Ligers, Zorses and Bottlenose Dolphins

Most people do not realise that there are numerous species of Bottlenose Dolphins; the exact number is a subject of strong debate and depending on the background of whom you ask will strongly influence the number they give. For example, a conservationist, a geneticist and a taxonomist are unlikely to agree on such matters. However, there are at least three species formally recognised by everybody, each with their own behavioural and physical characteristics. Of the three recognised species Tursiops truncatus, known as the Common Bottlenose Dolphin, is the species that most people are familiar with. This is owing to it’s near global distribution (it is found in every sea except those in the polar regions), its prevalence in popular culture – be it films or TV and of course its popularity in aquariums during the latter half of the 20th century. The second of the three species, Tursiops aduncus, is more often referred to as the Indo-Pacific Bottlenose Dolphin and as its name suggests is found in the Indian and Pacific Oceans only. Whereas T. truncatus is found in both coastal and offshore environments T. aduncus is principally only found in coastal waters. The third species, Tursiops australis, known as the Burrunan Dolphin is found only in coastal waters of parts of Australia.

One of the most controversial topics in in the field of biology is the subject of what defines a species. Certainly in most high schools, pupils are taught that animals belong to the same species if they can reproduce and form fertile offspring. This is undoubtedly complete fallacy and a recent topic of research that I have been involved in significantly proves this. This research, led by Dr. Tess Gridley of the University of Cape Town, has just been published and I provided the genetics elements included in the paper. The research focuses on the production of fertile hybrids by two species of Bottlenose Dolphin when kept together in captivity. Hybrids are the offspring of two different species – famous examples include the Liger (the offspring of a Lion and Tiger) and Zorse (yes you guessed it, the offspring of a Zebra and a Horse).

Hybrids
Hybrid animals are remarkably common. Here we see a Liger (Lion and Tiger hybrid) on the left and a Zorse (Zebra and Horse hybrid) on the right.

First of all, let’s deal with the elephant in the room. Yes, this research is based on dolphins kept in captivity. Let me be absolutely clear that I am in no way an advocate for keeping any species of cetacean in captivity. When the individual dolphins on which this research focusses were taken into captivity it was the 1970s, at which time our understanding of cetacean biology, in particular their emotional intelligence, was significantly inferior to our understanding today. Like all areas of knowledge, our understanding progresses through time and moral humans adapt their behaviour and actions to take account of this improved understanding. Flipping this on its head, we should be reticent to judge people who made decisions in the past with which we would normally condemn when judging by todays understanding, morality and societal will. Times and understanding were different then and as long as we are willing to, pragmatically and sensibly, adapt our actions today to take account of our improved understanding then we should look forward and not back. No, we should not be taking new cetaceans into captivity but those that currently are kept in aquaria, like those in this study, provide an opportunity to expand our knowledge of cetaceans such that we can continue to improve our decision making in the future; thus, having greater benefit for the conservation of wild cetaceans.

Our research focussed on two dolphins and their offspring. The first, a male Tursiops truncatus by the name of Gambit, and the second a female Tursiops aduncus by the name of Frodo. As well as physical characteristics (Frodo has speckling on her underside, a feature common in older Tursiops aduncus), we confirmed their species identity genetically. This is done using DNA extracted from blood taken from routine veterinary check-ups. The principal finding of this study revealed that hybrid and backcross offspring were fertile – proven by a second generation in both cases.

Backcross fertile
One of the apparently fertile backcross offspring featured in this research.
Picture1
The underside of Frodo, showing her belly speckling that is a common feature in mature Tursiops aduncus individuals.

This finding is important for two reasons. Firstly, it adds further weight to current scientific thoughts on evolution as a process. We like to think that evolution is a linear process and that once a new species is formed it is permanent until such time that it may go extinct due to some natural disaster or change in environment. We know, however, that this is not the case at all. There are a number of emerging examples that show species emerged in the past, likely as a result of physical separation, but then disappeared again when the physical barrier was removed because they simply merged and interbred with their parent species. A great example of this comes in the form of the Common Raven. We also know that reticulation, or the interbreeding of species during speciation is common and demonstrating the production of fertile hybrid offspring in this study provides a mechanism for this to happen.

