A long long time ago, in a galaxy not too far away, was a little blue planet called Earth, and on this world not a single mammal lived. However a lot of time has past since then and we now have lots of furry creatures that are collectively called mammals. How did they get their? Where did they come from? These are the kinds of questions that led me to my subject of choice. I will endeavor to provide examples, using specific transitional fossils, to show that mammals have evolved from a group of reptiles and were simply not placed here by unknown forces.
Before I begin, I would like to define some terms so that nobody gets eft in the dust. The term transitional fossil can be used in conjunction with the term general lineage, together they help explain the how one species became another. “General lineage”: This is a sequence of similar genera or families, linking an older to a very different younger group. Each step in the sequence consists of some fossils that represent certain genus or family, and the whole sequence often covers a span of tens of millions of years.
A lineage like this shows obvious intermediates for every major structural change, and the fossils occur roughly (but often not exactly) in the expected order. However, usually there are still gaps between each of the groups. Sometimes the individual specimens are not thought to be directly ancestral to the next-youngest fossils (e. g. they may be “cousins”” or “uncles” rather than “parents”). However they are assumed to be closely related to the actual ancestor, since the have similar intermediate characteristics.
Where Does It All Begin ? Mammals were derived during the Triassic Period ((from 245 to 208 million years ago) It began with relatively warm and wet conditions, but as it progressed conditions became increasingly hot and dry. ) from members of the eptilian order Therapsida. The therapsids, members of the subclass Synapsida (sometimes called the mammal-like reptiles),generally were unimpressive in relation to other reptiles of their time. Synapsids were present in the Carboniferous Period (about 280 to 345 million years ago) and are one of the earliest known reptilian groups.
Although therapsids were primarily predators by nature, some adaptations included a herbivorous species as well, they were generally small active carnivores. Primitive therapsids are present as fossils in certain Middle Permian deposits; later forms are known from every continent xcept Australia but are most common in the Late Permian and Early Triassic of South Africa. The several features that separate modern reptiles from modern mammals doubtlessly evolved at different rates.
Many attributes of mammals are correlated with their highly active lifestyle; for example, efficient double circulation of blood with a completely four-chambered heart, anucleate and biconcave erythrocytes (blood cells), the diaphragm, and the secondary palate (which separates passages of food and air and allows breathing during mastication (chewing) or suckling). Hair for insulation correlates with endothermy (being warm-blooded), the physiological maintenance of individual temperature independent of the environmental temperature, and endothermy allows high levels of sustained activity.
The unique characteristics of mammals thus would seem to have evolved as a complex interrelated system. Transitions to New Higher Taxa Transitions often result in a new “higher taxon” (a new genus, family, order, etc. ) from a species belonging to different, older taxon. There is nothing magical about this.
The first members of the new group are not bizzare, hey are simply a new, slightly different species, barely different from the parent species. Eventually they give rise to a more different species, which in turn gives rise to a still more different species, and so on, until the descendents are radically different from the original parent.
For example, the Order Perissodactyla (horses) and the Order Cetacea (whales) can both be traced back to early Eocene animals that looked only marginally different from each other, and didn’t look at all like horses or whales. (They looked more like small, dumb foxes with raccoon-like feet and simple teeth. ) But over the ollowing tens of millions of years, the descendents of those animals became more and more different, and now we call them two different orders.
Major Skeletal Differences (derived from the fossil record) The mammalian skeletal system shows a number of advances over that of reptiles. he mode of ossification (process of bone formation) of the long bones is one characteristic. In reptiles each long bone has a single centre of ossification, and replacement of cartilage by bone proceeds from the centre toward the ends. In mammals secondary centres of ossification develop at the ends of the bones. Mammalian skeletal growth is termed determinate, for once the actively growing zone of cartilage is used up, growth in length ceases. As in all bony vertebrates, of course, there is continual renewal of bone throughout life.
The advantage of secondary centres of ossification at the ends of bones lies in the fact that the bones have strong articular surfaces before the skeleton is mature. In general, the skeleton of the adult mammal has less structural cartilage than does that of a reptile. The skeletal system of mammals and other vertebrates is broadly divisible into axial and appendicular portions. The axial skeleton consists of the skull, the backbone and ribs, and serves primarily to protect the central nervous system. the limbs and their girdles make up the appendicular skeleton.
