Research Paper for Biology 301: Vertebrate Zoology

The Mercer University Honor Code was read and observed by Ahmad Rafati.

Introduction

Gigantism has evolved independently in many different vertebrate lineages. One common trait that was a prerequisite for the evolution of gigantism in all vertebrate lineages however was the acquisition of the myelin sheath (Zalc, 2015). The myelin sheath allowed for better transmission of nerve signals across the large body of gigantic vertebrates (Zalc, 2015). Since the acquisition of myelin, vertebrates now possessed the ability to grow much bigger. Having a very large body size through gigantism has many advantages as it can potentially reduce the risk of predation, permit tolerance to a greater range of environmental conditions, and allows large species to be the top consumers in their habitats (Vermeij, 2016). One major group of gigantic terrestrial vertebrates was the sauropods. Sauropods were a very successful group of herbivorous dinosaurs that flourished at the end of the cretaceous period (Cerrada et al., 2017). The now extinct sauropod dinosaurs were the largest living terrestrial vertebrates to ever live (Sander et al., 2010). The ability of sauropods to increase their body size is thought to have been due to specific evolutionary innovations such as a long neck, thermophysiology, rapid growth dynamics, and a specialized appendicular skeleton (Bonnan et al., 2013; Cerrada et al., 2017; Henderson et al., 2013). By comparing sauropods to the more primitive sauropodomorphs and to large eutherian mammals, we can come to a better understanding on how vertebrates evolved gigantism.

There are many groups of marine vertebrates that have also evolved gigantism. The biggest marine vertebrate is the blue whale (Ferrón et al., 2017). The blue whale is an endothermic filter feeder and is the largest living animal to ever inhabit Earth (Ferrón et al., 2017). The ability of the blue whale to become so gigantic may have been due to their thermophysiology, feeding strategy, low activity levels, and a lower metabolic rate (Anderson et al., 2012; Ferrón, 2017; Ferrón et al., 2017). The evolution of these traits allowed blue whales to sustain the largest body size on Earth. By studying the blue whale’s feeding strategy, thermophysiology, and metabolic rate in comparison to other marine vertebrates of varying body size, we can infer how they evolved to become so big and then maintained their size (Ferrón et al., 2017). By considering the similarities between both very large terrestrial and marine vertebrates, we can better understand how vertebrates evolved gigantism.

Inactive Filter Feeders and Active Marine Predators

Among the gigantic vertebrates on earth, aquatic vertebrates have attained the biggest sizes ever recorded (Ferrón et al., 2017). Balaenoptera musculus, or the blue whale, is the largest vertebrate to ever live (Ferrón et al., 2017). The evolution of gigantism in blue whales is thought to have been due to diet related niche partitioning, activity levels, and thermoregulation (Slater et al., 2017). Blue whales are filter feeders that consume large amounts of zooplankton, which allows them to inhabit nearly all oceans (Anderson et al., 2012).

There is clear evidence showing that the largest marine vertebrates appear to be inactive filter feeders, while marine predators always tend to be smaller (Ferrón et al., 2017). This is evident when comparing the blue whale to any large macro predatory sharks, namely the otodontids (Ferrón, 2017). The high activity levels necessary for being a predator can explain the difference in body sizes between the two gigantic marine vertebrates (Ferrón et al., 2017). Active marine predators have shifted to having a higher metabolic rate to sustain their feeding strategy and body size. There are many factors that directly increase an animal’s rate of metabolism. Higher ambient temperatures in their surroundings as well as a higher oxygen concentration in the atmosphere both increase the potential for a bigger body size in marine predators (Ferrón et al., 2017). However, Ferróna et al. predicted that as body mass increases past a certain threshold, the energetic costs of a predatory lifestyle becomes too high to maintain and only less active lifestyles, like filter feeding, can physiologically sustain a gigantic size. In other words, a vertebrate’s metabolic rate is one of the main factors driving the maximum size it can attain. Many vertebrates have mass-specific metabolic rates, which are roughly the same for all living organisms (Ferrón et al., 2017). There is a limit to which the mass-specific metabolic rate can reach as it decreases while an organism gets bigger (Ferrón et al., 2017). This is due to the fact that as an organism gets bigger its activity level begins to decrease, in turn decreasing its metabolic rate (Ferrón et al., 2017). Blue whales can maintain a much larger body size with decreased activity levels because they are slow filter feeders. Active marine predators cannot evolve the level of gigantism present in the blue whale due to their high activity levels (Ferrón et al. 2017).

