Transitional Fossils: How dinosaurs gave rise to birds

Everyone knows that dinosaurs are extinct. As children, many of us gazed in awe at the fossils of these magnificent beasts. As adults, a lot of us still do!

Except that dinosaurs aren’t extinct, at least based on the most recent interpretations of the fossil record and analyses of DNA. The collective evidence points to a conclusion that once seemed improbable: birds are dinosaurs. I still recall the awe and wonder that beheld me when I learned this in college, and when I’ve taught it to children (all of whom are bonafide dinosaurs experts), I can see the same amazement on their faces.

So what evidence is there in the fossil record for this supposed ancestor – descendant relationship?

Prior to the Triassic, there were a lot of reptilian looking animals that no longer exist today. One such animal, Protorosaurus, has been found in rocks dating to about 260-251 million years ago (Ma). Typical of its contemporaries, it walked around on four limbs, each of which terminated in five fingers or five toes, possessed a long tail and teeth, and was almost certainly covered in scales. At this point in time, Protorosaurus and its fellow reptiles had seemingly little in common with today’s birds.

Not too much later, about 245 million years ago, animals like Asilisaurus appear in the fossil record. Though this species likely walked on all fours, it had shorter arms, suggestive of an increased ability to walk and/or stand on its hindlimbs.

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Asilisaurus

In another nine million years, we see animals like Marasuchus (236-234 Ma): clearly reptilian in form, very dinosaur like, and notably bipedal, just like birds.

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Marasuchus

Eodromaeus and its kin are among the earliest true dinosaurs, popping up a mere five million years after Marasuchus (231.4-229 Ma). One of its typical dinosaurian traits is a hip socket with a hole in it (perforate acetabulum). What’s additionally notable about this species and some of its contemporaries is how its fingers have changed. Modern birds do not have fingers, but their wing bones terminate in what appears to be the remnants of three fingers. Starting this trend toward digit reduction, Eodromaeus has five fingers, but the ring and pinky are very reduced in size.

Fast forward about 30 million years, and we have more modern-looking dinosaurs on the scene. Coelophysis (203-196 Ma) is typical of the early carnivorous dinosaurs (theropods), and continues the march towards birdiness. The pinky finger is practically non-existent at this point, and the toes have also reduced in number. Whereas earlier dinosaurs and other reptiles have five toes, Coelophysis has only four, with a tiny remnant of the fifth high up on the foot. Notably, birds have four toes, three in the front and one in the back, so this trait had already appeared at least 200 million years ago.

Whereas Coelophysis had four fingers on its hands, Sinosaurus (201-196 Ma), appearing two million years later, only has three fingers, having lost the ring finger altogether.

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Sinosaurus

While other bird-like traits accumulated over time, perhaps the most significant change is found in rocks that date to 50 million years after dinosaurs like Sinosaurus. Archaeopteryx (150.8-148.5 Ma), the first documented transitional fossil, has a striking mix of bird- and reptile-like traits. Perhaps most significantly is the appearance of feathers in this species.

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Archaeopteryx

There is reason to think that feathers appeared prior to Archaeopteryx, however. Soft tissues like feathers don’t typically preserve well as fossils, but over the last 20+ years, a number of exceptionally preserved specimens have demonstrated that plenty of non-flying dinosaurs had feathers. This suggests that these structures didn’t appear for the purpose of flight, but rather for a simpler function, such as thermoregulation. Just like hair keeps us and other mammals warm, feathers provide a similar insulating layer for birds.

Archaeopteryx clearly had wings, though, suggesting some ability to glide or perform flapping flight. It had another bird-like feature, known as the furcula. Also known as the wishbone, the bone frequently broken apart as a Thanksgiving dinner ritual in the United States, this bone formed by the fusion of the two clavicle bones present in earlier species like Coelophysis. This fusion is thought to be important in withstanding the stressors caused by flight, but it is also present in many other carnivorous dinosaurs, suggesting it initially appeared for a different reason.

Furcula_evolution4

Despite these innovations, Archaeopteryx is still not as bird-like as you’d think. For one, it still very clearly had three fingers, along with claws, on its hand. It also had a long bony tail, a structure reduced to a nub (pygostyle) in modern birds. Last but not least, Archaeopteryx had teeth, whereas birds have lost their teeth entirely in favor of a beak made of a protein called keratin.