In addition, there are skeletal elements derived from gill arches of primitive vertebrates, collectively called the visceral skeleton. Visceral elements in the mammalian skeleton include jaws, the hyoid apparatus supporting the tongue, and the auditory ossicles of the middle ear. The postcranial axial skeleton in mammals general has remained the rather conservative during the course of evolution. The vast majority of mammals have seven cervical (neck) vertebrae, and do not have lumbar ribs, both characteristics are unlike reptiles.
The skull of mammals differs markedly from that of reptiles because of the great expansion of the brain. The sphenoid bones that form the reptilian braincase form only the floor of the braincase in mammals. In mammals a secondary palate, that is not present in reptiles, is formed by processes of the maxillary bones and the palatines. The secondary palate separates the nasal passages from the oral cavity and allows continuous breathing while chewing or uckling. The bones of the mammalian middle ear are a diagnostic of the class.
The three auditory ossicles form a series of levers that serve mechanically to increase the amplitude of sound waves reaching the tympanic membrane, or eardrum, produced as disturbances of the air. The innermost bone is the stapes, or “stirrup bone. ” It rests against the oval window of the inner ear. The stapes is homologous with the entire stapedial structure of reptiles, which in turn was derived from the hyomandibular arch of primitive vertebrates. The incus, or “anvil”, articulates with the stapes.
The incus was derived from the quadrate bone, which is involved in the jaw articulation in reptiles. The malleus, or “hammer”, rests against the tympanic membrane and articulates with the incus. The malleus is the homologue of the reptilian articular bone. The mechanical efficiency of the middle ear has thus been increased by the incorporation of two bones of the reptilian jaw assemblage. In mammals the lower jaw is a single bone, the dentary. The mammalian limbs and girdles have been greatly modified with locomotor adaptations.
The primitive mammal had well developed limbs and was five-toed. In each limb there two distal bones (radius and ulna in the forelimb; tibia and fibula in the hindlimb) and a single proximal bone (humerus; femur). The number of phalangeal bones in each digit, numbered from inside outward, is 2-3-3-3-3 in primitive mammals and 2-3-4-5-4 in primitive reptiles. Modifications in mammalian limbs have involved reduction, loss, or fusion of bones. Loss of the clavicle from the shoulder girdle, reduction in the number of toes.
The Transition This is a documented transition between vertabrate classes. Each group is clearly related to both the group that came before, and the group that came fter, and yet the sequence is so long that the fossils at the end are astoundingly different from those at the beginning. As Gingerich has stated (1977) “While living mammals are well seperated from other groups of animals today, the fossil record clearly shows their origin from reptilian stock and permits one to trace the orgin and radiation of mammals in considerable detail.
This list starts with pelycosaurs (early synapsid reptiles) and continues with therapsids and cynodonts up to the first unarguable “mammal”. Most of the changes in this transition involved elaborate repackaging of an expanding brain nd special sense organs, remodeling of the jaws and teeth for more efficient eating, and changes in the limbs and vertebrae related to active, legs-under- the-body locomotion.
Braincase floor shows first mammalian tendencies and first signs of stronger attachment to the rest of the skull. Lower jaw shows first changes in jaw structure. Body narrower, deeper, vertebral column more strongly constructed. Ilium further enlarged, lower-limb musculature starts to change. This animal was more mobile and active. Too late to be a true ancestor, must be a “cousin”. Haptodus (late Pennsylvanian) – One of the first known sphenacodonts, showing the initiation of sphenacodont features while retaining many primitive features of the ophiacodonts. Skull more strongly attached to the braincase.
Teeth become size differentiated, with the in the canine region and fewer teeth overall. Stronger jaw muscles. Vertebrae parts and joints more mammalian. Neural spines on vertebrae longer. Hip strengthened by fusing to three sacral vertebrae instead of just two. Limbs very well developed. – Dimetrodon, Sphenacodon (early Permian) – More advanced pelycosaurs, clearly losely related to the first therapsids. Dimetrodon is almost definitely a “cousin” and not a direct ancestor, but as it is known from very complete fossils, it’s a good model for sphenacodont anatomy.
