The group comprising the cartilaginous fish, known as the chondrichthyans, have, as a whole, survived numerous extinction events, and have withstood the tests of time. While they are often considered to be living fossils due to how seemingly little they have changed throughout time, this really isn’t an accurate statement and does a disservice to their astonishing diversity throughout their evolutionary journeys.
When trying to piece together the mysteries of ancient life and evolution, figuring out the age of fossilized remains is an essential and complex task. Through exhaustive relative and exact dating methods, geologists (scientists specializing in the study of the Earth’s physical structure and substance, its history, and the processes that act upon it) developed a series of units of time in order to break down Earth’s history into more palatable chunks. Earth’s over 4.5 billion year history is broadly split into 4 units of time known as eons. The 4 eons in Earth’s history, from oldest to youngest, include the Hadean, Archean, Proterozoic, and Phanerozoic. While single celled organisms have been around for billions of years, and animal fossils have been discovered in the Proterozoic, vertebrates (animals with backbones, which includes the Chondrichthyans) first appear in the latest eon, the Phanerozoic. The Phanerozoic is divided into three units of time, known as ‘eras’. From oldest to youngest, these eras include the Paleozoic, which translates to ‘ancient life’ in Greek, Mesozoic, which means ‘middle life’, and Cenozoic, which translates to ‘recent life’. These units of time are further split into smaller chunks known as periods.
In this article, we will be delving into the periods of the Phanerozoic in chronological order from oldest to youngest, and some of the important evolutionary events that chondrichthyans underwent throughout every time period! We will be basing all of the numbers of this video from the International Chronostratigraphic Chart, version 2023/09 created by the International Commission on Stratigraphy.
The following article is adapted from the video "A Review of Chondrichthyan Evolution by Geological Time Periods". This video can be found on YouTube on my channel "Elasmocast".
You can watch it here!
The oldest geologic period in the Paleozoic is the Cambrian, which lasted from approximately 538.8 +- .2 million years ago to 485.4 +- 1.9 million years ago. It was during this period that a tremendous biodiversity event took place, known as the Cambrian explosion. All sorts of weird and wonderful creatures evolved at this time: types that paved the way for life on Earth today, and others that were evolutionary dead-ends, like many of the oddities found in the famous Burgess Shale. The first Chordates, the phylum of animals that at some point in their lives possess a flexible rod that supports their back side, known as a notochord, first evolved at this time. The Chondrichthyans are a part of this phylum, though they have not yet evolved at this time period.
Cambian-age jawless fish, Haikouichthys
The next geologic period is the Ordovician, which spans from approximately 485.4 +- 1.9 million years ago to 443.8 +-1.5 million years ago. It was during this period that the first fossil evidence of possible Chondrichthyans appeared. No teeth have been published yet from the Ordovician, and all that has been found are Chondrichthyan-like scales. The oldest chondrichthyan-like scale as of now has been described as Tantalepis gatehousei, from the Middle Ordovician-age Stairway Sandstone of central Australia. The first of the “big five” mass extinctions marks the end of the Ordovician period. The culprit of this mass extinction has been traditionally attributed to an ice age that was then followed by subsequent warming, which has been estimated to have wiped out approximately 85% of marine species. New research, however, suggests volcanism as the cause of the mass extinction, followed by warming and anoxia, which is the lack of oxygen.
Scales of Tantalepis gatehousei (Sansom et al. 2012)
Following the Ordovician comes the Silurian period. This period spanned from 443.8 +-1.5 million years ago to 419.2 +-3.2 million years ago. During the Silurian, the chondrichthyans swimming around belonged to a paraphyletic group known as the acanthodians, which are commonly referred to as the “spiny sharks” for commonly possessing numerous fin spines. A paraphyle is a group that contains an ancestor and some of its descendents, but not all of them. This is the case for the acanthodians as evidence indicates that some acanthodians may have given rise to the other chondrichthyans, which aren’t typically referred to as acanthodians. This is also when the oldest-known toothed chondrichthyans appear in the fossil record. To date, the oldest-known chondrichthyan teeth belong to Qianodus duplicis, which has been described from the early Silurian of China. I made a detailed Elasmocast video on Qianodus duplicis; I’ll attach a link to it below, so be sure to check it out after you’re done with this article!
Qianodus duplicis. Credit: Zhang Heming
After the Silurian is the Devonian period, which ranged from approximately 419.2 +-3.2 million years ago to 358.9+-.4 million years ago. This was a time of major diversification for the cartilaginous fish, with several new tooth and body designs evolving. A common tooth design of Devonian chondrichthyans is known as the diplodont design, which has two large lateral cusps with either a significantly reduced or absent medial cusp. This tooth design is present in one of the earliest non-acanthodian grade chondrichthyans, Leonodus carlsi, known from the Early Devonian of Spain. Evolving later on in the Devonian is the cladodont design, which have a principal median cusp and lateral cusplets, as well as a broad basal platform. The cladodont tooth design winds up being highly successful for the duration of the Paleozoic. An example of a cladodontomorph chondrichthyan is the iconic Devonian genus Cladoselache, known from complete articulated remains. The type specimen was described from the Late Devonian Cleveland Shale of Ohio. Other oddball chondrichthyans that evolved in the Devonian include the Phoebodontiformes, which were eel-like in shape and had bizarre pronged teeth whose main cusps, usually three, are more-or-less equal in length. It was in this period that the holocephalans evolved, which is the superorder (more commonly classified as a subclass, which is broader grouping: this is debate is beyond the scope of this article) in which the modern day ratfish are a part of. Hybodontiformes and Protacrodontiformes also evolved in the Devonian, which were much closer related to the sharks and rays of today. The Devonian ended in the world’s second of the “big 5” mass-extinction events. Some hypotheses as to why this mass extinction occurred include global cooling, as well as oxygen deprivation due to anoxic conditions.