After another 15 million years, other dinosaurs, like Sinornis (135 Ma), were just a few steps away from modern birds. The tail bones had finally become shortened and fused into a pygostyle in Sinornis, providing a structure tail-feather attachment. The sternum also became keeled increasingly keeled, allowing for the attachment of more powerful flight muscles, suggesting that Sinornis was a better flyer than Archaeopteryx. Despite these innovations, Sinornis still retained teeth and three distinct clawed fingers, traits that are not present in modern birds.

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Sinornis

40 million years later, when tyrannosaurs and pachycephalosaurs were still roaming the earth, some extremely bird-like animals appear in the fossil record. At first glance, Ichthyornis (93-83.5 Ma) may be difficult to discern from a modern seabird. Many of the limb bones, including the fingers have become fused in Ichthyornis, resulting in a skeleton that was probably well-adapted for powered flight. Nonetheless, as one of the final holdouts of its reptilian past, Ichthyornis still had teeth. Interestingly, however, its jaw tip appears to be covered by an incipient beak, suggesting the transformation is nearly complete.

Ichthyornis_Clean
Ichthyornis

After most dinosaurs disappeared from the fossil record, coinciding with geological evidence for a disastrous meteor impact and intense volcanism, birds persisted and began to really thrive. Among the earliest species is Waimanu (60 Ma), which appeared just five million years after this mass extinction event. Not only does Waimanu have the appearance of a modern bird, paleontologists think it was the earliest known penguin, suggesting the surviving bird species have already begun to resemble some of their modern forms.

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Waimanu

Over a period of about 200 million years, the fossil record appears to document the gradual appearance birds from reptilian forebears. With the advent of bipedalism, the reduction in fingers and toes, derivation of feathers and wings, reduction of the tail, and loss of teeth, fossils seem tell the story that birds are, in fact, dinosaurs. Do not hesitate to remember this the next time you feed a duck or eat a chicken wing!

Question for Creationists

Where did all of these fossil animals go? Could they not fit on Noah’s ark? Wouldn’t species with some capacity for flight, such as Sinornis, Archaeopteryx, and Yi qi, be able to avoid the Flood? Why does the fossil record appear to document a transition between reptilian animals and birds? If Noah’s Flood is responsible for the placement of these fossils, why did they appear in this particular sequence? Is it just a coincidence that the fossil record appears to document birds descending from ancestors with teeth and birds have remnants of tooth genes in their genomes?

Photo credit

Protorosaurus, chickenAsilisaurus, Marasuchus, Eodromaeus, Eodromaeus handCoelophysis, Coelophysis handSinosaurus, Archaeopteryx, Sinosauropteryx, Beipiaosaurus,  Sinornithosaurus, Microraptor, Psittacosaurus, Epidexipteryx, Similicaudipteryx, Anchiornis, Changyuraptor, Yi qi, furcula, Sinornis, Waimanu

Transitional fossils: Manatees that walked on land

Manatees and the dugong, known collectively as sirenians, are herbivorous, aquatic mammals that are generally restricted to shallow, tropical waters. Like whales, sirenians have reduced the amount of hair on their bodies, have forelimbs shaped into flippers, lack external hindlimbs and have a paddle-like tail.

Despite their overall similarities, the DNA of sirenians is much more similar to that of elephants and the DNA of whales is much more similar to hoofed mammals like cows and giraffes. This suggests that both sirenians and whales independently adapted to living in an aquatic medium. In fact, the fossil record of whales suggests a transition from a land-based habitat to a completely aquatic way of life. Do sirenians show the same pattern?

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Pezosiren

Indeed they do! One of the earliest sirenian species appearing in the fossil record is Pezosiren, a mammal with an unmistakably sirenian-like skull, but the rest of its skeleton clearly suggests that it had the ability to walk on land. Unlike modern sirenians, this animal had forelimbs that are not shaped like flippers but instead were used to support their weight on land, as well as hindlimbs and a distinct hip bone (pelvis). Pezosiren has been found in rocks estimated to be ~47.8 million years in age, similar to the age that walking ‘whales’ were also found roaming the earth.

