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Drew Lyons
Micah Mansfield
Animal Design Project #1
Thrinaxodon
Thrinaxodon, a synapsid cynodont, was a small, mammallike reptile that lived
253 million years ago in the late Permian. It disappeared during the extinction event 245
million years ago at the end of the Olenekian portion of the Triassic period. The
discovery of Thrinaxodon was important as a transitional fossil in the evolution of
mammals.
Cladogram showing the relationship of Thrinaxodon to mammals (Botha and
Chinsamy, 2005).
Fossils of Thrinaxodon were found in modern day South Africa and Antarctica,
providing strong evidence that Thrinaxodon once roamed an area that combined these
land masses because the physiology of Thrinaxodon suggests it could neither swim
long distances nor fly. Current day separation of fossils by a vast ocean helped
scientists understand plate tectonics and the existence of a supercontinent called
Pangea.
Pangea: Image taken http://www.metafysica.nl/wings/wings_3a.html. The inserted
black box shows the location where Thrinaxodon fossils were found and where it likely
lived during the Late Permian and Early Triassic periods.
Thrinaxodon was 30 to 50 cm in length, 10 cm tall, had a large, flat head and
legs somewhat characteristic of fossorial animals that splayed out slightly from the
torso, creating a 15 cm wide stance. Indentations in fossils of its skull provide strong
evidence that Thrinaxodon had whiskers. Whiskers are a very beneficial adaptation for
predators at night because it would allow the animal to better sense its surroundings in
low light conditions, giving it a competitive advantage over its prey and other predators
that compete for similar resources. If it had whiskers then there may have been fur as
well, indicating that it was homeothermic since fur functions to insulate the animal from
the outside conditions, so the animal’s temperature is being driven more by internal
processes. Being one of the earliest mammallike organisms with fur, it was most likely
less dense than the fur modern mammals have (prehistoricwildlife.com, 2011).
Thrinaxodon had many mammalianlike adaptations that in ways allowed it to
function in similar ways as modern day mammals, suggesting it was a distant ancestor
of mammals. Key morphological innovations allowed for increased metabolic rates and
its survival through the PermianTriassic extinction event. These included features in
Thrinaxodon’s skeleton such as the addition of lumbar vertebrae on the spine and the
shortening of thoracic vertebrae, one additional occipital condyle, the presence of a
masseteric fossa, and a hardened secondary palate. The segmentation of the spine
allowed for increased weight bearing and movement in the lower back. Segmentation,
in combination with the absence of ribs in the lower abdomen, suggests the presence of
a diaphragm. The ribs now form a chest cavity that houses the lungs and provides an
attachment surface for the diaphragm, which allows for increased respiration efficiency
and minimum energy expenditure due to breathing (Cowen, 2000). The addition of an
occipital condyle functioned to increase articulation with the atlas vertebrae and
permitted more movement, which allowed it to be more aware of its surroundings and
potential predators. The masseteric fossa presented a larger surface area for muscle
attachment on the dentary bone to make chewing and processing food more efficient,
which in turn leads to a faster metabolism. One of the most important adaptations,
especially for carnivores, is the presence of the hardened secondary palate that allowed
for breathing through the nose while chewing, which is important in order to take down
struggling prey or chew for a longer period of time while still maintaining the ability to
breathe (prehistoricwildlife.com, 2011). Thrinaxodon also possesses the beginnings of
a brain case, which is shown by the epipterygoid bone expanding to alisphenoidlike
proportions, as well as nasal turbinates, which are “convoluted bones in the nasal cavity
that are covered by olfactory sense organs” (Cynodontia). The teeth of Thrinaxodon
display the mammalian traits of thecodontia (teeth present in the socket of the dentary)
and differentiated teeth. In its tooth differentiation, the three cusped post canines that
Thrinaxodon was named after were important so it could thoroughly chew its food and
decrease the time of digestion. This also suggests a faster metabolism that was more
like modern mammals, as well as an important evolutionary step towards the
tribosphenic molar (Estes, 1961). Due to this increased metabolism, Thrinaxodon was
eurythermic, meaning it was able to function in a broad range of temperatures, and was
essentially homeothermic.
Dimensions of Thrinaxodon: Hand drawn based on paper by Estes (1961).
Being able to tolerate a wide range of temperatures was extremely important for
the survival of Thrinaxodon. The late Permian and early Triassic periods in which it
lived had the hottest and most arid conditions the Earth has experienced. The
supercontinent Pangea fully formed at the end of the Permian, which led to both higher
sea levels and warmer inland temperatures (Natural History Museum). Enormous ash
plumes from volcanic eruptions and large volumes of CO2 released resulted in a
greenhouse effect that increased global temperatures. This resulted in large forest
dieoffs because photosynthesis does not function optimally above 35°C, and plants
cannot sustain themselves for long at temperatures above 40°C (Sun et al., 2012).