Perhaps more importantly, however, this study demonstrates the potential resilience of Bottlenose Dolphins to adapt to changing environments. By producing fertile offspring, the success of gene flow events between different species of Bottlenose Dolphin may allow them to adapt to a more coastal or more pelagic way of life more readily should the need arise. We should take encouragement in this new understanding; although life in our oceans is currently under a great many threats it is likely that, thanks to the plasticity of evolution, the famous smile of a Bottlenose Dolphin will continue to greet us for many generations to come.

 

 

This research is published in PLoS ONE, Sepember 2018. You can download a copy of the paper here.

Full paper citation:

Gridley, T., Elwen, S.H., Harris, G., Moore, D.M., Hoelzel, A.R. and Lampen, F., 2018. Hybridization in bottlenose dolphins—A case study of Tursiops aduncus× T. truncatus hybrids and successful backcross hybridization events. PloS one, 13(9), p.e0201722.

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The origins of great whites

There is great debate within the scientific community with regards to the true ancestral origins of the modern White Shark Carcharodon carcharias. There currently exist two main hypotheses on the correct phylogenetic placement of C. carcharias. Both hypotheses talk in terms of sharing a more recent common ancestor with an extinct species (Smith 1994). The older of the two hypotheses suggests that C. carcharias is descendant from the ancestral lineage of megatoothed sharks such as Carcharodon megalodon and that the megatoothed sharks should be placed within the family Lamnidae. The more recent hypothesis suggests that C. carcharias is descendant from the ancestral lineage of the extinct Mako shark Isurus hastalis and that the megatoothed sharks should be separately placed within the family Otodontidae. It is important to be clear that neither theory suggests an absolute direct descendancy from the afore-mentioned species as the scarcity of fossil evidence could not support such a claim.

4cm tall Fossil Carcharodon carcharias tooth from Miocene (~20 million year old) sediments in the Atacama Desert of Chile. (Photo credit: Wikipedia)
4cm tall Fossil Carcharodon carcharias tooth from Miocene (~20 million year old) sediments in the Atacama Desert of Chile. (Photo credit: Wikipedia)

Proponents of the megatoothed descendancy hypothesis (Applegate and Espinosa-Arrubarrena, 1996; Gottfried et al., 1996; Martin, 1996; Gottfried and Fordyce 2001; Purdy et al., 2001) base their conclusions upon aspects of shared tooth morphology between the modern C. carcharias and principally the extinct C. megalodon. These similarities include: similar tooth morphologies between juvenile C. megalodon and adult C. carcharias; fine tooth serration in both adult C. carcharias and C. megalodon; shared chevron-shaped neck area on the lingual surface of the upper anterior teeth, a mesially inclined large intermediate tooth and second anterior teeth symmetry (Gottfried et al., 1996; Gottfried and Fordyce, 2001; Purdy et al., 2001). Under this phylogenetic regime the megatoothed sharks are placed within the family Lamnidae and retain their genus Carcharodon.

The evolutionary history of Carcharodon megalodon is well understood from a large quantity of fossil evidence including many transitional specimens. The direct ancestry of C. megalodon can be traced at least as far back as the late Pliocene in the form of the large mackerel shark Otodus obliqus although if Cretolamna appendiculata is considered a chronospecies then this ancestry can be extended into the Lower Cretaceous. O. obliqus had large non serrated teeth with distinctive side cusps. During the middle Eocene the side cusps of O. obliqus reduced in size and the edges developed slight serration. This Eocene species is classified as Carcharocles auriculatus. A massive increase in shark body size accompanied by further reduction in teeth cusps and increased development in serrated edges marks the introduction of Carcharocles angustidens during the late Oligocene. Carcharodon megalodon finally evolved from C. angustidens during the early Miocene and is characterized by a further increase in body size, further development in serrated tooth cutting edges as well as a complete loss of teeth side cusps. The megatoothed descendancy hypothesis suggests that the origin of Carcharodon carcharias is derived in a form of dwarfism of Carcharodon megalodon (Ehret et al 2009).