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Halitherium

More recent sirenian fossils show evidence of fully committing to an aquatic lifestyle, including Halitherium, which is found in rocks estimated to be ~38 million years old. This species and similarly dated fossils had modified their forelimbs into flippers, possessed thick and dense ribs to provide ballast, and reduced their hindlimbs, presumably to minimize drag. Though it looked very much like modern manatees (see below) and dugongs, it had a better-developed, albeit very reduced, pelvis+hindlimb complex.

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In addition to their better-developed limbs, early species of sirenians had a number of other traits that manatees and the dugong have lost. Pezosiren and others had multiple vertebrae in the hip region (sacral), much like their land-dwelling elephant relatives, whereas most fossil and all modern sirenians have fused these into a single bone. Modern sirenians also have reduced their teeth to a great extent compared to their fossil forebears. Earlier species had permanent premolars, canines and incisors, whereas modern species lack these teeth (dugongs have only one incisor). See below for a diagram of a fossil sirenian (Protosiren, ~47.8 million years old) showing the positions of the premolars (P), canine (C) and incisors (I).

protosiren_750_labels

Questions for Creationists

Where did the dozens of other sirenian species go? Given their aquatic adaptations, wouldn’t they have survived Noah’s flood? Is it just a coincidence that molecular phylogenetics shows sirenians as being genetically similar to land mammals like elephants and there are sirenian fossils that appear to document a transition from land to water? Is it coincidence that we see parallel evidence of a land to water transition in whales?

References

1. Springer, M. S., Signore, A. V., Paijmans, J. L., Vélez-Juarbe, J., Domning, D. P., Bauer, C. E., … & Meredith, R. W. (2015). Interordinal gene capture, the phylogenetic position of Steller’s sea cow based on molecular and morphological data, and the macroevolutionary history of Sirenia. Molecular phylogenetics and evolution91, 178-193.

Photo credit

Manatees, dugong, Pezosiren, Halitherium, manatee skeleton, Protosiren

Transitional fossils: The earliest platypuses

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Platypuses are strange creatures, even to evolutionary biologists. They have hair, webbed feet, a beaver-like tail, and duck-like bills with which they can sense electrical currents, they lay eggs, and the males possess venom glands on their hind limbs. When a drawing and a pelt of the animal were first sent to British scientists at the end of the 18th century, the incredulous naturalists assumed it was a hoax. In college, I had a t-shirt that made light of their bizarre features:

platypus_fullpic_artwork

Given the idiosyncrasies of this animal, you might imagine that they’re a bit of an enigma to evolutionary biologists. In fact, I recall a creationist acquaintance of mine once declaring that they must be an “evolutionist’s worst nightmare”. To be sure, there are a lot of gaps in our knowledge regarding how they might have evolved. What the evidence points to pretty clearly is this: 1) they’re mammals, as they have hair, produce milk for their young, and their DNA is much more similar to other mammals than it is to, say, birds or lizards, and 2) they split from other mammals a long, long time ago, somewhere on the order of 220 million years. The fossil record of their relatives, known as monotremes, is extremely spotty, with the earliest dating to over one hundred million years ago. Unfortunately, the specimens are extremely fragmentary, frequently consisting of only part of a jaw or arm, preventing researchers from making many firm conclusions about their evolution.

However, at least one fossil animal gives some insights into their ancestry. Obdurodon includes several species of platypus-like monotremes that are estimated to have lived ~28.1 to 5.3 million years ago. Below you can see a comparison between Obdurodon dicksoni (left; 23-11.6 million years ago) and the modern platypus (right).

Obdurodon-and-Platypus-skulls

You’ll notice differences between the two, but overall they look extremely similar. Clearly the duck-like bill has been around for a while! However, at least one feature  points to Obdurodon being a transitional fossil, namely the fact that it has teeth. Modern platypuses have teeth when they are very young, but shed these by adulthood and replace them with horny pads (see below).

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Considering these horny pads are a very uncommon feature in mammals, and vertebrates as a whole, biologists interpret it as an evolutionary novelty. With Obdurodon, we have support for this hypothesis, with evidence that a toothed ancestor preceded the modern platypus dental condition.

Questions for creationists

Where did the other platypus species (i.e., Obdurodon) go? Could Noah not fit them or their eggs on his ark? Is it a coincidence that there are fossil platypuses that had teeth as adults, much like typical mammals, but modern platypuses replace them during development with horny pads?

