| Palaeos: | |
Amniota |
| The Vertebrates | Sauropsida |
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Abbreviated Dendrogram
REPTILIOMORPHA | `--AMNIOTA |--Casineria |--Westlothiana `--Crown Amniota |--Sauropsida (= Reptilia) | |--ANAPSIDA | `--EUREPTILIA `--SYNAPSIDA |
Contents
Overview |
Sauropsida includes most of what was clasically known as "Reptilia" along with birds (which are in effect glorified reptiles) and of course dinosaurs. Sauropsids therefore are the bulk of the reptiles plus birds, while Synapsids are the "mammal-like reptiles" plus mammals.
The
first Sauropsids were probably primitive lizard-like creatures rather similar
(and fairly closely related) to the protorothyrid Hylonomus, shown at
right. They scurried through the Carboniferous
undergrowth. In appearance and behaviour they would have been similar to
modern lizards, although anatomically they were more primitive. At this
time, the world was ruled by stem tetrapods. The only other amniotes around
were a few basal synapsids. Both groups
underwent a rapid evolutionary radiation as the Carboniferous and Permian
proceeded. The synapsid "pelycosaurs" grew into 1 to 3 metre
long predators, including specialized fin-back forms and two herbivorous
lineages. The sauropsids remained small and lizard-like. Thus the dominant
life-form during the Permian
were synapsids, and, during the Late Permian, the therapsids
which evolved from them. The end-Permian extinction, which saw off 95% of
lifeforms on Earth, decimated both synapsids and sauropsids, but the sauropsids
came back faster, with creatures such as the crocodile-like Proterosuchidae.
These sprawling predators were the advance guard of a great army of scaly and
armoured carnivores and herbivores (the Archosauriformes
), as well as the dinosaurs, during
the Triassic period.
Meanwhile the Triassic seas were the home of a variety of Sauropsid marine
reptiles - ichthyosaurs, nothosaurs, and thalattosaurs.
During the Late Triassic
period the dinosaurs well and truly took over. The Synapsids were reduced to the
role of mouse-sized rodents and insectivores; the mammaliforms
and mammals of the Late Mesozoic.
They remained in that lowly station, sharing the microvertebrate niches with a
variety of lepidosaurs (lizards and lizard relatives), until a huge asteroid saw
off the dinosaurs and the marine reptiles (the terminal Cretaceous
extinction event) and the mammals were able to inherit the Earth. The
surviving sauropsids include turtles,
lizards, crocodiles
and birds, all of which are still around
today. The crocodile-like Choristodera were a major group (order) of
reptiles that survive the terminal Cretaceous extinction but became extinct
before the modern era, while the sphenodonts,
the "lizards of the mesozoic" one could call them, are represented by
a single endangered species on a few small islands off New Zealand.
Sauropsida: All amniotes closer to snakes than to St. Patrick.
Range: from the Late Carboniferous.
Phylogeny: Amniota : Synapsida + * : Reptilia + Eureptilia
Characters: Little or no specialization along tooth row; $ maxilla separate from quadratojugal; $ single coronoid; some suborbital fenestra present; $ supinator process parallel to humeral shaft; $ 1 centrale in ankle; tail-based locomotion using lateral undulation; frequently bipedal; no glandular skin, uric acid waste, beta keratin. ATW010219.
Comment: With Mesosauridae now included in the Anapsida, Sauropsida and Reptilia become synonymous. If turtles are diapsids then Reptilia become synonymous with whatever "eosuchian" clade includes the most recent common ancestor of all extant Sauropsids. MAK111105,
Links: Phylogeny and Classification of Amniotes; Taxonomy browser (Sauropsida); Lecture 8 - Tetrapods; Amniota. ATW010219.
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