Introduction metabolic rates allow reptiles to feed only

Introduction

Reptiles evolved in the
Late Carboniferous with the widely accepted date of 315 Ma. The oldest identified
reptile is the Hylonomus lyelli (Sigurdsen & Carroll, 2016). To this date,
it is the oldest unquestionable reptile to have evolved, developing from reptiliomorphs.
However, there are reptile fossils that pre-date Hylonomus. Westlothiana lizziae
is one, which when discovered was classified as the oldest, evolving 350 Ma (Carroll,
1982).

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Reptiles are air-breathing
vertebrates descended from tetrapods and allowed the evolution of mammals and
birds. Reptiles belong to the group Amniota along with birds and mammals
(Benton & Spencer, 1995). The main characteristics of amniotes are their
eggs and the ability of internal fertilisation (Reisz, 1997). Reptiles do not maintain
a constant body temperature (Spelman, 2012). They are ectotherms (Shine, 2005),
that is, they move into sunlight to warm up and use shade and water to cool
(Spelman, 2012). When temperatures get too low, reptiles become inactive by
slowing down their metabolism, a process reptiles have control of (Spelman,
2012). The ability to regulate their body temperature is used to simply reproduce,
collect and digest food (Huey, 1982). The ability of reptiles to alter their metabolic
rate means they can go for long periods of inactivity followed by short periods
of activity (Shine, 2013). The control of their metabolic rates means reptiles
use less of the environment resources than mammals and therefore consume less
energy (Huey, 1982). The slow metabolic rates allow reptiles to feed only on a
few types of prey which may be available only at certain times of the year
(Shine, 2005).

The purpose of this
report is to outline how reptiles evolved. More specifically, I focus on the key
evolutionary advancements that were essential for the evolution of reptiles and
the point which reptiles evolved.

 

Evolutionary
advances leading to reptile evolution

The key evolutionary
steps essential for reptiles to evolve was the transition of life from water to
land along with the development of lungs and subsequent loss of gills, thus
allowing animals to breathe on land. Additionally, the development of limbs to
aid in movement across the terrestrial landscape and the ability to either lay
eggs on land without drying out or being able to give birth.

Fish

During the Devonian (400
Ma), bony fish were dominant in oceans. The two large groups were the lobe
finned fish and the ray finned fish. A key feature of lobe fins was that, along
with gills, they possessed lungs, allowing them to breathe in air and water
(Lambert et al., 2003). This became highly advantageous when water bodies
became stagnant resulting in the reduction of oxygen levels in the water, because
lobe and ray fins could swim above the water level and breathe in atmospheric
oxygen (Romer, 1957). Due to the development of lungs, the descendants of
Osteolepis (descendants of lobe fins) began developing limbs from some of their
fins whilst other fins began shrinking (Lambert et al., 2003). Through the
evolution of limbs, it enabled the fish to move to other water bodies when the
current one dried out during seasons of drought (Romer, 1957).

Tetrapods

Tetrapods
evolved from lobe fins. The early tetrapods still lived in the water and their
limbs were used to travel faster in water due to their paddle shape (as seen in
Figure 1). They still possessed gills and tail fins along with distinct digits which
developed from fin bones (Lambert et al., 2003). The Elginerpeton from Scotland
(as seen in Figure 1) is an early tetrapod possessing the paddle morphology. Eventually
the tetrapods migrated to the terrestrial landscape evidenced by footprints
found in Australia from the Late Devonian.

Reptiliomorphs (tetrapods
more related to amniotes than to amphibians) (Sigurdsen & Carroll, 2016) are
the ancestors of amniotes but are also amniotes (Lambert et al., 2003). Once on
land, their skeletons needed to adapt to the increased gravity, allowing them to
carry their own weight. From tetrapods, amphibians evolved who could live on
land but could not leave the water permanently.

Reptiles evolved from amphibians,
through the evolution of the amniotic egg. From amphibians, there was a split,
namely into synapsids and sauropsids, with the sauropsids splitting into anapsids
and diapsids.

