In this article we will discuss about:- 1. Introduction to Origin of Camels 2. Biological Organisation of Camel 3. Surviving Genera of Camels 4. Ancestry of Camels 5. Biological Trends in the Evolution of Camels 6. Palaeontological Procession of Camels 7. Pliocene and Pleistocene – Periods for True Camels.
Contents:
- Introduction to Origin of Camels
- Biological Organisation of Camel
- Surviving Genera of Camels
- Ancestry of Camels
- Biological Trends in the Evolution of Camels
- Palaeontological Procession of Camels
- Pliocene and Pleistocene – Periods for True Camels
1. Introduction to Origin of Camels:
The camels, the ships of desert, are of special interest to us because of their unique adaptation to the harsh and in hospital conditions in sandy deserts. Next to horses, the evolutionary history of camel is vividly known from the excellent collection of fossils. The remarkable fossils in the history of the earth give an instance of progressive evolution of camel in course of phylogenetic development.
Despite these facts, the entire evolutionary history of camels is recorded in North America. They were exclusively North American up-to Pliocene whence they migrated to South America, Africa and Asia where they are surviving to-day. Occurrence of many evolutionary side-lines in other continents does not make the phylogeny of camel so confusing as that of horses.
The camels are the representatives of the even-toed Artiodactyla. The camels occupy a transitional status and hold an isolated position. The biological relationship of camels with others is not yet clearly established.
2. Biological Organisation of Camel:
Two genera of camels are surviving today—Camelus and Llama or Auchenia. They are characterised by the possession of certain features. The limbs are two-toed and long. They are secondarily digitigrade showing an adaptation to sandy desert. The hoofs are reduced to nail-like forms. The metapodials are fused to form the cannon bone.
The lateral movement of limbs is restricted to a limited range. One or two cushion-like pads are present in the feet. The number of teeth is reduced to 34. Only one upper incisor is present on each side and it is more canine-like.
All the lower incisors are present. They are spatulate and procumbent. Canines are similar in structure and function as the incisors. The molars are high crowned, elongated and each has four crescentic cusps.
The stomach is three-chambered and ‘water- cells’ are present in the wall of the rumen. The red blood corpuscles are unique amongst the mammals. They are elliptical and not circular in outline. The placenta is of primitive type.
Presence of hump is a very characteristic feature in camel. It consists of a gelatinous fat and assumes a conical shape. It provides nourishment in time of scarcity of food. The hump is single in Arabian camel (Camelus dromedarius) and is double in central Asian camel (Camelus bactrianus).
3. Surviving Genera of Camels:
Two living genera are encountered— one, Camelus and the other, Llama or Auchenia. Two species of the genus Camelus are present. Liama is the South American genus and includes two wild species —Llama huanacus and Llama vicunia.
The domestic derivatives of these wild forms are the Llamas and Alpacas of South America. They are built on the same structural plan as the camels excepting the absence of hump and of much smaller stature. Two pads, instead of one are present.
4. Ancestry of Camels:
The primal evolutionary home of camels was North America and the ancestral source can clearly be traced back to the upper Eocene time.
Protylopus—holds the ancestral key:
The first possible ancestral stage in the evolution of camel is observed in the Upper Eocene time. Protylopus possessed many features which are certainly camel-like. This creature had a small stature. It had the normal number of mammalian teeth, i.e., 44.
The canines were slightly enlarged and the molars were low crowned. The facial portion of the skull was narrow and the orbit was incomplete. The limbs were subequal, the forelimbs were shorter than the hind-limbs. The forelimbs had four functional digits while the lateral digits of the hind-limbs became slender.
It is unanimously accepted that Protylopus is the earliest known stage in the evolution of camel (Fig. 1.25). Prior to Protylopus the ancestral line merges with the ancestry of other groups and cannot not be traced with certainty.
5. Biological Trends in the Evolution of Camels:
Between Protylopus and the surviving genera of this day, a large number of fossils is recorded. Throughout the evolutionary line of camel the following trends are recorded. Excepting the retrogression of the limbs, the evolutionary changes show close parallelism to the changes undergone by the horses.
The changes may be summarised in the following way:
(a) Gradual increase in size.