References

1. Musser, A. M., & Archer, M. (1998). New information about the skull and dentary of the Miocene platypus Obdurodon dicksoni, and a discussion of ornithorhynchid relationships. Philosophical Transactions of the Royal Society of London B: Biological Sciences353(1372), 1063-1079.

Photo credit

platypus, t-shirt image, platypus Obdurodon comparison, horny pads

Transitional fossils: How the ankylosaurs got their club-like tails

Ankylosaurs were a group of herbivorous dinosaurs that were covered with bony armor composed of osteoderms (bones formed in their skin), much like the mammalian glyptodonts. Many of the ankylosaurs had osteoderms flanking the tips of their tails, making a club-like structure that may have been used to ward off predators or compete for females.

Ankylosaurus_raul_martin

A recent paper [1] by ankylosaur expert Victoria Arbour showed that these ornate creatures appear to have evolved these club-like tails in a two-step transition.

Scelidosaurus 199-191 million years ago*

Scelidosaurus_harrisonii_(1)

Before talking about ankylosaurs, it’s first important to mention Scelidosaurus, a precursor of sorts to the ankylosaurs. The earliest dinosaurs were bipedal, but Scelidosaurus was quadrupedal, walking on all four legs. Also unlike earlier dinosaurs, Scelidosaurus possessed osteoderms, although not the large, fused types characteristic of ankylosaurs.

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Gastonia 127-124 million years ago

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After Scelidosaurus disappeared, the ankylosaurs appeared in the fossil record. They had much more elaborate osteoderms that had fused into plates and spines. The earliest fossils, such as Gastonia here, had flexible tails, often flanked with spikes, but lacked the mallet shape characteristic of later species.

Gobisaurus 93-90 million years ago

Gobisaurus_SydneyMohr711

Gobisaurus differs from earlier ankylosaurs in that it had a very stiff tail, achieved by having highly overlapping vertebral joints. These fossils have not been discovered with the tail clubs characteristic of the latest ankylosaurs, such as Euoplocephalus (78-77 million years ago) shown below:

euoplocephalus.skeleton

This suggests that a stiffened tail, which would have been strong enough for striking predators or opponents, predated the clubs of later ankylosaurs. The clubs then appeared at a later stage, presumably to amplify the power of this putative weapon.

To further illustrate this transition, here is a diagram created by Victoria Arbour**:

timeline_Arbour

*Dates are estimates from fossil distributions reported in figure 4 of Arbour et al. [2015]

**Note that she highlights different ages than I, due to her summarizing of multiple fossils

Questions for Creationists

What happened to the ankylosaurs? Why does the fossil record appear to record a transition from dinosaurs that were bipedal and lacked osteoderms, to quadrupedal species with elaborate osteoderms and club-like tails? Is it a coincidence that these species appear in this order?

References

1. Arbour, V. M., & Currie, P. J. (2015). Ankylosaurid dinosaur tail clubs evolved through stepwise acquisition of key features. Journal of anatomy227(4), 514-523.

Photo credit

Ankylosaurus by Raul Martin, Scelidosaurus 1 by FunkMonk, Scelidosaurus 2, Gobisaurus by Sydney Mohr, Euoplocephalus, Ankylosaur transition by Victoria Arbour

Transitional fossils: Before turtles had shells

Turtles are unique among reptiles in that they have a large shell, composed of a carapace on their back and a plastron on their belly. This shell largely develops from fused ribs and bones derived from the skin. Since turtles presumably evolved from a lizard-like ancestor to become the distinctively shelled creatures we know of today, we might expect that there would be transitional fossils that illustrate this link.

Pappochelys 242-237 million years ago

image_2952_2e-Pappochelys

At first glance, Pappochelys [1] might not look very turtle-like, as it had no shell. However, it did have what appears to be the beginnings of a shell. Specifically, it had very broad t-shaped ribs and little bones in its belly region known as gastralia. Beyond these, Pappochelys had various other features that it shares with other turtles, such as a process coming off of the pubis, a femur with an offset head, and a modified shoulder blade.

MID TURTLE

The overhead view of the reconstruction above gives the impression of a lizard-like animal with the beginnings of a shell.