Synapsids are animals with
a single temporal opening in the skull roof (Figure 2 (B)), and includes,
mammals. Whereas sauropsids and consequently anapsids had no temporal openings
in the skull roof (Figure 2 (A)), diapsids, had two temporal openings on either
side of the skull roof (Figure 2 (C)) (Sues, 2016; Sigurdsen
& Carroll, 2016). Reptiles, birds and dinosaurs evolved from sauropsids.

Amniotic egg

The main evolutionary
advancement that allowed the rise of reptiles was the hard-shelled amniotic egg
(Figure 3). Amniotic eggs evolved from aquatic animals as a way of protecting
the eggs from aquatic predators (Carroll, 1969). The amniotic egg evolved from
amphibian eggs by the addition of the chorion and the allantois, with the development
of the amnion following, forming the amniotic egg (Sigurdsen & Carroll,
2016). The primary reason why amniotic eggs evolved was to be protected in
droughts which were common in the Paleozoic. As water bodies evaporated,
amphibian eggs dried up and desiccated thus being destroyed, whilst amniotic
eggs remained unaffected (Romer, 1957). The egg contains four membranes that
surround the embryo: yolk sac, allantois, amnion and the chorion (Reisz, 1997; Benton
& Spencer, 1995). The hard shell, although porous, protects the embryo from
“mechanical shock” (Romer, 1957) and stops the egg from drying out, but still
allows gas exchange with the surrounding environment (Benton & Spencer,
1995). The albumin (egg whites) provides the embryo with water and protein,
whilst the large amount of yolk enables the embryo to grow (Romer, 1957). In
addition to this, the chorion (allows gas exchange and surrounds the embryo and
yolk sac), amnion (liquid-filled sac surrounding the embryo and is the
innermost membrane (Sigurdsen & Carroll, 2016)), and the allantois (sac
beneath the shell, functioning as a lung through gas exchange via the shell
pores, as well as acting as a bladder, storing waste products) are extra
membranes which are distinctive of amniotic eggs (Romer, 1957), and provide protection
to the egg (Smithson et al., 1994). The amnion as well as being present in
hard-shelled eggs is also present in animals that give birth (Sigurdsen &
Carroll, 2016). These embryonic membranes however, are unlikely to be preserved
as fossils and any eggs from the Carboniferous are rare to be found fossilised (Smithson
et al., 1994; Sigurdsen & Carroll, 2016). The amniotic egg provided access for
reptiles to spread across the terrestrial landscape without returning to water
for reproduction (Reisz, 1997).The key differences
between reptiles and amphibians is the ability of reptiles laying hard-shelled
eggs on land without them drying out whilst amphibians lay their eggs in or
near water, relying on the water to provide the embryo with oxygen and food
(Sues, 2016). Reptiles do not have skin glands but developed scales in the
epidermis made from keratin (Sues, 2016). The primary function of these scales is
to prevent water loss (Sues, 2016), therefore, enabling reptiles to live away
from water.

 

Environment
and climate needed for reptile evolution

The Carboniferous is
known for its Coal Forests, primarily because Europe and North America were at
the equator from 318-299 Ma and the tropical temperature allowed the growth of
vast amounts of vegetation creating the rainforests (Sahney et al., 2010). The rainforests
allowed more niches to become available in which animals could live in. Figure 4
highlights the high levels of oxygen during the Carboniferous; peaking at 32%,
much higher when compared to the present-day oxygen levels at 21% (National
Geographic, 2012). In the Late Carboniferous, the temperatures started cooling gradually,
shifting towards an arid climate due to the vast swamps taking in large amounts
of carbon dioxide. The annual sequestration of carbon at this time was 13-47 x
109 tonnes (Cleal, 2017), which drastically reduced the amount of
carbon dioxide in the atmosphere and thus resulting in cooler temperatures,
receding sea levels and glaciation. Ice caps formed over Gondwana and the
glacial deposits of this ice cap are found in South Africa as tillites
(Falcon-Lang et al., 2010). The climate cooling was advantageous for reptiles
as they are more tolerant of colder temperatures than amphibians. This climate factor
further allowed their evolution.When
did reptiles evolve

