Megalodon Shark Evolution by Lutz Andres - Carcharodon versus Carcharocles






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C. megalodon - Megatooth Shark
Carcharodon versus Carcharocles

By Lutz Andres



'Megalodon' - this sonorous name probably evokes fascination and respect in every elasmobranch aficionado because everyone knows those gigantic jaw replicas which give us an idea of the truly monstrous size of this shark. There are many myths and speculations about this Megatooth Shark which often is presented as a direct ancestor of the Great White Shark Carcharodon carcharias, almost looking alike but the size of a modern Blue Whale... Some even speculate that these animals are still living today, somewhere in the vast realms of the deep sea. But this is wishful thinking and ignoring existing evidence as one can easily guess. After thorough scrutiny many theories have to be strongly revised and with new fossil finds more and more 'hard facts' see the light of day which change our image of Megatooth completely... Since over 20 years D.E.G. member Lutz Andres is collecting fossil shark teeth. During recent years he has been studying the evolution of certain shark species. Here is his interpretation of what nowadays is under discussion in elasmobranch paleontologist circles.


Hardly anything is known about the Megatooth Shark. Many fossilised teeth, a few vertebrae and the locations of these finds are the only true facts with which one can work and derive all other information from. As has been noted earlier, the exaggerated visions of the Megatooth Shark have significantly to be corrected when looking more carefully at the evidence and many issues concerning this species are still under discussion.


Fig.1. Fossilised tooth of Carcharocles megalodon. Height: 13 cm. Middle to Late Miocene, Florida/USA. © Photo L. Andres


One of them is the name of this shark which roamed the seas many million years ago and explicitly has been marked as direct ancestor and sister species of the extant Great White Shark by naming it Carcharodon megalodon.

The Great White Shark (Carcharodon carcharias LINNEAUS, 1758) as a species can be traced back until the early Pliocene (5 mya). Its fossil teeth (Fig.2) can be found worldwide in almost all marine sediments of the correct age, e.g. in the Pliocene of Cadzand/Netherlands, Antwerp/Belgium, Almeria/Spain, Piacenza Province/N Italy, Aurora/NC/USA, Florida/USA, Sacaco/Peru, Caldera/Chile and Sapolwana/Kwazulu-Natal/South Africa as well as in the Pleistocene of Charlotte County/Florida/USA and Naganuma Yokohama/Japan. Teeth found in Miocene (15-13.5 mya) sediments are typically slightly rolled, have a more or less eroded root (e.g. those from Bakersfield/Kern County/CA/USA and Aurora/NC/USA) and have been reworked from Pliocene (5.3-1.6 mya) into Miocene (23.8-5.3 mya) sediments (links: www.elasmo.com/paleo/sth/sa-c_car.html and ..../heim/bh-c_car.html). Common synonyms for these fossil teeth are Carcharodon rondeletti and Carcharodon sulcidens, but the teeth are identical to those of the living species not regarding intraspecific variation.

'Megalodon', the supposed ancestor of the Great White Shark, appears as a distinct species at the beginning of the Miocene (about 20 mya) and is thought to have become extinct in the Pleistocene (120,000-10,000 ya). Fossil evidence of such a late extinction is seen in teeth from 'red mud' deep sea sediments (EASTMAN, 1906), but these teeth are not very well preserved and the applied method of determining their age is questionable and not generally accepted. 'Megalodon' is always one of several species of a typical Miocene fauna. and it is present in almost all Miocene deposits which yield cetacean (whale) bones.

The ancestry of 'Megalodon' can be traced back as far as the Cretaceous, starting with Cretolamna appendiculata - Otodus obliquus - O. mugodzharicus (transient form) - Carcharocles angustidens - C. chubutensis (transient form) and finally C. megalodon (AGASSIZ, 1843). Characteristical for this evolutionary sequence or lineage is the initial primary increase in tooth size, the acquisition of a serrated cutting edge of the crown and finally the loss of the secondary cusps (or cusplets) due to the secondary increase in tooth thickness (Fig.4).