The thermophysiology of large marine vertebrates is a major driving force in permitting the evolution of gigantism. Most of the extinct or extant gigantic marine vertebrates were endothermic, meaning they use their own metabolism to generate body heat to increase their internal temperature above their surroundings (Ferrón, 2017). This is evident when comparing the predatory endothermic sperm whale to the predatory ectothermic Greenland shark. Both of these marine vertebrates are gigantic, however the sperm whale is over four times bigger than Greenland shark (Ferrón et al., 2017). This is also noticeable when comparing the endothermic filter feeding blue whale that is more than 10 meters longer than its ectothermic filter feeding counterpart, the whale shark (Ferrón et al., 2017). Differences in the metabolic levels can serve to explain the size disparity between large endothermic and ectothermic marine vertebrates (Ferrón et al., 2017). Endothermic animals in general have higher levels of metabolism than ectothermic ones, which would allow them to be able to afford the high-energy costs of feeding and having a larger body (Ferrón et al., 2017).

Sauropods, Sauropodomorphs and Other Terrestrial Vertebrates

Basal Sauropodomorphs are the predecessors to the more derived sauropods of the early Cretaceous Period (Cerada et al., 2017). The Sauropodamorph taxon is comprised of non-sauropod sauropodomorphs that are more primitive than their sauropod counterparts (Cerada et al., 2017). The evolution of gigantism in sauropods from sauropodmorphs was accompanied by changes in their growth patterns and constant predation (Sander et al., 2010).

Most sauropodomorphs are characterized with having a cyclical growth pattern where bone development is slow and occasionally stops (Cerada et al., 2017). This entails that basal sauropodomorphs reached maximum size very late into their life cycles, which subjected them to predation (Sander et al., 2010). More derived sauropods instead experience unrestricted rapid growth by uninterrupted deposition of fibrolamellar bone tissue during their early life (Cerada et al., 2017). This uninterrupted rapid growth was a synapomorphy for sauropods and allowed them to reach maximum size early on in life (Cerada et al., 2017). This rapid growth is thought to have evolved in sauropods due to constant predation (Sander et al., 2010). For herbivores like the sauropods, the most significant selective advantage is for them to increase their body size. This is a common selective advantage as the biggest herbivores are commonly a magnitude larger than the biggest terrestrial carnivores (Sander et al., 2010). The sauropod eventually halted its growth due to its size being big enough for sufficient protection from predators (Sander et al., 2010).

Proboscideans (elephants) are extant eutherian mammals that evolved to be giant terrestrial herbivores (Bonnan et al., 2013). The level of gigantism exhibited by the sauropods however occurred much more rarely in eutherian mammals with only a small percentage of them reaching above 10,000kg (Bonnan et al., 2013). The frequency of gigantism in these two groups can be predicted by comparing their appendicular skeletons and the accompanying soft tissues. Both sauropods and proboscideans exhibit behaviors in which they reduce their mobility and minimize any kind of stresses on their skeleton with respect to their size (Bonnan et al., 2013). With increasing size comes a decrease in locomotion, and these two groups both share similar gaits (Bonnan et al., 2013). However the two clades do differ in their long bone dimensions and the formation of secondary centers of ossification (Bonnan et al., 2013). These differences have a drastic affect on the shape of the subchondral bone as well as the thickness of articular cartilage, which acts like a shock absorber (Bonnan et al., 2013). Sauropods possessed very thick articular cartilage in their joints that allowed them to distribute stresses from their massive size to their long bones and subchondral bones effectively (Bonnan et al., 2013). This allowed sauropods to be able to sustain their excessive size without many detrimental affects. Eutherian mammals instead have thin articular cartilages associated with their subchondral bones that prevent extensive distribution of stresses to their long bones and subchondral bones (Bonnan et al., 2013). In general, with increasing size came a decrease in the overall thickness of articular cartilage in eutherian mammals. The opposite holds true for the sauropods however with an increase in body size came an increase in the thickness of articular cartilage in their joints (Bonnan et al., 2013). Most eutherian mammals evolved more congruent joints in conjunction with thin articular joints to allow for better mobility and aid in deceleration of the joints during locomotion (Bonnan et al., 2013). Increased congruency and thin articular cartilage limits the amount to which a vertebrate can reduce stresses, which in turn limits maximum body size. The shape of subchondral bones and the thickness of articular cartilages play a major role in the attainment of gigantism in vertebrates (Bonnan et al., 2013).

