Essay/Term paper: Transitions of reptiles to mammals

Essay, term paper, research paper:  Science Reports

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.

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.

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.

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, 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 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. 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. -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.

- 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. 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.

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.