Major Subgroups
A simplified picture of the history of the Anapsida, showing the ranges of constituent groups. Some of these groups are argued to be elsewhere within Reptilia: see the note below. Note that most of the groups originated in, and went extinct at the end of, the PERMIAN. Time periods: Pen, Pennsylvanian (Carboniferous); Per, Permian; Tri, Triassic; Jur, Jurassic. Used with permission: UCMP
Millerettidae
The
millerittids originated by the upper Permian in South Africa. They were
small and lightly built, with sharp conical teeth suggesting an
insectivorous or carnivorous diet. Their hearing was still rather
basic. Some millerittids possessed temporal fenestrae.
Millerosaurus pricei,
a millerittid. The skull is around 50mm long, with temporal
fenestrae. It would have lived somewhat like a modern lizard. Watson 1957
This is the earliest known anapsid, from the lower Permian
of North America. It was small in size, and already showed signs of
being quite advanced, but its teeth were still fairly simple. See figure A below.
It is most closely related to the lanthanosuchids. It therefore is not
an ancestral form for the anapsids, indicating an earlier origin for the group - probably in the mid to late Carboniferous.
Lanthanosuchidae
This is an enigmatic group from the late Permian of Russia. They are closely related to Acleistorhinus,
but distant relatives of turtles. Because of their strange anatomy,
their position within Anapsida has been debated: they could be part of
a more ancient group of early tetrapods. Their skulls were broad
(around 15cm wide by 20cm long), and possessed temporal fenestrae, and bony ridges to add strength. They
were also very flat, so flat that the jaw musculature did not fit in
the skull and had to be attached outside it, through special openings
behind the eyes. They may
have used their flat skulls to push through leaf litter, feeding on
insects and grubs, or they could have been aquatic.
Nyctiphruretia
This
group is poorly known. They may be the group from which the
Procolophonoidae evolved, as they appear to be quite primitive.
Pareiasauria
The
position of the pareiasaurs within Anapsida is debated, but they may be
the closest relatives of the turtles. They are a well known and diverse
group from the upper Permian, and have been found in Europe, Asia
and Africa. They included the largest terrestrial anapsids that ever
lived, typically 2-3m long. The Russian Scutosaurus for example had massive elephantine limbs, and a broad, heavy skull (see B below)
with bumps and frills, characteristic of the pareiasaurs. All
pareiasaurs had bony scales ('scutes') over at least part of their
bodies, and these have been suggested to be the beginnings of the
turtle shell. They also had advanced hearing, and multi-cusped, leaf
shaped teeth, indicating herbivory.
Scutosaurus. Benton 2000
The procolophonids
are the only extinct group to have survived the end Permian extinction,
suviving from late Permian, for 50 million years through to the end of
the Triassic.
They are possibly, if not the pareiasaurs, the closest relative of
the turtles. They were very diverse, and have been found on nearly
every continent. Procolophonids were quite small, and had well
developed hearing. Most had sharp teeth for eating meat and insects,
but later forms, in the late Triassic, had more bulbous teeth
suggesting herbivory (see C below).
This group includes the only living members of the anapsids, the turtles (sometimes known as the Chelonia). Testudines originated in the late Triassic and radiated to include 25 families. One of the oldest known turtles is Proganochelys
from the upper Jurassic, by which time the teeth, common to all
previous anapsids, had been lost, and the familiar shell had been
gained. Its skull was exceptionally strong, and it was most likely terrestrial.
Various anapsid skulls, with
corresponding bones coloured: A, the Lower Permian genus Acleistorhinus. B, the
pareiasaur Scutosaurus.
C, the Triassic procolophonoid Procolophon.
D, Proganochelys,
one of the oldest known turtles. Scale bars equal 1 cm. Used with
permission: UCMP
Note on anapsid classification
Traditionally, many
taxa (animals) inferred to be basal (ancient) reptilian
forms
have been placed in the Anapsida, largely based upon the lack of
temporal fenestration. As this is a primitive (ancestral) character for
amniotes and cannot be used to constrain them, they are
technically an
artificial group.
This has caused many problems when new characters of the members of Anapsida have come to light: they may be moved within
the group,
or out of it altogether. When a basal group undergoes cladistic
(relational) reshuffling, this inevitably has wide reaching consequences -
for
anything inferred to have evolved from it, i.e. the turtles, and in
parallel to it, i.e. the rest of the reptiles, and even beyond.
The
relationships between the groups within Anapsida are still
hotly
debated, depending on which characteristics are used to define these
relationships, and how much importance is placed upon them. This has
resulted in controversy on whether groups in the subclass,
particularly the turtles, belong with Anapsida or elsewhere in
the
larger class of Reptilia.
Recent
studies have found that turtles are most closely related to
the diapsids - either the archosaurs (crocodiles and birds), or
lepidosuars (lizards and snakes). There is as yet no consensus on where
they truly belong. (If turtles are excluded from Anapsida, the
alternative term of Parareptilia is sometimes used for the remaining
members of the group.)
