Thalattosauria
(
Greek
for "sea lizards") is an extinct
order
of prehistoric
marine reptiles
that lived in the middle to late
Triassic
period. Thalattosaurs were diverse in size and shape, and are divided into two superfamilies:
Askeptosauroidea
and
Thalattosauroidea
. Askeptosauroids were endemic to the
Tethys Ocean
, their fossils have been found in Europe and China, and they were likely
semiaquatic
fish eaters with straight snouts and decent terrestrial abilities.
[1]
Thalattosauroids were more specialized for aquatic life and most had unusual downturned snouts and crushing dentition. Thalattosauroids lived along the coasts of both
Panthalassa
and the Tethys Ocean, and were most diverse in China and western North America.
[2]
The largest species of thalattosaurs grew to over 4 meters (13
feet) in length, including a long, flattened tail utilized in underwater propulsion. Although thalattosaurs bore a superficial resemblance to
lizards
, their exact relationships are unresolved. They are widely accepted as
diapsids
, but experts have variously placed them on the reptile family tree among
Lepidosauromorpha
(
squamates
,
rhynchocephalians
and their relatives),
[3]
[4]
Archosauromorpha
(
archosaurs
and their relatives),
[5]
ichthyosaurs
,
[6]
and/or other marine reptiles.
[7]
[8]
Description
Thalattosaurs have moderate adaptations to marine lifestyles, including long, paddle-like tails and slender bodies with more than 20 dorsal vertebrae. There are few unique traits of the postcranial skeleton shared by all thalattosaurs, but the skeleton is still useful for distinguishing between askeptosauroids and thalattosauroids. Askeptosauroids are characterized by elongated necks with short neural spines and at least 11 vertebrae, while thalattosauroids have shorter necks sometimes involving as few as four vertebrae. Thalattosauroids also have tall neural spines on their neck, back, and especially the tail vertebrae, increasing the surface area for swimming via
lateral undulation
. Thalattosauroids additionally possess short, wide limb bones poorly adapted for movement on land. In this superfamily, the
humerus
is widest near the shoulder, the
femur
is widest near the knee, the
radius
is reniform ("
kidney
-shaped"), and
phalanges
are long and plate-like. Askeptosauroids retain hourglass-shaped limb bones like land reptiles, but even they share specializations with thalattosauroids such as a short
tibia
and
fibula
, with the latter expanding near the ankle.
[1]
[9]
[2]
Skull
Thalattosaurs are
diapsid
reptiles, meaning that they have temporal fenestrae, two holes in the head behind the
orbit
(eye socket). However, many thalattosaurs have a vestigial upper temporal fenestra which is slit-like, and some have it fully closed up by surrounding bones.
[10]
Thalattosaurs lack a
quadratojugal bone
, leaving the lower temporal fenestra open from below. They also lack
postparietal
and
tabular
bones, while the
squamosal bone
is small, the
supratemporal bone
is extensive, and the
quadrate bone
is large. When seen from above, the rear edge of the skull bears a large, triangular embayment that reaches further forwards than the quadrates.
[5]
Thalattosaurs have a rostrum (snout) significantly longer than the portion of the skull behind the eyes. A majority of this length is formed from the
premaxillary
bones, and the nares (nostril holes) are shifted back close to the eyes. The premaxillae stretch back very far and are incised into the
frontal bones
. This leads to an unusual trait that is characteristic of thalattosaurs, where the left and right
nasal bones
are separated from each other and restricted to a small portion of the snout near the nares. The
lacrimal bone
is typically lost or fused to the large crescent-shaped
prefrontal bone
in front of the orbit, mirroring the
postfrontal bone
which is usually fused to the three-pronged
postorbital bone
behind the orbit.
[10]
[11]
[5]
Askeptosauroidea
have narrow, straight-edged snouts which are often elongated and filled with conical teeth. One askeptosauroid,
Endennasaurus
, is entirely toothless
[12]
while another,
Miodentosaurus
, has a short, blunt snout.
[13]
Most members of the second thalattosaur group,
Thalattosauroidea
, have more distinctive downturned snouts.
Clarazia
and
Thalattosaurus
both have snouts that taper into a narrow tip. Most of the snout is straight, but
premaxillae
at the tip are downturned.