Fig.2. Fossil tooth of Carcharodon carcharias. Height: 5 cm. Early Pliocene, Sacaco/Peru. © Photo L. Andres


When looking at the tooth morphology of the 'Megalodon' ancestral lineage some typical features for all family members (family Otodontidae) can be found. Especially outstanding is the so-called neck zone or collum, the large transitional area between crown and root on the lingual (facing towards the inside of the jaws) face of the tooth. The collum is an additional area (besides the root) where collagenous tissue helps to fix the tooth to the jaw. Because of the same reason the root branches of the anterior (front) teeth are elongated to resemble a 'V' or 'U' (Fig.1). For a better stability of the tooth itself its lingual face is convex what clearly can be seen in a cross section of the crown. The same is true for lateral (side) teeth, but these features may be less pronounced due to the morphology of the entire dentition. The teeth of the Great White Shark, however, have almost no collum, the roots of upper jaw teeth are nearly rectangular (without well developed root branches) and the crowns are not convex but flat in cross section. They obviously have a totally different morphology and with the exception of their serrated cutting edges resemble the teeth of the fossil mako shark Isurus hastalis. These significant differences caused the paleoichthyologist Henri CAPPETTA (1987) to place the Megatooth Shark and the Great White Shark in separate genera and even separate families.

Fossil tooth of Isurus hastalis. Height 5.4 cm. Middle Miocene. California/USA.
Fig.3. Fossil tooth of Isurus hastalis. Height 5.4 cm. Middle Miocene. California/USA. © Photo L. Andres


Another important clue ponting towards a different ancestry as has generally been assumed is provided by stratigraphical collections made in Peru (MUIZON & VRIES, 1985). The layers containing shark teeth and other fossils belong to the Pisco Formation and stretch along 400 km of coastline south of Lima, particularly at Pisco - Ica - Sacaco. These layers are up to 350 m thick. Geochronological Potassium-Argon dating revealed an Late Miocene age (8.8 mya) for the lower, older layers and a Early Pliocene age (3.9 mya) for the upper, younger ones. The older layers yield a typical Miocene fauna including Isurus hastalis, but contrastingly one finds Carcharodon carcharias instead of I. hastalis in the youngest Pliocene layers. However, two layers of 'medium' age yield transient forms of those two species. Some 'transients' have only a rudimentary serration, present only on the lower parts of the cutting edges near the crown basis, the rest of the cutting edges being smooth. Other teeth are serrated almost up to the crown tip, just a short portion of the tip being smooth-edged. The problem with 'transients' is designating a correct species name because the differences between those teeth are not clear-cut but gradual. In the Pisco layers the acquisition of a serrated cutting edge is exceptionally well documented in very narrow chronological and spatial limits.


Fig.4. Illustration of the development of secondary increase in tooth thickness and loss of the secondary cusps (or cusplets) in the Carcharocles lineage. Top row: Anterior upper jaw teeth. Bottom row: Lateral lower jaw teeth. © L. Andres redrawn after : BRETTON, W.K. (1994): Fossil Sharks of the Chesapeake Bay Region.


Isurus hastalis (Fig.3) itself developed from a more slender form as has been shown by stratigraphical collecting in Belgium and the Netherlands (BOSCH, 1978 and CEUSTER, 1976). There, slender narrow teeth have been found in Middle Miocene (about 15 mya) layers and large broad teeth in Late Miocene (about 9 mya) layers as well as corresponding transients. In some cases placing a particular tooth into one or another of the categories - regardless of stratigraphy - is a question of personal preference (see above), but the trend from "slank een weinig verbreed" (slender and a little broadened) to "groot een zeer breed" (large and very broad) is clear. The slender form of I. hastalis most probably is a descendant of I. desori from which the extant Shortfin Mako Shark I. oxyrhinchus (a 'Durchläufer', a term for a species that has been present for or still is present since a long continuous period of time*) has developed. Therefore the Great White Shark is a member of the genus Isurus rather than a descendant of 'Megalodon'. An unsolved issue in this discussion is the seemingly spontaneous appearance of broad, serrated Isurus teeth (I. escheri) in the Middle Miocene (15 mya). Recently also the idea has been published to place 'Hastalis' and 'Escheri' into Carcharodon (links: www.elasmo.com/heim/bh-i_has-sth.html and ..../paleo/sth/sa-c_car.html). During recent years almost complete dentitions of Carcharodon carcharias (Sacaco/Peru) (link: www.elasmo.com/paleo/sac/sac-gw.html), Isurus hastalis and Carcharocles megalodon (Lee Creek Mine/Aurora/NC/USA) (PURDY et al., 1996) and C. angustidens (New Zealand) (GOTTFRIED, 1997) have been found, providing the possibility to compare teeth from various positions. Before those finds some American scientists obviously saw a problem in combining or separating those species into families.