Cladoselache fyleri
Still, chondrichthyans as a whole survive this mass extinction and go on into the Carboniferous period, spanning from 358.9+-.4 million years ago to 298.9+-.15 million years ago. The Carboniferous is split up into two sub-periods, with the Early Carboniferous being the Mississippian and the Late Carboniferous being the Pennsylvanian. Many incredible and bizarre chondrichthyans called the Carboniferous waters home. It was during this time that the neoselachians first appeared, which is the group comprising the “true sharks” and rays. Some of the most bizarre taxa from the Carboniferous belong to the subclass Euchondrocephali, which is a more broad group that also contains within it the holocephalans. An iconic euchondrocephalan is Edestus. The genus Edestus are commonly referred to as the “scissor-tooth sharks” due to their unique tooth whorls, though they belong in the order Eugenodontiformes, which are more closely related to ratfish than they are to sharks. Other interesting euchondrocephalans that were successful in the Carboniferous were the petalodonts, which had weird dentitions and even weirder body plans. Their teeth can be petal-like in shape, hence their name. Yet another chondrichthyan order that made its debut in the Carboniferous were the Xenacanthiformes. These convergently resembled eels had teeth of diplodont design. Members of the Xenacanthiformes were apex predators of freshwater ecosystems.
Edestus heinrichi
Succeeding the Carboniferous is the Permian period, lasting from 298.9+- .15 million years ago to 251.902 +- .024 million years ago. Like the Carboniferous, a diverse array of strange chondrichthyans continued patrolling Earth’s Permian waters. Arguably the most famous is Helicoprion. Helicoprion is affectionately referred to as the “whorl-tooth shark”, and was a long-time favorite of mine as a child. I remember being in I think it was 1st grade, having my dad email a fossil dealer, asking if he had any Helicoprion tooth whorls, and we had received a response along the lines of “Do you have a gazillion dollars?”. Needless to say, I was a disappointed 7 or 8 year old. But anyways, Helicoprion, like Edestus, is a member of the order Eugenodontiformes, and is thus not a true shark, but rather more closely related to the ratfish. This was also when the first true sharks appear in the fossil record, with Synechodus antiquus teeth described from the Early Permian of Russia. The Permian ended in catastrophe, with the third and largest mass-extinction event wiping out approximately 96% of marine species. This wreaked havoc among the chondrichthyans, with entire orders wiped off the face of the Earth, and left others hanging on for dear life. The primary culprit, you may ask? The clues lie in the Siberian Traps. Here, evidence indicates an eruption of a flood-basalt volcano polluted the atmosphere, formed short-duration volcanic winters, long-term warming, shallow-water anoxia, and rapid CO2 buildup.
Helicoprion davisii
Chondrichthyans, while taking a massive beating from what has been coined as “The Great Dying”, chugged onwards and into the Mesozoic era, famous for being the age of the dinosaurs. The first period in the Mesozoic is the Triassic, ranging from 251.902 +- .024 million years ago to 201.4 +- .2 million years ago. Survivors of the Permian-Triassic mass extinction event include the Eugenodontiformes, the xenacanths, the Jalodontiformes, and, tentatively, the cladodontomorph chondricthyans (the Mesozoic 'cladodontomorphs' may actually represent sharks that convergently evolved teeth sharing similar attributes to the Paleozoic cladodonts). The two most prominent survivors, however, are the neoselachians, and the Hybodontiformes, which were the most diverse and abundant chondrichthyans during the Triassic. The true sharks began diversifying in the Triassic, though no orders that are alive today have been positively identified in the Triassic. The most abundant true sharks at the time belonged to an early-evolving order known as the Synechodontiformes. The end of the Triassic was brought forth by number 4 of the “big 5” of mass extinctions, and is thought to be caused by volcanics. Notice a trend, here? Our planet was once filled with awe-inspiring diversity within the cartilaginous fish, but by the end of the Triassic, only the Hybodonts, Neoselachians, Chimaeroids, which are the ratfish, and the "cladodontomorphs" (again, these are likely to not be closely related to the Paleozoic cladodontomorphs) trek forward into the next period.