It has been hypothesized that the sauropods were characterized by having tachymetabolic endothermy where heat from their metabolic processes generated an elevated basal metabolic rate (Sander et al., 2010). This type of thermoregulation allowed for the very high growth rate that evolved in the sauropods that was absent in sauropodomorphs (Sander et al., 2010). Tachymetabolic endothermy allowed for the bones of sauropods to grow rapidly. Eutherian mammals, like the giant proboscideans, also exhibit tachymetabolic endothermy (Henderson, 2013). This type of thermoregulation does have disadvantages in regards to an increase in body size. Gigantic vertebrates with tachymetabolic endothermy commonly experience overheating due to the low surface area to volume ratio (Henderson, 2013). Heat cannot readily escape from the body causing internal body temperature to increase dangerously high. Sauropods and large eutherian mammals both evolved mechanisms to control overheating. Proboscideans evolved a method to control overheating by using the surface area of their highly vascularized ears to dissipate excess heat (Henderson, 2013). It is speculated that the long necks of sauropods was used as cooling structures. The jugular veins and the carotid arteries may have exchanged heat with each other as the blood traveled from the body to the neck, cooling the animal (Henderson, 2013). Also sauropods were herbivores that did not chew their food, which allowed them to expend less energy and lower their metabolism (Sander et al., 2010). The surface area of their necks increased with an increase of their metabolic rate. This shows congruency between large eutherian mammals and sauropods in terms of their thermal biology.

Conclusion

The evolution of gigantism occurred in many terrestrial and aquatic vertebrate lineages. There are certain advantages to having a large body size. Gigantism allows vertebrates to access new niches, reduce predation, and become the top consumers in their habitats (Vermeij, 2016). There are many groups of marine vertebrates that evolved gigantism, namely the blue whale. The blue whale is a slow filter feeding, endothermic, giant that holds the record for being the largest animal to ever inhabit Earth (Ferrón et al., 2017). Slow inactive filter feeders like the blue whale are always larger than marine predators because they can maintain their large size by decreasing their activity level (Ferrón et al., 2016). The blue whale also had a much lower basal metabolic rate when compared to other large marine predators that allowed them to sustain their gigantic body (Ferrón et al., 2017). One major group of giant terrestrial vertebrates is the sauropods. The herbivorous sauropods were the largest terrestrial vertebrates to ever live (Sander et al., 2010). They possessed many key adaptions that allowed for the evolution of gigantism. When compared to most other terrestrial vertebrates, sauropods had a larger and more structured appendicular skeleton even when adjusting for relative size variations (Bonnan et al., 2013). They also had much thicker articular cartilage in their joints that helped them mitigate their bulk (Bonnan et al., 2013; Cerrada et al., 2017). Sauropods also exhibited rapid growth dynamics, which was absent in all vertebrates of that time period (Cerrada et al., 2017).