Xinpusaurus
and
Concavispina
also have downturned
premaxillae
, but the end of the
maxillae
are sharply upturned, forming a notch in their skull. In
Hescheleria
(and potentially
Nectosaurus
and
Paralonectes
), the premaxillae are abruptly downturned at the end of the snout, nearly forming a right angle with the rest of the jaw. In these forms, the end of the snout is a toothy hook separated from the rest of the jaw by a space called a
diastema
. Thalattosauroids also have
heterodont dentition
, with pointed piercing teeth at the front of the snout and low crushing teeth further back.
[11]
The exception to this rule is
Gunakadeit
, which has a straight snout and many slender teeth.
[2]
Thalattosaurs often have a pronounced retroarticular process at the rear of the mandible. Thalattosauroids are more specialized than askeptosauroids in jaw anatomy, as they have evolved a large peak-like
coronoid bone
and an
angular bone
that extends far forwards along the lower edge of the jaw.
Palatal
dentition is extensive in thalattosauroids but absent in askeptosauroids.
[14]
[2]
Paleobiology
Thalattosaurs are only known from marine deposits, indicating that they were all primarily aquatic reptiles. The retracted nostrils and long, paddle-shaped tail are further evidence for aquatic habits. Thalattosauroids seemingly spent all of their time in the water, with short, wide limbs, poorly developed wrist and ankle bones, and tall vertebrae adapted for swimming via
lateral undulation
. Even so, they retained strong claws and functional
digits
which had not transformed into flippers, in contrast to
ichthyosaurs
and
sauropterygians
. Unlike these other marine reptiles, there is no evidence that thalattosaurs fully adapted to a pelagic life out in the open ocean, and instead they probably all lived in warm waters close to the coast. Askeptosauroids had stronger limbs more typical of terrestrial reptiles, indicating they would have been capable of moving around on land to some extent. They likely primarily used their tails when swimming, while thalattosauroids may have utilized their body and tail in conjunction.
[3]
[1]
[12]
[2]
[15]
Thalattosaurs had diverse diets, though they probably all involved marine animals in one way or another.
Endennasaurus
probably predated small animals like fish fry or small crustaceans due to its lack of teeth.
[12]
Various thalattosauroids (like
Thalattosaurus
,
Xinpusaurus
, and
Concavispina
) had large fang-like teeth at the front of the mouth and thick button-like teeth at the back of the mouth. Based on
Massare
(1987)'s
[16]
technique of correlating diet with tooth shape, the taller teeth were suited for a "crunching" diet, involving armored fish, large
crustaceans
, and thin-shelled
ammonites
. The low, robust teeth would have been useful for a "crushing" diet specialized in large
molluscs
or other thick-shelled prey.
[14]
[17]
Gunakadeit
's slender teeth correlated with the "Pierce II" guild of Massare (1987), indicating it likely fed on soft, fast-moving fish and squid. It also had a large
hyoid apparatus
which may have played a role in
suction feeding
.
[2]
Thalattosaurs also fell prey to other marine reptiles: the torso of a ~4 meter (13 feet) long
Xinpusaurus xingyiensis
has been found within the body cavity of a 5 meter (16 feet) long skeleton of the predatory ichthyosaur
Guizhouichthyosaurus
.
This is the oldest known predatory interaction between marine reptiles, and
Xinpusaurus
may also be the largest prey item preserved within another marine reptile.
[18]
Distribution
It is not certain where thalattosaurs originated from. During the Triassic period, the earth had one giant supercontinent,
Pangaea
, which was surrounded by the superocean
Panthalassa
. The eastern portion of Pangaea was incised by a massive tropical inland sea, the
Tethys Ocean
, which extended all the way from China to Western Europe. While thalattosauroids are known from worldwide Triassic marine deposits, askeptosauroids are only known in Tethyan deposits. Assuming
Endennasaurus
and
Askeptosaurus
were the most
basal
askeptosauroids, Askeptosauroidea would have originated in the Western Tethys Ocean, now the Alpine region of Europe.
[1]
However, if
Miodentosaurus
is more basal, a Western Tethys (European) origin would be significantly less likely.
[19]
Although the
sister group
to Thalattosauria is still debated, one possibility, the
icthyosauromorphs
, seemingly evolved in the Eastern Tethys (China) during the early Triassic or earlier.
