reasons,
perhaps only coming to the surface to breath. While smaller than many dinosaurs and marine
reptiles during the Mesozoic era, Hyphalosaurus filled an important role in its ecosystem.
It also demonstrates that the Age of Reptiles was even more diverse than is commonly thought.
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