Odontochelys 237-227 million years ago

Fig-2-paratype

Not too long after Pappochelys appears in the fossil record, we find Odontochelys [2]. This extinct species apparently fused those belly bones (gastralia), making up a plastron that is very similar to that of modern turtles. In other words, this turtle only had half of a shell (the bottom half). In addition to this new feature, Odontochelys had a shorter tail than Pappochelys, a larger forelimb to hindlimb ratio, new bones called neurals, and it lost two holes in the sides of its skull that formerly allowed jaw muscles to bulge out during chewing.

Proganochelys 227-208.5 million years ago

Proganochelys_Quenstedti

Next to appear in the fossil record is Proganochelys, a species very close to modern turtles in overall form. It has a complete shell, with both the lower plastron and upper carapace. Additionally, whereas Pappochelys and Odontochelys had typical reptilian teeth on the margins of its mouth, Proganochelys lost these and probably replaced them with a keratinous beak. However, Proganochelys did retain teeth on its palate, as well as other traits related to the skull, shell, shoulder and pelvic girdles that are not found in turtles today.

Notably, this overall pattern of tooth reduction in turtle history (i.e., marginal teeth in Pappochelys -> marginal+palatine teeth in Odontochelys -> palatine teeth in Proganochelys -> no teeth in modern turtles) is paralleled by genetic data. Specifically, turtles retain remnants of tooth genes, suggesting that they formerly possessed teeth.

Questions for Creationists

Where did the turtles without shells and the turtles with half shells go? Or even turtles like Proganochelys, which had palatine teeth? Some of these species, such as Odontochelys, were likely aquatic, so should they not have survived Noah’s flood? Is it a coincidence that the species found in lower (=older) rocks are less like modern turtles than the species in higher (=younger) rocks? Is it coincidence that the reduction in teeth in the turtle fossil record mimics the loss of tooth genes in modern turtles?

References

1. Schoch, R. R., & Sues, H. D. (2015). A Middle Triassic stem-turtle and the evolution of the turtle body plan. Nature.

2. Li, C., Wu, X. C., Rieppel, O., Wang, L. T., & Zhao, L. J. (2008). An ancestral turtle from the Late Triassic of southwestern China. Nature456(7221), 497-501.

Photo credit

Rainer Schoch 1Rainer Schoch 2,  Li et al. 2008, Claire Houck 

Transitional fossils: Back when snakes had legs

As I recently discussed, snakes are genetically nested within lizards. One of the major evolutionary implications of this fact is that snakes used to have legs. Just this week, researchers [1] heralded the discovery of a major fossil, helping to bridge the putative transition from legged-lizard ancestors to modern legless snakes.

Tetrapodophis 125-113 million years ago

Tetrapodophis whole

A casual glance at Tetrapodophis reveals that it has a very snake-like body, possessing a typical snake-like count of over 150 vertebrae before the tail. To a trained morphologist, there are also a number of other features that are notably serpentine including recurved teeth and an intramandibular joint to allow for the widening of the gape, among others.

The most noteworthy feature, however, is the retention of all four limbs.

Tetrapodophis forelimb

Above is a forelimb

Tetrapodophis hind

and here are the hindlimbs.

Eupodophis 101-94 million years ago

After the appearance of Tetrapodophis, there are at least four known species of snakes that show evidence of a major modification compared to their predecessor: they have completely lost their forelimbs, but still retain hindlimbs. In Eupodophis, the example species I highlight here, the hindlimbs are further reduced, eliminating the foot bones altogether.

Eupodophis limb

By contrast, modern snakes of course do not have legs. Some species, such as pythons and boas, have pelvic spurs, apparent remnants of the pelvis and femur, but the transition from legged to legless lizards appears to be mostly complete.

Questions for Creationists

Is it possible that God created snakes with legs? Where did they go? Is it a coincidence that the species with four legs appears in rocks that are older than the species with two legs, and before any snake species that have no legs? Is it also a coincidence that snakes are genetically nested within legged lizards?

References

1. Martill, D. M., Tischlinger, H., & Longrich, N. R. (2015). A four-legged snake from the Early Cretaceous of Gondwana. Science349(6246), 416-419.