The Isurus hastalis descendancy hypothesis argues that owing to a shared overall tooth shape and labio-lingual flattening in tooth morphology in both the later ‘broad-form’ I. hastalis (specifically the suggested transitional fossil Isurus xiphodon as described by Purdy et al. 2001) and Carcharodon carcharias, the megatoothed sharks should be viewed as a distinct and non related taxon (Casier 1960). This phylogenetic regime would see the megatoothed sharks placed in the separate family Otodontidae, containing other extinct genera such as Otodus and Parotodus, with Carcharodon megalodon being renamed as Carcharocles megalodon (Casier, 1960; Glickman, 1964; Capetta, 1987). Proponents of the Isurus hastalis descendancy hypothesis also argue that not only are tooth serrations in the megatoothed sharks much finer than those found in C. carcharias but that megatoothed sharks teeth lack enameloid in the neck area whereas C. carcharias does not (Nyberg et al. 2006).

The evolution of Isurus hastalis itself is reasonably well documented in the fossil record. All Mako sharks can find their ancestry in the Eocene epoch (approximately 50mya) with the arrival of Isurus praecursor. During the Oligocene epoch a new Mako shark, Isurus desori, appears in the fossil record. Fossils of Isurus desori are found to be almost cosmopolitan in their distribution and it is from this species that Isurus hastalis is likely to have evolved during the early Miocene. Initial forms of Isurus hastalis are relatively small and considerably longer than their width, thus these initial forms are often referred to as ‘narrow-form’ with later examples being referred to as ‘broad-form’ as their width increases in the mid to late Miocene (Alter 2013).

Casier (1960) makes suggestion of a possible transitional fossil in Isurus escheri where teeth were found to show slight fine marginal serration. Isurus escheri inhabited the waters of the Atlantic Ocean around the mid Miocene (approximately 10mya) and likely derived from the ‘narrow-form’ Isurus hastalis. Unfortunately it would appear that Isurus escheri would be an evolutionary dead-end as fossil evidence of their existence disappears within just a few million years.

True Carcharodon carcharias fossils with all modern characteristics represented have been dated back to the late Miocene with specimens being recovered from California, Maryland and Japan showing that by this time C. carcharias was already thriving across the Pacific and Atlantic Oceans (Gottfried and Fordyce, 2001; Stewart, 1999, 2000, 2002; Hatai et al., 1974; Tanaka and Mori, 1996; Yabe, 2000). A fossil recovered from Peru and described by Muizon and DeVries (1985) was suggested as another transitional fossil in favour of the Isurus hastalis descendancy hypothesis on account of weak tooth serrations but this evidence is countered by Purdy (1996) and Purdy et al. (2001) who observe that this fossil (known internationally as the Sacaco sp. on account of its discovery in the Pisco formation of the Sacaco basin, Peru) is predated by the aforementioned C. carcharias discoveries.

Based upon the simplified evidence presented it would seem that the decision is of only two possible phylogenies. It seems this view is now over-simplified as new fossil and genetic evidence (Martin 1996; Martin et al 2002) shows that the Carcharodon lineage may have split from that of the Mako sharks much earlier than suggested here and that Isurus was not in fact a true Mako but truly of the Carcharodon lineage and should therefore be placed in the genus Cosmopolitodus to indicate as such (Glikman 1964).

For now there remains great scope for research into the ancestry of Carcharodon carcharias and it is likely that debate over the matter will continue for decades to come.

