Megalodon: The Classification Debate Continues. . .

Today we will be exploring the classification of the largest predatory shark that has ever lived!  The following is an adapted version of a script for my video entitled "Megalodon: The Classification Debate Continues" on the YouTube channel Elasmocast. If you'd like to watch this informative highly detailed video, you can find it here:

The infamous megalodon was a fierce predator that swam the Earth’s oceans throughout the Miocene, and became extinct in the Pliocene, approximately 3.6 million years ago.  With teeth exceeding 7 inches in length, this was a massive animal that is typically estimated to reach between 50 and 65 feet in length.  But what exactly was the megalodon?

Megalodon tooth. Read on to learn what the best genus placement for megalodon is!

Megalodon belongs to the Lamniformes, an order of sharks commonly referred to as the Mackerel Sharks.  Extant members of this order include the Goblin Shark, Megamouth, and the Great White Carcharodon carcharias.  In fact, megalodon was initially placed within the Great White’s genus Carcharodon!

Since its initial placement within the genus Carcharodon, megalodon has undergone several revisions in classification, including Carcharocles, Procarcharodon, Megaselachus, and Otodus.  But which is the valid genus for megalodon? 

Firstly, let’s look at why this shark does not belong within the genus Carcharodon. While both sharks are considered to be large Lamniformes (granted one was A LOT larger than the other), and both had evolved serrated teeth, these similarities aren’t enough to combat the vast differences within their dentitions.  The Great White lineage can be traced back to the Paleocene with the genus Isurolamna; older than that, the evolution of this lineage becomes murkier.  Megalodon’s lineage can be tracked down to the Cretaceous with the genus Cretalamna.  Significant differences within these two lineage’s dentitions include the possession of symphyseal teeth which are present in Cretalamna and the other sharks within the evolution of megalodon; these teeth are not present in Carcharodon and its ancestors, nor are they typically found in the other sharks within the family Lamnidae.  There was a Russian publication released in 1988 by L.S. Glikman and V.N. Dolganov on the discovery of symphyseal teeth present in a Salmon Shark specimen, which does belong within the family Lamnidae.  I have not been able to find access to this publication as of the making of this article, and the vast majority of this species’ jaws don’t have the presence of this tooth position, so I can’t really specify if this is an atavistic trait (an ancestral trait that re-emerged after a period of dormancy), a mutation, or something else, but I figured it is worth noting.

Symphyseal position tooth from an early megalodon ancestor, Otodus obliquus. This tooth is available for sale NOW @ onpointfossils.com

Upper jaw section of a Great White Shark. Note the lack of symphyseal position teeth in the center of the jaw. Copyright @ Simon De Marchi

Another key difference is the significantly reduced intermediate position tooth that sits between the anterior and lateral tooth positions, which is present in the oldest-known ancestors of the Great White. This upper 3rd anterior intermediate position is also still present in the Great White’s closest extant relatives, the Mako Sharks.  Other Cretaceous sharks, such as Cretoxyrhina, have reduced intermediate teeth, as do some modern Mackerel Sharks like the Sand Tiger. The megalodon’s ancestors in the genus Cretalamna, which go all the way back to the Early Cretaceous, do not possess this trait.  The presence of symphyseal teeth and the lack of a highly reduced intermediate position tooth suggests a fairly distant relationship between megalodon and Great Whites.  While the Great White still has this reduced position that was plesiomorphic, or ancestral, to its family, it is far less reduced than its relatives and Paleogene ancestors, which may be attributed to its macropredatory lifestyle pursuing larger prey at a higher trophic level, meaning it sits higher up in the food chain.  

Great White jaw. Note the reduced size of the third anterior position tooth. Copyright @ Simon de Marchi

A fundamental basis for the inclusion of megalodon into the Great White’s genus Carcharodon was the presence of serrations on the teeth of both taxa.  Serrations begin to develop within the megalodon lineage in the Early Eocene, with transitional forms preserved detailing the evolution of a serrated crown.  The Great White Shark, however, develops serrations in the Miocene when Carcharodon hastalis begins to serrate in the Pacific, evolving into the transitional form Carcharodon hubbelli, before eventually becoming the shark we know and love today.  What does this mean?  Serrations on both lineages evolved due to convergence, and don’t bear any solid foundation for linking these two taxa together.  Evolution of serrations is common within the fossil record of Lamniformes.  Genera such as Palaeocarcharodon, Carcharoides, and even the Thresher Sharks Alopias, among others, have all evolved serrations independently of each other.  

Transitional Palaeocarcharodon tooth. Note the robust, jagged serrations primarily closer to the base of the tooth, with other sections of the crown being unserrated. With Palaeocarcharodon being a Paleocene genus, and unserrated Otodus obliquus being present into the Early Eocene (Ypresian) before eventually evolving serrations itself, it is evident that serrations evolved independently within these two separate lineages nestled within Otodontidae.

Okay, so we now know that megalodon isn’t directly related to the Great White, and isn’t even within the same family as them. What could they possibly be?

To find this out, let's refer to basic taxonomic principles.  Seniority of description is a hallmark of taxonomic classification.  Procarcharodon was created by Edgar Casier in 1960 to correct the classification issues of megalodon’s lineage.  This is invalid due to it being a junior synonym of Carcharocles, which was described previously by David Starr Jordan and Harold Hannibal in 1923 to separate the lineage.  The genus Megaselachus was a split proposed by Russian paleontologist L.S. Glikman in 1964 to separate megalodon and its direct ancestor chubutensis from the rest of the lineage due to the reduction and eventual loss of lateral cusplets.  Since the earliest development of serrations within megalodon’s ancestors, it is clear that the lineage is a chronospecies, which essentially means that one species evolves into a new species through uniform changes over time, without splitting off into separate lineages.  For this reason, Megaselachus being separated taxonomically from the rest of the lineage is not a necessary split.  