There are two proposed
dates as to when and which animal reptiles evolved from. One is the Hylonomus
lyelli which evolved 315 Ma and is the more widely accepted of the two. The
other is the Westlothiana lizziae which evolved 350 Ma. Although it was similar
to reptiles, the morphology of Westlothiana was closer to that of amphibians.
Early reptiles in general, were similar to modern primitive lizards; they were
small in size (Carrol, 1982). To first identify whether a vertebrate is an
amniote (including reptiles) the vertebrate must have an astragalus in the
ankle that is formed through the fusion of tarsals, along with ossified
vertebrae with large pleurocentra (Sigurdsen & Carroll, 2016).

Reptile trackways have been
located in the Lower Pennsylvanian Grande Anse Formation of New Brunswick, which
underlies the assemblage in which the Hylonomus was found by 1 km, and so, this
site shows the earliest evidence of amniotes (Falcon-Lang et al., 2007). The trackways
represent true tracks because the preservation was on the underside of the
bedding plane and in convex hyporelief, along with transverse scale impressions
which were interpreted to be made from keratin: a distinctive reptilian feature
(Falcon-Lang et al., 2007). Table 1 shows the measurements of key features used
to compare amniotes (reptiles) and non-amniotes. The trackways found in New
Brunswick correlates with the measurements of amniotes rather than
reptiliomorphs. This means that the trackways represent early reptiles. The
lithology in which the trackways were found are associated to an alluvial plain
with an eastward palaeocurrent flow (Falcon-Lang et al., 2007). This implies
that early reptiles of the Carboniferous lived on dryland like dry river beds
or dry alluvial plains (Falcon-Lang et al., 2007). These trackways provide
evidence for the existence of reptiles to coincide with the existence of the
Hylonomus.

Westlothiana
lizziae

The articulated skeleton
of Westlothiana lizziae (Figure 5) found at East Kirkton quarry, near Bathgate,
West Lothian, Scotland in 1988, was considered the oldest known reptile
(Smithson & Rolfe, 1990), and is a holotype. The skeleton was found in the
Upper Oil Shale Group of the Upper Viséan, and was considered the most
primitive reptile, with it being 20 cm long, containing 48 simple conical teeth
in each jaw and having forelimbs shorter than its hindlimbs (Smithson et al.,
1994). The site itself is dated to 335 Ma and is a part of the lower Brigantian,
from the Viséan in the Lower Carboniferous (Sigurdsen & Carroll, 2016).

The reasons why Westlothiana
was considered a reptile is partly because the skull resembles those of other
amniotes: the large size of the parietal, and the lack of the intertemporal,
squamosal notch and labyrinthine infolding of the teeth (Smithson et al., 1994).
Westlothiana had a short tibia and fibula, characteristic of amniotes (of which
reptiles are part) (Smithson & Rolfe, 1990). The skull roofing bones had a smooth
surface typical of reptiles and did not have a dermal ornament which is
typically found in tetrapods (Smithson et al., 1994). The skull itself resembled
that of both tetrapods and amniotes (Sigurdsen & Carroll, 2016). It also
shared a trait with Late Carboniferous amniotes: the cheek was loosely attached
to the back of the skull table (Smithson et al., 1994). Westlothiana had
overlapping oval shaped dorsal scales which were relatively thick and covered
the back of the skull and parts of the tail, however, due to the thickness and heavy
layering of the scales, it is classed as a primitive development which was not
in amniotes (Smithson et al., 1994).

