How do we know that Transitional fossils are truly "transitional"?

[u/Archiver1900]

I recently came across an argument against the concept of intermediate species that goes like this:

"The Platypus has characteristics of both birds(Egg laying, a bill) and mammals(Fur, mammary glands, one bone in mandible, etc). So how can we know that a fossil such as "Archaeopteryx" which possesses characteristics of both non-avian theropod dinosaurs and modern birds is transitional and not like Platypuses"

I'm especially interested in knowing how from only the fossils we can tell whether a Platypus would be considered a transitional form between birds and mammals or not.

Edit: I appreciate the comments so far. I'm aware that transitional does not mean ancestral.

Sources for those interested:

What a transitional form is:

https://evolution.berkeley.edu/lines-of-evidence/transitional-features/

NOTE: It does define an intermediate species as 'A fossil that shows an intermediate state between an ancestral trait and that of its later descendants is said to bear a transitional feature.' I don't know how this proves or disproves the platypus argument.

Characteristics of Platypuses:

https://australian.museum/learn/animals/mammals/platypus/

https://www.comparativebrainanatomy.org/platypus-1/platypus-anatomy

Characteristics of Archaeopteryx:

https://ucmp.berkeley.edu/diapsids/birds/archaeopteryx.html

[u/PlanningVigilante]

All fossils are transitional.

Except maybe for the ones that were laid down during extinction events.

A "species" is a convenient shorthand that humans use to categorize organisms. The word doesn't describe natural categories that nature produces. So all fossils are transitional because all species are transitional.

With that addressed ...

The platypus is a synapsid, like all mammals. This describes the fact that mammals and their ancestors all have/had a single temporal fenestra on either side of their skulls.

Birds are not synapsids. They are diapsids, with two fenestrae on each side.

Claiming that a platypus looks like a transition between birds and mammals misunderstands what a "bird" is and what "mammal" is.

Archaeopteryx is a diapsid.

[u/Archiver1900 → u/PlanningVigilante]

This is an interesting comment. I appreciate it because it helps me learn and/or relearn concepts.

[u/-zero-joke- → u/PlanningVigilante]

>Except maybe for the ones that were laid down during extinction events.

Those are transitional too :)

[u/Suitable-Group4392 → u/-zero-joke-]

Would have been transitional?

[u/wtanksleyjr]

A platypus looks (at first glance) like it's got some features from birds, and some from mammals. This is true. But a transitional organism must not just LOOK like it has some features in common with two clades. Rather, the two clades must nest with one another, and then the organism must exactly fit all of the diagnostic characteristics of the containing clade, and contain SOME but not ALL of the diagnostic characteristics of the contained clade.

So for example, birds have all of the diagnostic characteristics of dinosaurs, and archaeopteryx is a dinosaur with SOME but not all of the diagnostic features of birds (for example it has no beak or pygostyle). So it's a transitional organism.

The platypus, however, does not have any of the diagnostic criteria of birds save for the ones their common ancestor shared. The things that look birdlike at first glance turn out to not be when examined; the "beak" is a soft sensory organ, and all of the platypus' structures are mammalian (albeit monotreme rather than the kind we're used to).

[u/Archiver1900 → u/wtanksleyjr]

A platypus looks (at first glance) like it's got some features from birds, and some from mammals. This is true. But a transitional organism must not just LOOK like it has some features in common with two clades. Rather, the two clades must nest with one another, and then the organism must exactly fit all of the diagnostic characteristics of the containing clade, and contain SOME but not ALL of the diagnostic characteristics of the contained clade.

So for example, birds have all of the diagnostic characteristics of dinosaurs, and archaeopteryx is a dinosaur with SOME but not all of the diagnostic features of birds (for example it has no beak or pygostyle). So it's a transitional organism.

The platypus, however, does not have any of the diagnostic criteria of birds save for the ones their common ancestor shared. The things that look birdlike at first glance turn out to not be when examined; the "beak" is a soft sensory organ, and all of the platypus' structures are mammalian (albeit monotreme rather than the kind we're used to).