Evolutionary lineages of Carcharodon carcharias and Carcharocles megalodon
Fig.5: Evolutionary lineages of Carcharodon carcharias and Carcharocles megalodon. © L. Andres
(Click on the image for the full size version)


The preference for the denomination by AGASSIZ (1843/44 status of knowledge!) who simply placed all large, serrated shark teeth into the genus Carcharodon is seemingly bigger than the acceptance of modern scientific proof mostly based on morphology and stratigraphy.

Until 1960 the genus name Carcharodon as proposed by AGASSIZ had been valid. Then CASIER (1960) demonstrated that 'Megalodon' was only a distant cousin of the 'Great White' and proposed the genus name Procarcharodon for the entire C. auriculatus - C. megalodon lineage. The central problem of the discussion, the presence of a serrated cutting edge, was now interpreted as having evolved independently. This view was accepted by most of the scientists. In the 1980's it was discovered that JORDAN & HANNIBAL already in 1923 had proposed a new genus name for the Megatooth Shark: Carcharocles. Due to the rules of nomenclature (being the older name) Carcharocles became the valid genus name for the lineage mentioned above. Meanwhile most modern paleontologists, convinced by the existent complex of evidence accept the genus name Carcharocles rather than Carcharodon.

New ideas by ZHELEZKO & KOZLOV (1999) base on a derivation by GLYCKMAN (1964) to apply the genus name of the ancestor of C. auricaulatus, Otodus obliquus, to the entire lineage. They argue that the acquisition of a serrated crown by the descendants of O. obliquus is not enough to place them in a separate genus. Hence, the Megatooth Shark would have to be renamed Otodus megalodon and the Great White Shark Isurus carcharias. At present, however, Carcharocles megalodon is the scientific name generally accepted for the Megatooth Shark.

The discussions mentioned above are only meant to make plain how difficult it can be to find correct names or reveal species relationships, a typical problem of paleontological research. Many species names, especially those by AGASSIZ (1843/44), have been given to single teeth representing just different positions in the dentition (Carcharocles disauris) or even pathological teeth (Carcharocles debrayi).

Finally, some modern views concerning size, duration in time and probable biology of C. megalodon shall be mentioned. The first reconstruction of a 'Megalodon' dentition was made in 1909 by Bashford Dean of the Museum of Natural History in New York. The body length of the animal behind it would have been 20-30 m. Based on the find of a partially preserved dentition the Smithsonian Institution in Washington DC in 1985 produced a revised version which was a third smaller than the original reconstruction. After comparison with the 'closely related' Great White Shark and new calculations the body length of 'Megalodon' was corrected to be about 13-15 m. Meanwhile, Michael D. Gottfried even reconstructed the entire skeleton of 'Megalodon' which can be admired in the Calvert Marine Museum in Maryland among others. The largest 'Megalodon' teeth are found in Late Miocene and Early Pliocene (9-3 mya) layers. The largest tooth hitherto known was found in 1987 in the Pisco Formation of Aguada de Lomas/Peru by Peter Larson of the Black Hills Institute of Geological Research. This tooth measures 17.3 cm (6.8 inches) in vertical height! Examples of the youngest layers in which the occurrence of Carcharocles megalodon is confirmed are the Pisco Formation (see above) and the Yorktown Formation (Chesapeake Bay region, East coast of the USA), both are of Early Pliocene age (about 5-3 mya). Finds pointing towards a younger age (see above) are dubious, but led to the assumption that a few specimens might have survived until today.