However, there is a
debate over whether Westlothiana is a reptile because it has not been directly determined
the type of eggs it laid (amphibian or amniotic). Key features of the skull
were not well preserved and so it was inconclusive what precise group Westlothiana
belonged to, other than that it was not an amniote. To accurately place Westlothiana,
more specimens are required (Reisz, 1997). With further research, it has been
determined that Westlothiana is a reptiliomorph and is a stem group from amniotes
(Smithson et al., 1994). It is a stem group because it does not have an otic
notch, gastrocentrous vertebrae or the pedal phalangeal formula similar to amniotes,
but cannot be classified as an amniote because the structure of the palate and
tarsus are too primitive than defined early amniotes (Smithson et al., 1994).
Also, Westlothiana has lamellae along the vertebrae which is not present in
early amniotes like the Hylonomus (Smithson et al., 1994). When Westlothiana
was first described, it was stated to have two large proximal elements in each
tarsus: the astragalus and the calcaneum (Smithson, 1989), a distinctive feature
of all amniotes. However, further study showed a third proximal tarsal: tibiale,
as seen in Figure 6. What was previously regarded as the astragalus and
calcaneum were actually the intermedium and fibulare respectively (Smithson et
al., 1994). Another feature which Westlothiana does not possess, is the pterygoid
flange which is key in early amniotes (Smithson et al., 1994). Westlothiana is therefore,
a reptiliomorph because it lacks the pterygoid flange and an astragalus and
calcaneum which are key features of amniotes (Smithson et al., 1994), instead,
Westlothiana is a transition animal from amphibian to reptile, where is possesses
both amphibian and reptilian features.

Hylonomus
lyelli

The more widely regarded
first reptile is the Hylonomus lyelli (Figure 7), found in the Westphalian
stage of Joggins, Nova Scotia, Canada (Lambert et al., 2003; Sigurdsen &
Carroll, 2016) by William Dawson in 1859 (Falcon-Lang et al., 2010) and was
classed as the earliest known amniote and possessed slender limbs (Meyer &
Anderson, 2013; Sigurdsen & Carroll, 2016). The reasons being, because the
Hylonomus had a transverse flange on the pterygoid, an astragalus on the ankle
joint, phalangeal formula of 23453 based on the manus and pes (Sigurdsen &
Carroll, 2016). All of these characteristics are diagnostic for reptiles, thus,
naming the Hylonomus as the oldest reptile.  Some of the specimens found at this site were
complete skeletons and in some cases, scales were found intact. Reasons for the
preservation of these specimens is from forest floor traps where the specimens
were found. The formation of these traps occurred by the smothering of tree
roots via floods or salty water that results in the rotting and collapsing of
trees, leaving the stump, that over time, hollowed, forming a trap through the
rise in the forest floor by continued floods (Lambert et al., 2003). As a
result, this site only contains terrestrial fossils which were found inside
hollow tree stumps of lycopods (Smithson et al., 1994). Although, another
reason why vertebrates were found in hollowed tree stumps is because they may
have been used as shelter, especially during forest fires, which inevitably,
burned the animals. This is because charcoal was found on some of the specimens
(Falcon-Lang et al., 2010).

Conclusion

Reptiles evolved in the
Carboniferous, approximately 315 Ma, with the first undisputed reptile being
the Hylonomus lyelli, which was found in 1859 by William Dawson. Older
vertebrates that resembled reptilian morphologies have been regarded as transition
vertebrates from amphibians to reptiles (reptiliomorphs). The Westlothiana
lizziae being regarded as a reptiliomorph. The key evolutionary advancement
that allowed the evolution of reptiles was the amniotic egg, which enabled eggs
to be laid on the land without drying, but still allowing oxygen and carbon
dioxide exchange with the environment and water retention. This key
evolutionary step allowed life to transition out from the water towards land. The
entire terrestrial landscape therefore was eventually inhabited by reptiles through
their tolerance of low temperatures, slow metabolism and their ability to use
their surroundings to alter their body temperatures. Through the evolution of
reptiles, dinosaurs, birds and mammals also evolved.