There were also extensive marine extinctions due to anoxic conditions, and the loss of
most large terrestrial animals due to lack of food and the inability to tolerate extreme
temperatures and dryness (Natural History Museum). The discovery of extreme
abundances of fossil fungal cells in land sediments at the PermainTriassic boundary
suggests fungi were decomposing the remnants of a mass extinction event of
gymnosperms and coalgenerating floras (Visscher H, 1996), leaving the majority of
plant life that survived to be characterized by “weedy” traits, with almost no forest
remaining. This was especially prevalent around the equator, where conditions were
the harshest and temperatures were the hottest. This was the PermianTriassic mass
extinction event, and Thrinaxodon was able to survive it.
Thrinaxodon was fossorial and able to thermoregulate by utilizing its burrowing
capabilities. Several of its adaptations made it a very effective burrower, such as a flat
top of the skull that could be used for patting down the roofs of a tunnel as it dug, thus
making the burrow more structurally sound. The lumbar vertebrae increased the
mobility of the lower spine and allowed it to turn around in the tighter spaces found in
burrows. Furthermore, it had a body that was proportionally longer than wide, making it
easier to travel down narrow tunnels. By burrowing, Thrinaxodon was able to escape
the hot temperatures during the day, therefore preventing itself from overheating. It also
evolved nasal turbinates as an adaptation to extreme arid conditions. Nasal turbinates
retain moisture well when exhaling as well as moisturizing the dry air when inhaling,
which leads to less net water loss, as shown by the graph below (ChinsamyTuran,
2012).
Figure showing effectiveness of nasal turbinates reducing evaporative loss
(ChinsamyTuran, 2012).
A small animal like Thrinaxodon has a large surface area to volume ratio,
suggesting it had a high metabolic rate per kilogram. Staying in its burrow during the
day would avoid direct exposure to solar radiation, which would allow it to remain much
cooler and minimize its water loss. Thrinaxodon would hunt for small lizards and
invertebrates at night rather than during the heat of the day, maximizing its chances of
surviving the harsher conditions that characterized the PermianTriassic extinction and
the early Triassic. As a small carnivore, it did not depend on large amounts of plants
that were not available due to the environmental conditions. Instead, it was able to prey
on smaller organisms that were more successful during these extreme environmental
conditions.
Although the time period had high seasonality, fossil records show that, “all the
Thrinaxodon limb bones exhibit fibrolamellar bone to varying degrees, which indicates
that this animal deposited bone at a relatively rapid rate” (Botha and Chinsamy, 2005).
This sustained rate of growth suggests that there is no state of torpor or hibernation to
avoid certain unfavorable seasons. Fernandez et al. (2013) were able to find a
fossilized Thrinaxodon that was curled up in a burrow. Fossilized alongside it was an
injured amphibian Broomistega, which leads researchers to believe that there was an
intruder tolerated in its burrow. The most plausible explanation to this finding was that
Thrinaxodon was probably in a brief state of aestivation. Aestivation is a state of
dormancy characterized by inactivity and a lowered metabolism found in animals that
live in regions with hot temperatures and arid conditions. Since there are no lines of
arrested growth present in Thrinaxodon’s bones, it is thought that aestivation was a
short but frequent occurrence. Therefore, we speculated that Thrinaxodon only
aestivated in the burrow during the day when it would have to compensate for high
outside temperatures. By doing this it would be able to decrease its metabolic rate and
heat gained from the environment.
In order to find Thrinaxodon’s approximate body mass, we were able to use the
brain mass
encephalization quotient (EQ= .055(body mass)
3/4 ). The EQ of a typical mammal is around 1,
and most reptiles are about 1/10 of the value. According to Jerison (1973), the EQ of
Thrinaxodon was that of a typical reptile, EQ equals 0.1. From a paper by Rowe et al.
(2011) that analyzed CT scans of early cynodont fossils, including those of
Thrinaxodon, we determined that the endocranial volume is 1.462 ml. Assuming the
brain is mostly water, and that it took up the majority of the endocranial space, then the
weight of the brain would be about 1.462 g. Using the equation from Jerison (1973)
with c equaling body mass and s equaling brain mass in grams, we were able to solve
3/4
3/4 = 265.82 ,
using EQ = .1 = 1.462g(s)
.055c3/4 , 1.462 g = .0055 c , c
c = 265.824/3 = 1709.15 g. Based on the Thrinaxodon skull that was scanned by Rowe
et al. (2011), its mass would be around 1709 g.
CT scan of Thrinaxodon skull (Rowe et. al., 2011).