Fig.6. Comparing sizes of 'Megalodon' (length about 13 m) and Great White Shark (length about 6.5 m). © L. Andres
(Click on the image for the full size version)


The development of Megatooth Sharks can be traced back until the Cretaceous period as has been mentioned earlier. It is directly linked with the development of other animals. In the Cretaceous not only sharks, but also marine reptiles ruled the waters. This condition changed, however, after the extinction of the dinosaurs (65 mya) in favour of the sharks. Sharks now occupied the ecological niches for predators. Additionally, the basis for a more energy-rich nutrition was created by the rise of marine mammals in the Eocene. Hence, for reasons of functional morphology the appearance of serrated cutting edges in Megatooth Sharks is strongly linked to the evolution of whales, e.g. the ancestors of baleen whales (cetotheriids). During the Late Oligocene (30-25 mya) cetotheriids have been exceptionally numerous at times, the climatic conditions being much more favourable. Temperatures were significantly higher than they are today, tropical and subtropical waters reached much further into the higher latitudes of the polar regions. During the following epoch, the Miocene (25-5 mya), modern baleen whales (Mysticeti) developed and spread more and more. There was an obvious increase in size in whales and simultaneously in Megatooth Sharks which can be explained by a better nourishment of the whales. During the spreading out of the whales they presumably reached cooler polar waters that provided them with a richer food supply to which the whales adapted themselves. The whales were migrating between cool water feeding grounds and warm water breeding grounds. The climate became colder at the end of the Miocene and the beginning of the Pliocene (about 5 mya). The ice cover of the Antarctic polar region grew bigger and the mean sea level dropped. Living conditions and habitat for 'Megalodon', who provably loved warm waters, obviously were restricted to such a degree that the species became extinct during the Pliocene. At least this is what the fossil record says, meaning that fossils of Megatooth Sharks are lacking in the corresponding layers and places. This is where the evolution into a specialised 'super predator' obviously led to a dead end.

Lutz Andres
B. Frentzel-Beyme
Translation: Ralf Michael Hennemann

(*) 'Durchläufer' (literally: 'runners through') are all shark species which exist unchanged since e.g. the Miocene. Examples are Tiger Shark (Galeocerdo cuvier), Sandtiger Shark (Carcharias acutissima/taurus), Bluntnose Sixgill Shark (Hexanchus gigas/griseus) and also Shortfin Mako Shark (Isurus desori/oxyrhinchus). Some of the fossil species bear different names only because of their occurrence during earlier times, but they are virtually identical with the extant forms. Other species/genera, however, have developed only since the Miocene, e.g. the genus Carcharhinus: 3-5 species in the Miocene, about 30 today!

References:

BOSCH, M. van den (1978) : On Shark Teeth and Scales from the Netherlands and the Biostratigraphy of the Tertiary of the Eastern Part of the Country. - Meded. Werkgr. Tert. Kwart. Geol., vol.15 nr.4.

CAPPETTA, H. (1987) : Chondrichthyes II, Mesozoic and Cenozoic Elasmobranchii. - Handbook of Paleoichthyology, Vol.3B.

CEUSTER, J.de (1976) : Sratigrafische Interpretatie van Jong-Cenozoische Afzettingen bij Rumst (Belgie, Provincie Antwerpen) en Beschrijving van de in een Post-Mioceen Basisgrind aangetroffen Vissenfauna, II. Systematische Beschrijvingen en Conclusies. - Meded. Werkgr. Tert. Kwart. Geol., vol.13 nr.4.

GOTTFRIED, M.D. & FORDYCE, E.(1997) [wissenschaftliche Beschreibung in Bearbeitung persönliche Mitteilung, Feb. 2000]

MUIZON, C. de & VRIES, T.J. de (1985) : Geology and paleontology of late Cenozoic marine deposits in the Sacaco area (Peru). - Geologische Rundschau 74/3.

PURDY, R., McLELLAN, J.H. SCHNEIDER, V.P., APPLEGATE, S.P., MEYER, R.L. & SLAUGHTER, B.H. (1996) : Preliminary study of the Neogene fish faunas from the Texasgulf, Inc., Lee Creek Mine, North Carolina. - Smithson. Contrib. Paleobiol. ["in press", 1998/99 noch nicht erhältlich].

ZHELEZKO & KOZLOV (1999) : Elasmobranchii and Palaeogene Biostratigraphy of Trans Urals and Central Asia. (Ekaterinburg: RussianAcad. of Sciences, Urals Branch, Materials and Stratigraphy and Paleontology of the Urals, Vol. 3) 321 pp., 61 plates (of teeth), 39 figs.




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