(b) Subsequent loss of lateral digits.
(c) Elongation and fusion of metapodials to form the cannon bone.
(d) Change over from unguligrade to digitigrade form.
(e) Reduction in the number of teeth.
(f) Gradual elongation of teeth.
6. Palaeontological Procession of Camels:
From the Protylopus stage to the present-day form, a large number of fossil forms have been recorded in the successive strata of the earth.
Oligocene—the influx of camel-like animals:
During Oligocene period a number of fossil forms were recorded. Camel-like animals were quite abundant during Oligocene. They constituted a characteristic fauna of the time. Poebrotherium—the middle Oligocene form. The remains of the Poebrotherium were abundant in the middle Oligocene time. Poebrotherium attained the size of a sheep having longer limbs and neck.
The facial portion of the skull was elongated and tapering. In the upper jaw, the grinding teeth were short-crowned and in the lower jaw they tend to become elongated. The number of teeth was 44. Digital reduction was observed in the limbs. The lateral toes were reduced to small vestiges. The hoofs were like that of deer. Protomeryx—the upper Oligocene form.
Arising from Poebrotherium, Protomeryx directed the main evolutionary line of camel and continued up to the lower Miocene time. Protomeryx had all the features of Poebrotherium excepting the orbit which was completely encircled by bone. Protomeryx possessed full mammalian quota of teeth and the grinding teeth exhibit deepening tendency. Two digits were present and the hoofs were pointed.
Miocene—the period of divergence:
Like that of horses, a number of well- defined divergent groups of camels was recorded. Of these divergent lines, the main line of evolution was directed through the lower Miocene form (Protomeryx) and the upper Miocene form (Procamelus).
Procamelus—directed the main line of evolution:
The upper Miocene s-rata shows the fossil camel—the Procamelus showing for the first time the tendency towards the reduction of the number of teeth and the loss of the first and second incisors of the upper jaw in the adult stage. The meta-podiah showed the tendency towards fusion to form the cannon bone. The first sign of desert adaptation was quite evident by the presence of pads on the limbs.
Side-line evolution. Gazelle-camels:
A large number of fossil remains of Stenomylus was recorded in the lower Miocene of Western Nebraska. The Oligocene ancestry of Stenomylus was not known but it continued as Rakomylus and became extinct in the lower Pliocene. Stenomylus was designated as the Gazelle-camel and had a very brief evolutionary career. Stenomylus had many anomalous characteristics, especially in dentition.
The lower jaw had ten incisors like teeth. Six true incisors and the canines and first premolars assume the appearance of incisors. The molars were low crowned. The body was delica telly built and the limbs were two-toed. The metapodials were separate.
Giraffe-camels:
The group is represented by the upper Oligocene Paratylopus, the lower Miocene Oxydactylus and the upper Miocene Alticamelus. The particular name of the group does not imply any relationship with Giraffes.
Paratylopus in the upper Oligocene was regarded as the ancestor of the group, Giraffe-camels. The lower Miocene derivative of Paratylopus was the Oxydactylus having small stature than its successor. It had shorter limbs and neck.
The metapodials were separate and had sharply pointed hoofs. It retained the full quota of mammalian teeth and the teeth were short crowned. The late Miocene and early Pliocene form, Alticamelus, originating from Oxydactylus became far advanced than its predecessors in a number of ways.
Presence of cannon-bone and pads was some of the most remarkable features. Alticamelus had a very large size and the teeth were low crowned.
7. Pliocene and Pleistocene – Periods for True Camels:
The direct descendants of the upper Miocene Procamelus were the Camelops and Eschatius of the Pliocene and Pleistocene times. The fossil records of the Camelops and Eschatius were quite rare. They failed to survive beyond Pleistocene period.
In Pleistocene period, camels were quite, abundant. The typical camels were the members of the genus Camelus. Fig. 1.26 shows the scheme of evolution of camel. It is well developed in comparison to Procamelus.
In Camelus further reduction of one premolar in the upper jaw and two in the lower jaw was encountered. Camelus was the first genus to migrate to the old World and the first record was found in the Siwalik formation of India. The cause of disappearance of North American camels is not known.