Hybodont Lissodus sp. (Fischer 2005)
Things change in the favor of the true sharks in the next period, the Jurassic, which ranges from 201.4 +- .2 million years ago to around 145 million years ago. During this period, many of the extant (still alive) shark orders start appearing. The true sharks split up into two superorders, the Galean Sharks and the Squalean Sharks. Sharks become more recognizable, with early forms of cow sharks, catsharks, and angel sharks, among others, appearing in the fossil record. Most sharks were still smaller than what their chondrichthyan predecessors have been able to attain, and to what the future ones have in-store for them. The earliest unequivocal ray teeth are also known from the Early Jurassic period, though they likely go back further in the fossil record.
Late Jurassic Angel Shark Pseudorhina alifera
Chondrichthyans once again start dominating the seas in the next period, the Cretaceous. The Cretaceous spans from around 145 million years ago to 66 million years ago. In the Cretaceous period, one order of shark in particular, the Lamniformes, also known as the mackerel sharks, began diversifying and taking up roles as apex predators across the globe. The most famous of these macropredatory mackerel sharks has to be Cretoxyrhina mantelli, aka the Ginsu Shark, which appropriately obtained that common name from their large and sharp teeth superficially resembling a ginsu knife. This shark held its own amongst a sea teaming with mosasaurs, massive predatory fish, and of course, other large sharks, around the world in the late Cretaceous. Meanwhile, the rest of the extant shark orders also evolved in the Cretaceous. The Cretaceous period ended with perhaps the most famous mass extinction event of all: the one that wiped out the non-avian dinosaurs 66 million years ago (yes, birds are dinosaurs). This extinction event took a heavy toll on sharks, and wiped out the long-lasting Hybodonts. The supposed cladodontomorph sharks, which were hypothesized to have sought refuge by occupying deep sea niches, have no evidence of surviving past the Early Cretaceous, though their post-paleozoic fossil records are quite spotty, with long ghost-lineages leading up to the Cretaceous. This leaves only the modern day sharks, rays, skates, and ratfish to move on to the next period. The commonly held belief is that a massive asteroid crashed into the Earth in what is now the Gulf of Mexico's Yucatán Peninsula. Another opposing view is that the flood-basalt volcano eruptions that happened shortly before the mass extinction in what is now the Deccan Traps in India caused the event. What do you think took out the non-avian dinosaurs and hybodonts? Let us know and explain your response in the comments section below!
Cretoxyrhina mantelli
With the end of the Cretaceous period came the end of the Mesozoic era, and the beginning of the Cenozoic. The Cenozoic era’s first period is known as the Paleogene, which took place from 66 million years ago to 23.03 million years ago. In the Paleogene, Lamniformes dominate and obtain massive sizes. Among the most famous lineages that is well represented in the Paleogene fossil record is the Otodus-Carcharocles lineage, with clear evolutionary stages being marked at set geologic times. Another order becomes increasingly successful, as well: the Carcharhiniformes, also known as the ground sharks. The once-prominent Synechodontiformes become extinct during the Paleogene, leaving the world with just the extant orders we see today.
After the Paleogene comes the Neogene period, from 23.03 million years ago to 2.58 million years ago. Macropredatory sharks meet new records in the Neogene. Lamniformes and Carcharhiniformes continue increasing in size, with the most famous prehistoric shark of all time evolving during this period: Carcharocles megalodon; and yes, I still use the genus Carcharocles rather than Otodus for this animal.
If you are interested in an in-depth discussion regarding the classification of C. megalodon, check out this blog post:
Or, you can watch the video here:
Anyways, C. megalodon is infamous, and rightfully so, for its monstrous teeth that can exceed 7 inches in length! Another example of Neogene gigantism in Lamniformes is Parotodus benedeni, commonly known as the false mako, as it is more closely related to Carcharocles megalodon than it is to the Makos. A carcharhiniform (commonly known as the requiem sharks) that was able to reach massive sizes in the Neogene was the extinct snaggletooth shark Hemipristis serra, which is known to have teeth that exceeded 2 inches in length. Not even C. megalodon can survive the Neogene, however: Carcharocles megalodon is believed to have become extinct well before the upper boundary of the Neogene, which brings us to our last period, the Quaternary.
Carcharocles megalodon
As you are reading this article, you are living in the Quaternary period! So what sharks are alive during the Quaternary? Whatever is still swimming around today. The Quaternary period is everything from 2.58 million years ago to present day. The most iconic modern shark is by far the Great White, Carcharodon carcharias. While it may have been dwarfed by Carcharocles megalodon, and it did evolve in the Neogene, the Great White survived and is top dog in the shark world today. As a whole, however, Lamniformes aren’t doing so well: the real kings of the Quaternary are the carcharhinids, aka the ground sharks. Lamniform diversity is a mere fraction of what it once was, and this was a trend taking place even before human intervention.
Great White Shark Carcharodon carcharias
Some people argue that we are living amidst a sixth mass-extinction event: this one being caused by us. We as a species have to be more diligent and focus on conservation, as it is a very real possibility that we can cause or expedite the extinctions of countless shark species, let alone even larger groups that have stood the tests of time.
I hope you enjoyed this article! What is your favorite geologic time period? Let us know in the comments below!
Comments