Transitional fossils: A bat with claws

Bats are unique among mammals in that they are capable of powered flight. While we do not have evidence of flightless bats, we do have fossils of bats with primitive features. The oldest known species* is Onychonycteris finneyi, which Nancy Simmons and colleagues discovered in Wyoming rocks that date to approximately 52.5 million years ago [1].

Simmons_fig1_2007-09-10114B

If you’ve ever looked at a bat’s wings in detail, you’ll notice that the scaffold for the wing membrane is actually a hand, with very long fingers extending to the edge.

batskeleton

Despite these being fingers, bats only have claws on their thumbs, with the exception of some old world fruit bats which also have claws on their index fingers. Onychonycteris is more similar to earlier, non-flying mammals in that it possessed claws on every one of its fingers, hence it’s name: “clawed” (Onycho) “bat” (nycteris).

Another transitional feature involves the relative sizes of its limb bones. Bats are typically characterized by having particularly long forearms and legs that are short relative to their arms, features thought to be adaptations for flight. Just by comparing the photos above, it should be apparent that Onychonycteris is not proportioned the same as modern bats. Simmons et al. [1] compared its limb proportions to modern bats and various non-flying, tree-dwelling species, and they discovered that Onychonycteris was proportioned much more like non-bats than bats (see below). nature06549-f3.2

Though Onychonycteris was almost certainly capable of powered flight, it may have been less adept at flying than modern bats and perhaps spent ample time climbing trees as opposed to just taking to the air.

In short, Onychonycteris, the earliest known bat, had some traits that are more similar to non-flying mammals than bats, consistent with the hypothesis that bats evolved from non-flying species.

*Another bat, Icaronycteris, is approximately the same age, but it has more modern features than Onychonycteris.

Questions for Creationists

Why would God create most bats with only a claw on the thumb, but give Onychonycteris claws on every finger? Is it just a coincidence that this fossil was found in rocks that correlate with 52.5 million years ago and it has some features that are more similar to other mammals than bats? If it could fly, shouldn’t it have survived Noah’s flood?

References

1. Simmons, N. B., Seymour, K. L., Habersetzer, J., & Gunnell, G. F. (2008). Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature451(7180), 818-821.

Transitional fossils: From the ancestor of modern whales to baleen whales

Finishing off these transitional whale fossils, I will now discuss the second major lineage of modern whales, the baleen whales. Baleen whales are characterized by having sheets of keratin (baleen) used for filter feeding instead of teeth.

baleen-whale-5

Though some of these whales are relatively small, many of them are among the largest animals of all time, including the enormous blue whale.

blue-whale-the-largest-species-in-animals

Let’s refer to McGowen et al’s (1) phylogeny (evolutionary tree) one more time

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Janjucetus 28.4-23.03 million years ago* Janjucetus_Melb_Museum_email

Mammalodon 25.2-23.03 million years ago

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The first thing you should notice is that these are both found in rocks that are dated younger than the most recent predecessor to modern whales (that I’ve discussed), Basilosaurus (40.4-33.9 million years ago). The next thing you might notice is that they both have teeth. Janjucetus and Mammalodon are unique baleen whales in that they actually have no evidence of having possessed baleen. It might cause one to wonder why paleontologists have suggested that these are somehow transitional baleen whales. One feature that they share in common with other baleen whales is a broadened rostrum, the bones of which meet the braincase in a way that is particular to baleen whales.

Aetiocetus 33.9-23.03 million years ago

i-800672a4d9865241f89bf3c41d25a603-toothedbaleen

Aetiocetus is an even more convincing transitional baleen whale because it not only has evidence of baleen (arrows on right point to blood vessels that nourished baleen), but it had baleen side-by-side with teeth! Whether or not Aetiocetus fed with both teeth and baleen is a matter of speculation, but it’s an intriguing hypothesis nonetheless.

Eomysticetus 28.4-23.03 million years ago

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Eomysticetus then dealt away with teeth altogether and probably fed via filter feeding exclusively. It differs from modern baleen whales in that modern species have bowed mandibles and a telescoped skull.

