T
he living reptiles o f South Africa can be divided into three groups, vis. (1) the tortoises and turtles, (2) the crocodiles, and (3) the lizards and snakes. It is less well known that a fourth group o f reptiles, the so-called mammal-like reptiles, also once roamed the South African plains. These animals are now found as fossils in the Karoo layers (see map below).The upperm ost tw o layers o f the Karoo are known as the Beaufort Group and the Stormberg Group respectively. The Beaufort covers the period 250 to 215 million years ago, while the Stormberg dates from 215 to 190 million years ago.
During the whole o f the Karoo period the major part o f South Africa was a very vast and very flat plain. More or less along the present eastern coast there was a long m ountain range, while a shal low sea was to be found in the central part, approximately in the present posi tion o f the Orange Free State. The rivers flowed mainly from the high eastern
m ountain range into the shallow sea in the centre.
At the beginning o f the Beaufort period it m ust have been fairly wet and marshy conditions prevailed. This was followed by ,alternating periods of low and high rainfall, but the general tendency over the whole Beaufort period was towards a dry climate. However, as the country was very flat, even occasional torrential rains caused the shallow rivers to over flow their banks and this inundated the plains. Animals were trapped in the mud, died and sank into it, and through the years their skeletal remains were fossilized.
In the Beaufort formations the remains of a variety of reptile fossils are found. Early relatives o f the m odem turtles and
tortoises, like Procolophon, were
common. These reptiles form one main branch o f the reptiles and are characte rized by having no temporal openings behind the orbits. The earliest represen tatives o f the lizards and the 'crocodiles (e.g. Euparkeriaj are also found in the
Beaufort; these reptiles all have two tem poral openings behind each orbit. The mammal-like reptiles, like the living mammals, have only one temporal ope ning on each side. This is the basic simi larity between the mammal-like reptiles and the mammals.
What did these mammal-like reptiles look like and how did they live?
The oldest representatives o f the group are found in North America and a prime example is the “ sail lizard” (Dimetro- don - see illustration on next page). A large number o f specimens o f different sizes have been found to date and it has been established that the size o f the sail was determined by the volume of the animal, not by its length. This again in dicates that the sail was used to radiate and/or absorb heat - it was a simple way to regulate' the body temperature. The South African mammal-like rep tiles, like Lycaenops, Lystrosaunis, Thrinaxodon and Cynognathus, do not have a sail, but many have a “ third eye”
3Tammal~like Reptiles
Dimetrodon, the “sail lizard" o f North America. I t was a carnivore and grew to a length o f three metres. The “sail” helped to regulate the body tempera ture.
Top: Side view o f the skull o f Procolophon, an early relative o f the turtles. I t had no temporal opening behind the orbit. Bottom: Side view o f the skull o f a w o lf (Canis sp) o f 2 million years ago. Mammals have one temporal opening behind each orbit, b u t the postorbital bar (between orbit and temporal opening) often disappear.
Top: Side view o f the skull o f Euparkeria, which had two temporal openings behind each orbit. I t is related to the m odem crocodiles and the extin ct dinosaurs.
Bottom: Side view o f the skull o f Dimetrodon, the “sail lizard”, an early mammal-like reptile with a single temporal opening behind each orbit.
1 meter
Above: Lystrosaurus, an herbivorous mammal-like reptile one to tw o metres long. I t inhabited the marshy areas. between the tw o temporal openings. This absorbed the sun’s heat and kept
the pineal organ and hypophysis
directly below the “ eye” warm and ac tive. The microscopic structure o f the bones is similar to that found in mam mals, w ith an intricate system o f blood vessels and this again is correlated with warm-bloodedness. Several specimens o f
Thrinaxodon have also been found curled up, which is a natural pheno menon in warm-blooded animals to con serve body heat.
The mammal-like reptiles are by far the dom inant group among the Karoo rep tiles. They can be divided into tw o main groups, viz. one where m ost (if n o t all) the teeth were lost and functionally replaced by a hom y beak, and a second group where there is a progressive diffe rentiation o f the teeth.
Lystrosaurus (above) is representative o f the first group. It retained the two tusks o r canines in the upper jaw and these teeth probably served for defence only. Lystrosaurus is the most abundant o f land-living vertebrates among the Karoo fossils and m ust have been a marsh- dweller, where it fed on soft marsh plants.
Below: Lycaenops, a carnivorous mammal-like reptile about the size o f a large dog. Its teeth were all conical.
L yca en o p s (previous page) is an early
re p resen tativ e o f t h e s e c o n d m ain g ro u p . T his c a rn iv o ro u s reptile was m o r e o r less th e size o f an A lsatio n do g a n d it h a d a w hole series o f conical te e t h , th o s e in th e ca n in e region bein g s o m e w h a t lo n g e r th a n th e rest.
Trinaxodon and C ynognathus (above)
are la te r m e m b e r s o f t h e same gro up. H ere we find w ell-develo ped incisors, canine s a n d “ m o l a r s ” . In C ynognathus
t h e “ m o l a rs ” w ere dev elo p e d as carnas- sial te e t h , similar in s t r u c tu r e to th o s e fo u n d in m o d e m jack a ls, dogs an d wolves. T h e “ m o l a r s ” o f T hrinaxodon
h a d a b r o a d e r surface w ith small cu sp s o n th e inside.
C o rre l a te d w i t h th e d if f e r e n t ia t io n o f t h e t e e t h is th e d e v e lo p m e n t o f th e se c o n d a r y p ala te. T h is se p a r a te d th e nasal ca vity f r o m th e buccal ca vity a n d e n sured t h a t the anim al co u ld chew its fo o d . In anim als w i t h o u t a se condary p alate, th e fo o d is sw allow ed i m m e d i a te l y , as it w o u ld o th e r w ise p re v e n t the passage o f air d u rin g b re a th in g .
Drawings and photographs o f the skull o f th e m am m al-like reptile Thrinaxodon (specim en n u m b er R 2 3 in the N ational M useum ). N o te th e single tem poral opening b eh in d each o rb it and the d ifferen tia tio n o f the teeth.
We do n o t k n o w w h e t h e r th e m a m m a l like reptiles suckled t h e i r y o u n g , a nd it is u n lik e ly t h a t we will ever have p r o o f o f this, as soft tissues are on ly fossilized u n d e r m o s t e x c e p t io n a l co n d it io n s . Y o u n g specim ens o f T hrinaxodon have,
h o w e v e r, b e e n f o u n d w i t h fully-gr own sp e c im e n s ( p r e s u m a b l y fem ales), w hic h p o i n t s t o th e fact t h a t these anim als at least care d for t h e i r yo u n g . T h is is qu ite unlike th e c o n d i t i o n in tu r tle s, lizards a n d crocodile s, w h ere th e y o u n g have t o fe n d f o r them se lves.
A b o ve: C y n o g n a th u s , an advanced carnivorous m am m al-like reptile w ith camassial-like teeth behind the canines. I t was m ore or less th e size o f a leopard, b u t n o t as high o f f th e ground.
T e x t an d line drawings b y Jacques van H eerden, Karoo Palaeontologist o f the M useum. R eco n stru ctio n drawings b y A u d re y van E eden o f the M u se u m ’s A r t D epartm ent.
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