Medium sized fenestra. Teeth further differentiated, with small incisors, two huge deep-rooted upper canines on each side, followed by smaller cheek teeth, all replaced continuously. Fully reptilian jaw hinge. Lower jaw made of multiple bones and first signs of a bony prong later involved in the eardrum, but there was eardrum yet, so these reptiles could only hear ground-borne vibrations (they did have a reptilian iddle ear). Vertebrae had still longer neural spines (especially so in Dimetrodon, which had a sail), and longer transverse spines for stronger locomotion muscles. Procynosuchus (late Permian) – The first known cynodont – A famous group of very mammal-like therapsid reptiles, sometimes considered to be the first mammals. Probably arose from the therocephalians, judging from the distinctive secondary palate and numerous other skull characters.
Enormous temporal fossae for very strong jaw muscles, formed by just one of the reptilian jaw muscles, which has now become the mammalian masseter (muscle). Secondary palate now omposed mainly of palatine bones, rather than vomers and maxilla as in older forms.
Lower incisor teeth were reduced to four per side, instead of the previous six. Dentary now is 3/4 of lower jaw; the other bones are now a small complex near the jaw hinge. Vertebral column starts to look mammalian: first two vertebrae modified for head movements, and lumbar vertebrae start to lose ribs. A diaphragm may have been present. -Thrinaxodon (early Triassic) – A more advanced cynodont. Further development of several of the cynodont features seen already. Temporal fenestra still larger, larger jaw muscle attachments.
Bony econdary palate almost complete. Functional division of teeth: incisors (four uppers and three lowers), canines, and then 7-9 cheek teeth with cusps for chewing. The cheek teeth were all alike (no premolars and molars). The whole locomotion was more agile. Number of toe bones is 2-3-4-4-3, intermediate between the reptile number (2-3-4-5-4) and the mammalian (2-3-3-3-3), and the “extra” toe bones were tiny. – Exaeretodon (late Triassic) – True bony secondary palate formed exactly as in mammals. Mammalian toe bones (2-3-3-3-3).
Lumbar ribs totally lost. – Sinoconodon (early Jurassic) – Proto-mammal. Eyesocket fully mammalian now closed medial wall). Hindbrain expanded. Permanent cheek teeth, like mammals, but the other teeth were still replaced several times. Mammalian jaw joint stronger, with large dentary condyle fitting into a distinct fossa on the squamosal. This final refinement of joint automatically makes this animal a true “mammal”. – Peramus (late Jurassic) – An advanced placental-type mammal. The closest known relative of the placentals and marsupials.
Has attained a fully mammalian three- boned middle ear with excellent high-frequency hearing. – Steropodon galmani (early Cretaceous) – The first known monotreme (egg laying ammals). – Pariadens kirklandi (late Cretaceous) – The first definite marsupial. – Kennalestes and Asioryctes (late Cretaceous) – Small, slender animals; eyesockets open behind; simple ring to support eardrum; primitive placental-type brain with large olfactory bulbs; basic primitive mammalian tooth pattern. Canine now double rooted.
Still just a trace of a non-dentary bone (the coronoid process), on the otherwise all-dentary jaw. “Could have given rise to nearly all subsequent placentals. ” says Carroll (1988) So, by the late Cretaceous the three groups of modern mammals were in lace: monotremes, marsupials, and placentals. Placentals appear to have arisen in East Asia and spread to the Americas by the end of the Cretaceous. In the late Cretaceous, placentals and marsupials had started to diversify a bit, and after the dinosaurs died out, in the Paleocene, this diversification accelerated.
For instance, in the mid-Paleocene the placental fossils include a very primitive primate-like animal (Purgatorius – known only from a tooth, though, and may actually be an early ungulate), a herbivore-like jaw with molars that have flatter tops for better grinding, and also an insectivore (Paranygenulus). Because the characteristics that separate reptiles and mammals evolved at different rates and in a response to a variety of interrelated conditions, at any point in the period of transition from reptiles to mammals there were forms that combined various characteristics of both groups.
Such a pattern of evolution is termed “mosaic” and is a common phenomenon in those transitions marking the origin of major new adaptive types. To simplify definitions and to allow the strict delimitation of the Mammalia, some authors have suggested basing the boundary on a single character, the articulation of the jaw between he dentary and squamosal bones and the attendent movement of accessory jaw bones to the middle ear as auditory ossicles.
The use of a single character allows the placement in a logical classification of numerous fossil species, other mammalian. characters of which, such as the degree of endothermy and nursing of young and the condition of the internal organs, probably never will be evaluated. It must be recognized, however, that if the advanced therapsids were alive today, taxonomists would be hard-put to decide which to place in the Reptilia and which in the Mammalia.