Summary and questions for creationists

I have highlighted 16 putatively transitional fossils in my last three posts, documenting a lineage of land mammals shifting from a terrestrial habitat, to semiaquatic forms, to an obligately aquatic habit and thereafter transitioning into modern toothed and baleen whales respectively. The fossils that are still tied to land are found in rocks that are dated older (55.8-37.2 million years ago) than the rocks in which primitive obligately aquatic whales are found. These rocks are still older than those in which the whales that resemble more modern toothed (33.9-25.2 million years ago) and baleen whales (33.9-23.03 million years ago) are found. Each fossil shows traits that are a mix of primitive characters, uncharacteristic of modern whales, and some that are similar to modern whales. It’s important to ask yourself: is this all a coincidence? Where are all of these primitive whales today? Shouldn’t those that are aquatic still be around since they could have survived the Flood? Why would God have created a whale with both baleen and teeth, but all modern whales have baleen or teeth? Are truly none of these fossils convincingly transitional?

*all fossil ranges based on the paleobiology database

References

1. McGowen, M. R., Gatesy, J., & Wildman, D. E. (2014). Molecular evolution tracks macroevolutionary transitions in Cetacea. Trends in ecology & evolution

Transitional fossils: From the ancestor of modern whales to toothed whales

In my previous post, I described McGowen et al.’s [1] summary of fossil evidence for a transition between a terrestrial mammal and the ancestor of modern whales. Here I will begin to discuss their review of the transition from that hypothetical ancestral whale to today’s modern whales.

Many people are probably unaware that there are two major lineages of whales: the toothed whales and the baleen whales. Toothed whales, as their name implies, have teeth whereas baleen whales have baleen, fibers of keratin used for filter feeding. Additional differences include one blowhole in toothed whales with two in baleen whales, and toothed whales have the ability to echolocate. Examples of toothed species include dolphins, porpoises, belugas, narwhals, beaked whales and sperm whales.

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Here’s the phylogeny (evolutionary tree) from McGowen et al. [1] again, showing the transitions.

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Let’s start with transitional fossils in the toothed whales.

Simocetus 33.9-28.4 million years ago*

Simocetus appears after Basilosaurus and Dorudon, the last fossils I described appearing before the ancestor of modern whales. Simocetus possessed an elongated skull, a feature characteristic of toothed whales. Though not shown in the figure in McGowen et al., it also appears to have had a melon, a feature found in toothed whales to direct their echolocating clicks.

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Waipatia 27.3-25.2 million year ago

Slightly younger than Simocetus, Waipatia has the same features, uniting the two of them with toothed whales.

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What both species lacked, however are asymmetrical skulls and homodont dentition. The former is a characteristic though to be advantageous for underwater hearing. The right side of the skull is always larger than the left side. You can see it clearly in the bottlenose dolphin diagram below:

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Homodonty (“same teeth”) is defined as having teeth that look approximately identical. This trait is found in many fish-eating vertebrates such as toothed-whales, sea lions, and gharials. Most other mammals have heterodonty (“different teeth”). Whereas we have incisors, canines, premolars and molars, dolphin teeth are extremely similar. Early whales had heterodonty, a trait still preserved, albeit slightly, in Simocetus and Waipatia.

Compare the teeth in this sperm whale:

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with those of the ancient whale Basilosaurus:

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In short, fossils appear to document a transition between early whales and modern toothed whales.

Question for Creationists

Why would God create whales with homodonty and heterodonty, but only those whales with homodonty have survived into the present? Shouldn’t both groups have been able to survive Noah’s flood?

*all fossil ranges based on the Paleobiology database

References

1. McGowen, M. R., Gatesy, J., & Wildman, D. E. (2014). Molecular evolution tracks macroevolutionary transitions in Cetacea. Trends in ecology & evolution.

 

Transitional fossils: From land mammals to the ancestor of modern whales

Whales are remarkable in the number of transitional fossils that they demonstrate. Based on molecular evidence, whales descended from hoofed mammals, with their closest living relatives being the hippos. This makes sense in that both whales and hippos share numerous features, such as dense limb bones (reduced buoyancy to help stay underwater), they lack sebaceous glands (the glands that make most mammals’ hair oily), have reduced hair, have a semi-aquatic/aquatic lifestyle, and nurse their young underwater. Based on this hypothetical common ancestor, whales next have a series of transitional fossils that document changes in their body plan from a probable semi-aquatic ancestor to a fully aquatic one.

