Evidences from Living Mammals
Some of the primitive living mammals give the strongest support of reptilian affinity of mammals. The monotremes and the marsupials occupy intermediate status between reptiles and mammals.
Monotremes Offer Strongest Support
Monotremes constitute a sort of connecting link between the mammals and their reptilian ancestor and serve to bridge the gap between the two. Monotremes are undoubtedly mammalian, but possess many reptilian features too.
The reptilian features are
- Pectoral girdle is typically reptilian with a large coracoid, an interclavicle and without scapular spine.
- Pelvic girdle possesses an epipubic bone.
- Ribs excepting the cervical ones are single headed.
- Presence of cervical ribs.
- Bones of the skull are typically reptilian. LacrymaL bone is absent, jugal is reduced or absent, auditory bulla is absent, presence of reptilian provomer.
- Corpus callosum is absent.
- Reproductive system is based on reptilian plan.
- Presence of shallow cloaca.
- Meroblastic segmentation.
- They are not fully homiothermous.
It appears that the monotremes are roughly fifty per cent, mammal and fifty per cent, reptile in their organisation. They are very primitive and represent the end product of an independent evolutionary line of mammal-like reptilian stock.
Marsupials—also Offer Support
The marsupials also show many reptilian features.
These features are:
- Brain lacks corpus callosum and the olfactory part is comparatively large.
- Presence of epipubic bone.
- Palate is deficient in ossification.
- Epicondylar foramina occur frequently.
Evidences from Serology Support Reptilian Ancestry of Mammals
Serological tests have revealed that the reptilian blood shows close affinity with that of mammals which are warm blooded.
Palaeontological Evidences in Support of Reptilian Ancestry:
The fossil remains of the mammal-like reptiles offer a sound evolutionary basis of mammalian origin through reptiles. The mammal-like reptiles are:
Synapsida comprises of a very interesting group of reptiles that appeared in the rocks of late Carboniferous time. They form a bridge between the primitive- reptiles and the primitive mammals. The latest genera approached so close towards mammalian stage that it is really questionable whether they should be included amongst mammals or reptiles.
They showed the tendency of the loss of bones in the skull. Teeth showed differentiation. The lateral temporal opening was situated behind the eye and it became enlarged gradually. The synapsida had a large number of genera. The early members were very close to the ancestral Cotylosaurs, while some of the latest genera approached very close to the mammals.
Pelycosaurs—the first synapsid
They initiated a line of evolutionary development which is most completely recorded in geological history. The first Pelycosaurs are the Ophiacodon of which Varanosaurus was a primitive member. In Varanosaurus, the skull was narrow, marginal teeth were socketed and showed little regional specialisation.
Bifurcation of Pelycosaurs. The Pelycosaurs evolved along two lines—the large plant eating Edaphosaurs and the large aggressive carnivorous Sphenacodonts, Edaphosaurs.
Springing from an Ophiacodon ancestry in Permian age, the Edaphosaurs diverged into a different line. The skull was small, stout and shallow. The teeth were not differentiated. Sphenacodonts. The second evolutionary line from the Pelycosaurs was the Sphenacodonts which led the channel of evolution. Teeth were strongly differentiated which made the refinement of the skull.
The fossils of these forms were recorded in the middle and late Permian and in Triassic age. They were the forms that took the road leading towards mammalian organisation. Possession of the following characters certainly supports the contention.
- There was a strong trend of lateral temporal fossa to enlarge.
- Presence of a secondary palate.
- Dentary was enlarged at the expense of other bones.
- Double occipital condyles were present.
- Teeth were differentiated.
- Quadrate and quadratojugal were reduced and loosely attached with the skull.
The therapsids followed two lines of adaptive radiations.
They were archaic therapsids who retained many primitive features of the Pelycosaurs. They lack secondary palate and the pineal opening was very large. The dentary was moderately developed. The typical representative of this group is exemplified by Tapinocephalid (Moschops).
They were the most successful therapsids of phylogenetic longevity. They appeared in the middle of Permian to lower Triassic. Bones of the temporal region of the skull were thin due to enlargement of temporal opening. Acromion process was present on the scapula. The phalanges were reduced to mammalian count (2-33-3-3).
Thecodonts. When the Dinocephalians were evolving in their own way, the theriodonts developed rapidly in a direction that was led directly to the mammals. The theriodonts were varied in shape. Cynognathus was a typical genus of the theriodonts. Cynodontia had many mantmalian features.
They possessed the following mammalian characters:
- Double occipital condyles.
- Lower jaw was almost made up of a single bone—dentary.
- Heterodont dentition.
- Presence of hard palate.
- Double headed ribs.
- Scapular spine was present.
Ictidosauria formed a bridge between reptiles and mammals. They had almost crossed the threshold line that separates the reptiles from mammals.
- Skull was highly organised and mammal-like. The cranium was spacious.
- Quadrate was greatly reduced.
- Lower jaw was made up of dentary.
- Presence of double occipital condyles.
- Heterodont dentition.