[8]
The oldest known thalattosauroids (
Thalattosaurus
,
Paralonectes
, and
Agkistrognathus
of
British Columbia
's
Sulphur Mountain Formation
) lived in eastern Panthalassa, along what is now the western coast of North America. Müller (2005, 2007) argued that at least one branch of thalattosauroids had managed to spread worldwide early in their evolution.
[1]
[20]
However, this is based on the hypothesis that
Nectosaurus
(from California),
Xinpusaurus
(from China), and an unnamed species from
Austria
formed a
clade
basal to other thalattosaurs, a classification scheme which contrasts with many other studies.
[9]
The worldwide distribution of Thalattosauroidea is intriguing considering that thalattosaurs are considered to be poorly adapted for traversing open oceans, which would have been a necessity for spreading between the eastern coast of Panthalassa and the Tethys Ocean.
[20]
Coastal "refuges" such as
volcanic island arcs
and
guyots
may have facilitated the ability of thalattosaurs to spread between ocean basins.
[10]
Hescheleria
-like forms were previously only reported from North America and Europe,
[21]
but in 2021 a
Hescheleria
-like snout fragment was reported from China, indicating that they also had a widespread distribution.
[22]
Trans-Panthalassa connections are also observed in other Triassic marine life such as
pistosaurs
and
ammonites
.
[10]
Evidently thalattosaurs were capable of dispersing throughout major marine regions multiple times before the group's extinction, with thalattosauroids likely more prolific at spreading than askeptosauroids due to their greater aquatic adaptations.
[2]
Classification
Early hypotheses
When first named by Merriam in 1904, Thalattosauria was only known by the species
Thalattosaurus alexandrae
. Based primarily on the overall skull shape, it was hypothesized to have been close to the reptile order
Rhynchocephalia
, which includes
Sphenodon
(the living
tuatara
). Nevertheless,
Thalattosaurus
was recognized as distinct enough to be given its own order, and was tentatively grouped along with Rhynchocephalia in the group
Diaptosauria
, a collection of various "primitive" reptiles now known to be
polyphyletic
. Within Diaptosauria, thalattosaurs were also considered very closely related to
choristoderes
and "Proganosauria" (
parareptiles
). Comparisons were also made with Parasuchia (
phytosaurs
), Lacertilia (
lizards
), and
Proterosuchus
, but dismissed as incompatible with proposed evolutionary schemes.
[23]
Further discussion by Merriam (1905) considered a relationship with
ichthyosaurs
due to their similar ecology, but questioned why their skull and vertebral anatomy would diverge so widely if they had a close common ancestor. He proposed that potential similarities were best explained as convergent evolution. The possibility that thalattosaurs diverged from reptiles close to lizards (such as
Paliguana
) was described in more detail, with thalattosaurs serving as an short-lived early attempt for near-lizards to return to the sea, an evolutionary process later repeated more successfully when
mosasaurs
evolved from true lizards. Nevertheless, Merriam found no clear evidence that any previously known reptile group was directly ancestral to thalattosaurs or vice versa. They were probably descended from land-dwelling Permian reptiles, and not closely related to other marine reptile groups which first evolved in the Triassic.
[3]
Later 20th-century workers typically placed thalattosaurs close to rhynchocephalians or
squamates
as part of the group now known as
Lepidosauromorpha
.
[14]
Modern classification and outgroup relations
The rising popularity of
cladistics
in the late 1980s had some effects on thalattosaur classification. Continued research has helped cement some aspects of reptile classification, such as how
Sauria
(a major
clade
of
diapsids
including all living reptiles) split in the Permian into two branches: Lepidosauromorpha (which leads to lizards, snakes, and the tuatara) and
Archosauromorpha
(which leads to crocodilians and dinosaurs, including birds). While many paleontologists still consider thalattosaurs probable lepidosauromorphs, a few studies (such as a
phylogenetic analysis
by
Evans
, 1988) have instead suggested that they may be on the archosauromorph branch of Sauria.
[5]
Rieppel
(1998)'s re-evaluation of the thalattosaur-like
pachypleurosaur
Hanosaurus
argued that thalattosaurs have affinities with the aquatic reptile order
Sauropterygia
, which itself is aligned with turtles within an expansive interpretation of Lepidosauromorpha.
[4]
An analysis by
Müller
(2004) has even considered thalattosaurs to belong just outside of Sauria. Unusually, thalattosaurs have an affinity to shift near
ichthyosaurs
(in the group
Ichthyosauromorpha
) when certain basal saurians or near-saurians are excluded from the data set.