(http://bio.sunyorange.edu/updated2/pl%20new/55%20CYNODONTS.htm)
To calculate basal metabolic rate (BMR) we used the calculation E = M 3/4 , as
they are essentially homeothermic and their metabolic rate is not as dependent upon
outside temperature as reptiles. The typical difference between the active metabolic
rate of a reptile and a mammal is 15 to 20 fold (Bennett), but as there are many
mammalian features that contribute to the increased metabolism of Thrinaxodon, we
assumed that its active metabolic rate (AMR) would be on the slightly lower end of the
range of 20 70% that we were presented in class (although we calculated both the
lower and upper bound). Therapsids, a monophyletic group containing Thrinaxodon,
“improved their breathing enough to maintain a fairly high basal metabolic rate
(diaphragm, perhaps the ribs of Thrinaxodon)... they were not erect athletes the way
dinosaurs were, and they could not support sustained high speed because of Carrier’s
Constraint” (Cowen, 2000). Thrinaxodon did, however, evolve more upright motility
(shown by more erect posture in diagram above) than other cynodonts. Their widened
ribs, short legs, and “femur angled 55 degrees outward, but capable of dorsoventral
movement on angled humeral head” (Cynodontia). They were likely to move only in
short bursts due to the difficulty of breathing while undergoing sideways body flexion
(the morphological problem known as Carrier’s Constraint). We estimated their
maximum sustainable rate (MMR) to be on the lower end of the metabolic scope
because of these factors. According to Peterson et al. (1990), most sustained
metabolic scopes (ratio of MMR:RMR) fall between 1.5 and 5, and the sustained
metabolic scope of a lizard that falls in the same size category as Thrinaxodon was
measured at around 1.7. With the breathing innovations possessed by Thrinaxodon
and its improved skeletal structure, we estimated it to be above the ectotherm
measured by Peterson et al. (1990), but below most of the mammals also listed on their
table.
Thrinaxodon Metabolic Rates Chart
cal hr1
J hr1
kJ hr1
BMR
1494.7
6277.8
6.28
RMR (105% BMR)
1569.4
6591.7
6.59
Lower AMR (120% BMR)
1793.7
7533.3
7.53
Upper AMR (170% BMR)
2541.0
10672.2
10.67
MMR (220% BMR)
3288.3
13811.2
13.8
Burrowing is Thrinaxodon’s most effective way of thermoregulation. By
burrowing and staying in the hole during the day, it creates its own microclimate that
allows it not only to protect itself from solar radiation, but also to cut down the loss of
water through evaporation in an extremely arid climate, as it reduces its need to lose
water in order to cool itself. Based on the hot and arid nature of at least some of the
year (there was high seasonality that brought a lot of rainfall during part of the year in
the Early Triassic), Thrinaxodon most likely experienced conditions similar to that of a
modern desert.
“The hottest microclimates in the biosphere coincide largely with the driest
microclimates. Animals inhabiting hot deserts therefore face the competing
demands of thermostasis, which calls for the evaporative loss of water from
the body to the environment, and of hydrostasis, which calls for the bodily
retention of water that is environmentally scarce” (Hoffman, 2007).
We assumed a daytime temperature of 38°C because a paper published in
Science Magazine by Sun et al. (2012) stated that “critically high temperatures may also
have excluded terrestrial animal life from equatorial Pangea, and with SSTs [sea
surface temperatures] approaching 40°C the land temperatures are likely to have
fluctuated to even higher levels. Our compilation of tetrapod fossil occurrences reveals
them to be generally absent between 30°N and 40°S in the Early Triassic”. With almost
no terrestrial life living between 30°N and 40°S, Thrinaxodon was able to occupy some
of the more extreme environments at latitudes of around 60°S, since it was in the upper
area of the inhabitable land mass and most likely still had to deal with higher
temperatures.
Hc =
20.5(1709g)0.574(TbTa)
(J hr1)
Hrad
(J hr1)
He
(J hr1)
Hmeta
(J hr1)
Hnet
(J hr1)
* Day @ 27.53°C
(in burrow)
3632.1
~0
134.6
3766.7
0
* Night @ 23°C
(active)
10290.7
~0
381.5
10672.2
0
Day @ 38°C
(active)
11760.8
12864.8
381.5
10672.2
34916.3
Night @ 25°C (in
burrow)
7350.5
~0
134.6
3766.7
3718.4
If we were to use assumptions made from the findings of Fernandez et al. (2013)
that Thrinaxodon is an animal that aestivates but for only relatively short periods of time,
then its metabolic rate when it aestivates during the day would be lower than its BMR.
For aestivating animals, typically the metabolic rate decreases substantially. In a
certain species of frog, aestivation causes a 40% decrease in BMR, which is the lower
limit of metabolic rates while aestivating amongst various species. We assumed this
lower limit as Thrinaxodon’s metabolic rate during aestivation, since it did not lower its
metabolic rate substantially enough, or long enough, to temporarily slow down its bone
growth.
In order to determine how Thrinaxodon remained in heat balance despite the
warmest temperatures on record, we calculated total heat flux by finding the sum of
ThrinaxodonFinal.pdf (PDF, 538.58 KB)
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