Many paleontologists question the necessity of a separate genus being used to describe the serrated Carcharocles and the ancestral, unserrated genus Otodus.  Because Otodus obliquus is known to be the direct ancestor to the megalodon lineage, Otodus, a genus erected by the famous natural scientist Louis Agassiz in 1838, is currently the most widely-accepted genus for megalodon.

Now, before we all get too comfortable with Otodus megalodon, there still is convincing reasoning for the name Carcharocles megalodon.

Let me explain, and later, I will reveal my final thoughts as to what genus I think is most valid.  

If following modern taxonomic principles that focus on holophyly (an ancestral species + all of its descendants: monophyly is often used interchangeably with holophyly, though holophyly is the more accurate term for this classification structure), Otodus would be paraphyletic (an ancestor plus some, but not all of its descendants) if the genus is used for the megalodon lineage, but not for the lineage of Parotodus, and possibly for the elusive genus Megalolamna: both of which may have also derived from Otodus. Otodus obliquus can only be included in the chronospecies of megalodon if it doesn’t split into other lineages, yet in the Early Eocene we see both the emergence of Parotodus, as well as teeth serrating into a lineage separated from the Parotodus lineage, indicating a divergence of Otodus obliquus into different taxa. Modern taxonomic principles would state that all lineages would be bound by the same genus, Otodus, which would include the lineages of megalodon, Parotodus, and possibly Megalolamna, or that each diverged lineage would be given its own genus: both of which will maintain holophyly.

The issue isn’t that Otodus obliquus was the ancestor to the megalodon lineage, but more so is an issue with modern taxonomy. Paraphyly, which is a single ancestor and some, but not all of its descendents, can exist in nature, and is actually quite common. Think about what must happen for speciation to occur: individuals of an ancestral species may evolve new traits, and those traits get passed on to their offspring: this doesn’t happen overnight for most animals: instead, speciation may occur while the ancestral species is still present.  Sometimes, geographical isolation of a population may result in more rapid speciation, while the less constrained populations may still be around and evolve more gradually.  This can often complicate taxonomy and classification structures.  Reptilia is a paraphyle, as it doesn’t include birds, which share the same common ancestor as is the stem for reptiles.  Fish are also paraphyletic, as we don’t typically refer to tetrapods, such as our own species Homo sapiens, as a fish, yet any clade containing fish as a whole also contains the tetrapods.  For better or for worse, these common groups aren’t true clades; it must be stated though that unjustified paraphyly should be corrected when possible.  An example of paraphyly in the megalodon lineage is with the genus Cretalamna. Paleocene Cretalamna would be a paraphyletic taxa, as it coexisted with diverged taxa (Palaeocarcharodon and Otodus), without it, the ancestral taxa, becoming extinct. Are the differences between Cretalamna and the unserrated Otodus species enough to warrant a genus change?  Cretalamna consists of a slew of described and undescribed species, which are mostly distinguished by their isolated teeth.  With such variation and species-level diversity, separating Otodus and Palaeocarcharodon from that genus may be questionable.  It would essentially take lumping everything from the family Otodontidae, with the exception of the possible sister genus to Cretalamna, Kenolamna, into the genus Cretalamna to support a holophyletic clade, or, taking into account that Otodus was described before Cretalamna was, Otodus would be considered a senior synonym and would thus encompass all of Cretalamna, Parotodus, Palaeocarcharodon, Megalolamna, what is currently considered to be Otodus, and megalodon’s lineage. In this case, Otodus would in fact be the best genus placement for megalodon.  With that said, should we lump all of these taxa together to be united into one genus?  Does that really make the most sense?  Well, while Otodus and Cretalamna may not have significant morphological differences in tooth design, the descendents certainly did.  If we were to look at Palaeocarcharodon, which, with its heavily serrated crown that evolved in the Paleocene, as a sister taxa to Otodus, it would make a lot of sense to keep Cretalamna and use Otodus to comprise of, at minimum, the unserrated species within that genus currently described.  And even if this weren’t the case, because the teeth of sharks like Parotodus benedeni and megalodon are so significantly different from Cretalamna, it would still make sense to keep Otodus for those currently described unserrated teeth.  And again, because these Otodus descendents are so different based on tooth morphology, it would make sense to separate these lineages by placing them into different genera: I would use Carcharocles to differentiate the lineage that derived from Otodus obliquus, marked by the earliest instances of serrations appearing on the cusplets, as that marks the beginning of a chronospecies that leads to megalodon and doesn’t diverge into different offshoots.  I would then retain the genera Parotodus and Megalolamna.  While certainly not perfect, this would maintain holophyly from Otodus onwards, and thus reduces an unnecessary paraphyle, even though it retains a paraphyle within the genus Cretalamna.  

Three different potential classifications of the family Otodontidae and their effects on the genus of megalodon.

What I am trying to get at is that both Otodus and Carcharocles are not necessarily invalid, as there are arguments for both.  This is why I still say Carcharocles megalodon, even when much of the world has moved on to Otodus megalodon.

I hope this helps!  What are your thoughts on the classification of megalodon?  Let us know in the comments below!

If you are looking to purchase rare and high quality fossilized shark teeth from around the world, On Point Fossils is your go-to source! To learn more about sharks and their relatives, subscribe to http://www.youtube.com/@elasmocast!

Ben Goode

Owner, On Point Fossils