[6]
Some analyses derived from Müller (2004) group thalattosaurs in a "marine superclade" with ichthyosauromorphs and sauropterygians, and sometimes with turtles, archosauromorphs, or lepidosauromorphs as well. For example, Simões
et al
(2022) classify thalattosaurs as the
sister group
of the sauropterygians, with their clade being sister to the ichthyosauromorphs, and all three being basal
archosauromorphs
. However,
cladograms
generated by these analyses change in unpredictable ways through alterations to their methodology (such as including or excluding aquatic adaptations or switching between
parsimony
and
bayesian inference
), leading some to have concerns over the validity of the "marine superclade".
[7]
[8]
[24]
[25]
[26]
While thalattosaurs are almost certainly diapsids, the large degree of uncertainty surrounding their
outgroup
relations has led most modern paleontologists to classify them as Diapsida
incertae sedis
.
Ingroup relations
One of the first phylogenetic analyses specifically focusing on thalattosaurs was part of
Nicholls
(1999)'s reevaluation of
Thalattosaurus
and
Nectosaurus
. She used a restricted definition of Thalattosauria which referred to a
clade
including all reptiles more closely related to
Nectosaurus
and
Hescheleria
than to
Endennasaurus
or
Askeptosaurus
. The more inclusive group including
Askeptosaurus
,
Endennasaurus
, and traditional thalattosaurs was given the name
Thalattosauriformes
.
[14]
[1]
[20]
However, most studies focusing on the group have preferred to retain a broader definition of Thalattosauria equivalent to Nicholls' Thalattosauriformes clade, including reptiles close to both
Askeptosaurus
and
Thalattosaurus
. In these studies, Thalattosauria is divided into two branches, one leading to relatives of
Askeptosaurus
and the other leading to relatives of
Thalattosaurus
. The clade containing reptiles closer to
Thalattosaurus
than to askeptosaurids is given the name
Thalattosauroidea
(and sometimes called Thalattosauridea
[9]
[19]
). Meanwhile, the clade containing reptiles closer to askeptosaurids is termed
Askeptosauroidea
[10]
[13]
[2]
or Askeptosauridea.
[9]
[19]
Subsequent studies since Nicholls (1999) started to include more taxa, including newly described Chinese taxa such as
Anshunsaurus
and
Xinpusaurus
.
[10]
[27]
However, uncertainty over
Endennasaurus
's thalattosaurian ancestry led to it being excluded from these analyses. After Müller
et al
. (2005) re-affirmed that
Endennasaurus
was closely related to
Askeptosaurus
,
[12]
all thalattosaurs known at the time were finally combined into phylogenetic analyses.
[9]
[20]
Studies by Rieppel,
Liu
,
Cheng
,
Wu
, and others continued to identify new Chinese taxa such as
Miodentosaurus
and various species of
Anshunsaurus
and
Xinpusaurus
, though
homoplasy
in these new taxa has led to little resolution in the structure of the two major branches of Thalattosauria.
[13]
[28]
In an attempt to remedy this problem, new phylogenetic analyses were developed by Liu
et al
. (2013) during the description of
Concavispina
,
[19]
and
Druckenmiller
et al.
(2020) during the description of
Gunakadeit
.
[2]
The internal relationships of thalattosaurs is still considered tentative and inconclusive, although the fundamental structure of the group (a monophyletic Thalattosauria clade split into askeptosauroids and thalattosauroids) is very stable. Some paleontologists have attempted to divide thalattosaurs into families. One family,
Askeptosauridae
, is typically considered to include
Askeptosaurus
and
Anshunsaurus,
[9]
with a few studies also placing
Miodentosaurus
[13]
or
Endennasaurus
[12]
within it. Another family, Thalattosauridae, was originally used to group
Thalattosaurus
and
Nectosaurus
,
[3]
was later redefined to exclude
Nectosaurus
,
[14]
and later still encompassed practically all thalattosauroids.
[19]
Many thalattosaur-focused paleontologists avoid using family names due to their inconsistent usage and questionable validity.
The
cladogram
presented here is based on the largest and most recent analysis of thalattosaur ingroup relations, Druckenmiller
et al.
(2020). It shows all thalattosaur genera except for the fragmentary
Agkistrognathus
.
[2]