 

 

In addition to the literature cited below I would refer the reader to an excellent online article by Steven A. Alter based on his many years as a collector/dealer:

http://megalodonteeth.com/origin-of-the-modern-great-white-shark

In addition to the many excellent articles published online by Jim Bourdon at:

http://www.elasmo.com/

 

 

Cappetta, H. 1987. Chondrichthyes II. Mesozoic and Cenozoic Elasmobranchii; in H.-P. Schultze (ed.), Handbook of Paleoichthyology. Volume 3B. New York, NYVerlag Dr. Gustav Fischer193 pp.

Casier, E. 1960. Note sur la collection des poisons Pale´oce`nes et E ´ oce`nes de l’Enclave de Cabinda (Congo). Annales du Muse´e Royal du Congo Belge (A.3) 1, 2:1–48.

Ehret, D. J., G. Hubbell, and B. J. Macfadden. 2009. Exceptional preservation of the white shark Carcharodon (lamniformes, lamnidae) from the early pliocene of peru. Journal of Vertebrate Paleontology.  29:1 1-13.

Glickman, L. S. 1964. [Sharks of the Paleogene and their Stratigraphic Significance] . Nauka Press, Moscow: , 229 pp. [Russian].

Gottfried, M. D., and R. E. Fordyce. 2001. An associated specimen of Carcharodon angustidens (Chondrichthyes, Lamnidae) from the late Oligocene of New Zealand, with comments on Carcharodon interrelationships. Journal of Vertebrate Paleontology 21:730–739.

Gottfried, M. D., L. J. V. Compagno, and S. C. Bowman. 1996. Size and skeletal anatomy of the giant “megatooth” shark Carcharodon megalodon; pp. 55–89 in A. Kimley, and D. Ainley (eds.), Great White Sharks: the Biology of Carcharodon carcharias. San Diego, California: Academic Press.

Martin, A. F. 1996. Systematics of the Lamnidae and origination time of Carcharodon carcharias inferred from the comparative analysis of mitochondrial DNA sequences; pp. 49–53 in A. Kimley and D. Ainley (eds.), Great White Sharks: the Biology of Carcharodon carcharias. San Diego, California: Academic Press.

Martin, A. F., A. T. Pardini, L. F. Noble, and C. S. Jones. 2002. Conservation of a dinucleotide simple sequence repeat locus in sharks. Molecular Phylogenetics and Evolution 23:205–213.

Nyberg, K. G., Ciampaglio, C. N., and G. A. Wray. 2006. Tracing the ancestry of the great white shark, Carcharodon carcharias, using morphometric analyses of fossil teeth. Journal of Vertebrate Paleontology 26:806–814.

Purdy, R. 1996. Paleoecology of fossil white sharks; pp. 67–78 in A. Kimley, and D. Ainley (eds.), Great White Sharks: the Biology of Carcharodon carcharias. San Diego, California: Academic Press.

Purdy, R., Schneider, V. P., Applegate, S. P., McLellan, J. H., Meyer, R. L., and B. H. Slaughter. 2001. The Neogene sharks, rays, and bony fishes from Lee Creek Mine, Aurora, North Carolina; pp. 71–202 in C. E. Ray, and D. J. Bohaska (eds.), Geology and Paleontology of the Lee Creek Mine, North Carolina, III. Smithsonian Contributions to Paleobiology no. 90.

Smith, A. B. 1994. Systematics and the fossil record: documenting evolutionary patterns. Blackwell Scientific Publications, Oxford, England.

Stewart, J. D. 1999. Correlation of stratigraphic position with Isurus-Carcharodon tooth serration size in the Capistrano Formation, and its implications for the ancestry of Carcharodon carcharias. Journal of Vertebrate Paleontology 19(3, Supplement):78A.

Stewart, J. D. 2000. Late Miocene ontogenetic series of true Carcharodon teeth. Journal of Vertebrate Paleontology 20(3, Supplement): 71A.

Stewart, J. D. 2002. The first paleomagnetic framework for the Isurus hastalis-Carcharodon transition in the Pacific Basin: The Purisama Formation, Central California. Journal of Vertebrate Paleontology 22(3, Supplement):111A.