Transitions of Reptiles to Mammals
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 left 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.
-1-
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 reptilian 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 except 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.
-2-
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, they 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 following 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.
the 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.
-3-
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 suckling.
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,
-4-
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 after, 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
-5-
involved elaborate repackaging of an expanding brain and 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.
Here are some differences to keep an eye on:
Early Reptiles Mammals
1. No fenestrae in skull Massive fenestra exposes all of braincase
2. Braincase attached loosely Braincase attached firmly to skull
3. No secondary palate Complete bony secondary palate
4. Undifferentiated dentition Incisors, canines, premolars, molars
5. Cheek teeth uncrowned points Cheek teeth premolars and molars crowned and cusped
6. Teeth replaced continuously Teeth replaced once at most
7. Teeth with single root Molars double-rooted
8. Jaw joint quadrate-articular Jaw joint dentary-squamosal
9. Lower jaw of several bones Lower jaw of dentary bone only
10. Single ear bone (stapes) Three ear bones (stapes, incus, malleus)
11. Jointed external nares Seperate external nares
12. Single occipital condyle Double occipital condyle
13. Long cervical ribs Cervical ribs tiny, fused to vertebrae
14. Lumbar ribs Lumbars are rib free
15. No diaphragm Diaphragm present
16. Limbs sprawled out from body Limbs under body
17. Scapula simple Scapula with big spine for muscles
18. Pelvic bones unfused Pelvis fused
19. Two sacral (hip) vertebrae Three or more sacral vertebrae
20. Toe bone #'s 2-3-4-5-4 Toe bones 2-3-3-3-3
21. Body temperature variable Body temperature constant
-6-
- Paleothyris (early Pennsylvanian) - An early captorhinomorph reptile, with no temporal
fenestrae at all.
- Protoclepsydrops haplous (early Pennsylvanian) - The earliest known synapsid reptile.
Little temporal fenstra, with all surrounding bone intact. Fragmentary. Had amphibian-
type vertebrae with tiny neural processes. (reptiles had only just separated from
amphibians)
- Clepsydrops (early Pennsylvanian) - The second earliest known synapsid. These early,
very primitive sysnapsids are a primitive group of pelycosaurs collectively called
"ophiacodonts".
- Archaeothyris (early-mid Pennsylvanian) - A slightly later ophiacodont. Small temporal
fenstra, now with some reduced bones (supratemporal). Braincase still just loosely
attached to skull. Slight hint of different tooth types. Still has some extremely
primitive
amphibian features.
- Varnops (early Permian) - Temporal fenestra further enlarged. 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.
-7-
Hip strengthened by fusing to three sacral vertebrae instead of just two. Limbs very
well
developed.
- Dimetrodon, Sphenacodon (early Permian) - More advanced pelycosaurs, clearly closely
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
middle 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 composed 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.
-8-
-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 secondary 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
mammals).
- Pariadens kirklandi (late Cretaceous) - The first definite marsupial.
-9-
- 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 place:
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 the 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.
-10-
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.
-11-
References
Carroll, R. 1988. Vertebrate Paleontology and Evolution. W.H.
Freeman and Co., New York
Gingerich, P.D. 1977. Patterns of Evolution in the Mammalian Fossil Record.
Elsevier Scientific Pub. Co.
Gingerich, P.D. 1985. Species in the Fossil Record: Concepts, Trends, and
Transitions. Paleobiology.
Rowe, T. 1988. Definition, Diagnosis, and Origin of Mammalia.
J. Vert. Paleontology.
|