A figure from McGowen et al. [1] documents this transition:

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Indohyus 55.8-40.4 million years ago*

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The first transitional fossil the authors describe is Indohyus, a small, herbivorous (plant-eating) mammal that appeared about 55.8 million years ago. This strange creature possessed dense ear bones which likely facilitated underwater hearing. Sound does not travel underwater the same way it does through air, so modifications for hearing underwater are consistent with a transition from land to water-based ecosystems. Its otherwise typical terrestrial features show that it’s still a long way away from whales.

Pakicetus 55.8-40.4 million years ago

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The next transitional fossil shows evidence of abandoning an herbivorous diet in favor of a carnivorous/piscivorous (i.e., fish-based) diet. It also has a more robust tail than Indohyus, which may have facilitated better swimming.

Ambulocetus 48.6-40.4 million years ago

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Ambulocetus appears more recently in the fossil record than Pakicetus and Indohyus, and it’s not difficult to tell that it looks better adapted for an aquatic lifestyle than those two species. A new feature that Ambulocetus shows is evidence for a fat pad in its jaw, likely to aid in underwater hearing.

Remingtonocetus 48.6-40.4 million years ago

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Ambulocetus and Remingtonocetus together show two important changes in whale history. The first is the loss of the vomeronasal organ. The vomeronasal organ is a sensory structure used to detect chemical signals, particularly pheromones. If you’ve ever seen a  cat do this:

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it’s using its vomeronasal organ. Underwater, chemical signaling is not quite as useful, so the loss of the vomeronasal organ suggests a further commitment to living in the water. Additionally, Ambulocetus and Remingtonocetus show evidence of a move to saltwater, the medium that the majority of whales live in today.

Rodhocetus 48.6-40.4 million years ago

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Artiocetus 48.6-40.4 million years ago

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Protocetus 48.6-40.4 million years ago

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These three species, plus Georgiacetus and Dorudon (see below) document another crucial transition in the whale body plan. First off, their external nares (=nostrils) have moved more posteriorly than the earlier whales and hoofed mammals. If you’ve never thought about it before, where are the nostrils on a dolphin or humpback whale? It’s their blowhole(s)! These fossils show the beginning of that transition from forward facing nostrils to nostrils on the top of their head.

Another critical transition is the novel trait of tail flukes, the characteristic horizontal tail fin found in whales which aids in aquatic propulsion.

Georgiacetus 48.6-37.2 million years ago

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Your pelvis is a structure that connects your legs to your spine, allowing you to walk or run while maximizing stability. The same is true for animals with four legs. Georgiacetus, however, detached its pelvis from its spine. This makes sense for an animal that is aquatic and no longer needs to support its body weight on four legs. Interestingly, we see the opposite transition in fossil fishes that began leaving water for land.

Dorudon 40.4-33.9 million years ago

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Basilosaurus 40.4-33.9 million years ago

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Dorudon and Basilosaurus, early whales that appeared later than the eight fossil species I’ve described so far, are the most whale-y looking species at this point. An obvious feature is the major reduction in the hind limbs into tiny vestiges. This is indicative of a fully-aquatic lifestyle, as opposed to an amphibious habit like that found in seals and sea lions.

They differ from modern whales however, in that they have greater elbow mobility and they still possess hind limbs.

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This is in contrast to modern whales which have retained a vestigial pelvis and occasionally a femur but no lower limb elements, further evidence of a gradual loss in hindlimb structure and function.

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These fossils very neatly document the transition in the body plan of a terrestrial mammal to the ancestor of modern whales. In my next two posts, I’ll highlight the transitional fossils that display the transition from this hypothetical common ancestor to the two lineages of modern whales: toothed and baleen whales.

*all fossil ranges based on the paleobiology database 

Questions for creationists

Is it simply a coincidence that there are fossils that seem to document a transition between a terrestrial mammal and whales? Since many of them could swim, shouldn’t these animals have survived Noah’s flood? Why would a whale need a tiny, hindlimb, such as those found in Basilosaurus and Dorudon?

References

1. McGowen, M. R., Gatesy, J., & Wildman, D. E. (2014). Molecular evolution tracks macroevolutionary transitions in Cetacea. Trends in ecology & evolution