There are certain features that are present in all gigantic vertebrates. All gigantic vertebrates have a low basal metabolic rate that limits their activity levels (Ferrón, 2017; Ferrón et al., 2017; Sander et al., 2010). The thermophysiologies of large terrestrial and aquatic vertebrates are also similar as they are all endotherms (Ferrón, 2017; Ferrón et al., 2017; Sander et al., 2010). Feeding strategies that do not require much energy expenditure such as an herbivorous diet or filter feeding are also similar in all large vertebrates (Sander et al. 2010; Slater et al., 2017; Ferrón et al., 2017). These similarities show some of the necessary features that may play a major role in the evolution of gigantism. Overall, many different factors independently, or altogether, seem to facilitate the evolution of gigantism in vertebrates.

Studying the similarities of different gigantic vertebrate groups could shed light on how vertebrates evolved gigantism. By comparing other types of large vertebrates with small vertebrates within the same group, more information on the mechanisms that allowed such a variation in size can be understood. Moving forward, instead of just looking at the extreme cases of gigantism, more studies should compare groups who have more recently evolved moderate levels of gigantism.

References

Matthew F. Bonnan, D. Ray Wilhite, Simon L. Masters, Adam M. Yates, Christine K. Gardner, Adam Aguiar. (2013). What Lies Beneath: Sub-Articular Long Bone Shape Scaling in Eutherian Mammals and Saurischian Dinosaurs Suggests Different Locomotor Adaptations for Gigantism. PLoS One. 8(10).

P. Martin Sander

1

,AndreasChristian

2

,MarcusClauss

3

, Regina Fechner

4

,Carole

T. Gee

1

, Eva-Maria Griebeler

5

, Hanns-Christian Gunga

6

,J¨urgen Hummel

7

,Heinrich

Mallison

8

,StevenF.Perry

9

, Holger Preuschoft

10

,OliverW.M.Rauhut

4

,Kristian

Remes

1,4

, Thomas T¨utken

11

,OliverWings

8

and Ulrich Witzel

12

P. Martin Sander

1

,AndreasChristian

2

,MarcusClauss

3

, Regina Fechner

4

,Carole

T. Gee

1

, Eva-Maria Griebeler

5

, Hanns-Christian Gunga

6

,J¨urgen Hummel

7

,Heinrich

Mallison

8

,StevenF.Perry

9

, Holger Preuschoft

10

,OliverW.M.Rauhut

4

,Kristian

Remes

1,4

, Thomas T¨utken

11

,OliverWings

8

and Ulrich Witzel

12

Ignacio Alejandro Cerda, Anusuya Chinsamy, Diego Pol, Cecilia Apaldetti, Alejandro Otero, Jaime Eduardo Powell, Ricardo Nestor Martínez. (2017). Novel insight into the origin of the growth dynamics of sauropod dinosaurs. PLoS One. 12(6).

Humberto G. Ferrón, Carlos Martínez-Pérez, and Héctor Botella. (2017). The evolution of gigantism in active marine predators. Historical Biology. 10, 1-5.

Humberto G. Ferrón. (2017). Regional endothermy as a trigger for gigantism in some extinct macropredatory sharks. PLoS One. 12(9), 1-32.

Donald M. Henderson. (2013). Sauropod Necks: Are They Really For Heat Loss? PLoS One. 8(10), 1-8.

P. Martin Sander, Andreas Christian, Marcus Clauss, Regina Fechner, Carole T. Gee, Eva-Maria Griebeler, Hanns-Christian Gunga, Jürgen Hummel, Heinrich Mallison, Steven F. Perry et al. (2010). Biology of the sauropod dinosaurs: the evolution of gigantism. Biol. Rev. 86, 117-155.

Slater G.J., Goldbogen J.A., Pyenson N.D. (2017) Independent evolution of baleen whale gigantism linked to PlioPleistocene ocean dynamics. Proc. R. Soc. 284, 1-8.

Geerat J. Vermeij. (2016). Gigantism and Its Implications for the History of Life. PLoS One. 11(1).

B. Zalc. (2015). The acquisition of myelin: An evolutionary perspective. Brain Research. 1641, 4-10.

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