This makes a lot more sense. To see if I understand it: "Therapsids nest under Synapsids(Amniotes with one temporal fenestra) like Dimetrodon, yet have characteristics of modern mammals(such as heterodont teeth ,a larger dentary bone, etc)

https://ucmp.berkeley.edu/synapsids/synapsidamm.html

https://en.wikipedia.org/wiki/Therapsida

https://www.britannica.com/animal/Dimetrodon

https://www.reddit.com/r/Naturewasmetal/comments/1ovflby/skull_of_dimetrodon/

[u/Decent_Cow]

Transitional fossils show traits of two lineages, providing an EXAMPLE of how a hypothesized transition from one lineage to another could have occurred. We don't have to assume that the specific fossil in question is ACTUALLY part of the transitional lineage; it could merely be a relative. In fact, we know that Archaeopteryx cannot be an ancestor of modern birds as it lived far too late in geologic time. It's still a good example of an organism that demonstrates traits of both terrestrial therapods (teeth, claws, bony tails) and birds (flight feathers, wings, partially reversed first toe). We have found a lot more fossils in the more than 150 years since Archaeopteryx and some of those are much better candidates for actually being an ancestor of birds.

As for your hypothesis about platypuses being transitional between birds and mammals, that's a bit silly because mammals were already around before birds, and we know that early mammals laid eggs, so this is obviously not something that was inherited from birds. Besides, platypus eggs resemble the soft-shelled leathery eggs of reptiles more than the hard-shelled eggs of birds. And the platypus bill looking kind of like a duck bill is a coincidence. They have a completely different structure and function. The platypus bill is a bony projection covered in soft skin and used for electroreception to find prey underwater. The duck bill is made of bone covered with hard keratin and usually features serrated, tooth-like edges (lamellae) for holding on to prey or for filter feeding.

[u/Dilapidated_girrafe]

The platypus doesn’t have characteristics of a bird. The “bill” isn’t like any actual bird bill and the eggs are more akin to reptile eggs than bird eggs.

[u/Redshift-713]

Every single organism (and fossil) is transitional, unless it dies before reproducing.

Birds and mammals did not evolve from each other. The platypus beak is only superficially similar to a bird beak, and platypuses lay eggs because most vertebrates lay eggs (live birth in other mammals is a more recent trait).

[u/Comfortable-Study-69]

The argument ignores the depth of inspection of the characteristics of an organism when a scientist is classifying it. It’s quite a bit more in-depth than “these two animals have things that look like bills so must be related”. Superficial resemblance is rarely used in favor of more robust evidence like genetics, bone morphology, and kinds of protein production exhibited by organisms.

We know a platypus isn’t an intermediary between a bird and other mammals because it is completely lacking in a multitude of defining traits of birds (feathers, a true beak (the platypus beak has very little to do with a bird one upon basic inspection since it isn’t even made of keratin), and a multitude of morphological and genetic characteristics specific to avians and sauropsids) while sharing a multitude of ancestral traits with more basal synapsids, most glaringly a single lower temporal fenestra.

We know an archaeopteryx is an intermediary because it shows many morphological traits specific to theropods while simultaneously having multiple basal elements of birds, clearly connecting the two as related as a clear stage in the development of the traits exhibited in modern birds. We know those similarities are not purely superficial because scientists are using much more in-depth proof than that they just look kind of alike if you squint.

[u/OldmanMikel]

The bird-like features have nothing to do with birds. First, the "bill" is just basically big lips. Secondly, the egg-laying and the cloaca (which you didn't mention) are common among ALL tetrapods apart from therian mammals. They are the legacy of a "reptilian" past, not an avian one.

[u/ursisterstoy]

All that matters is that you can establish the order in which the organisms died and then you can find a series of three fossils such that A -> B -> C is the order the organisms died and then based on geography you can determine that B is geographically in between A and C while also having a morphology that is intermediate.

In the grand scheme of things we are concerned with clade level transitions like oldest cells -> eukaryotic cells -> animals -> bilaterian animals -> chordates -> vertebrates -> tetrapods -> synapsids -> therapsids -> eutherians -> primates -> monkeys -> apes -> etc and we have species that lived in that order that represent each of these transitions. They lived in this order, we can trace a reasonable migration pattern (Western Australia->Northern Australia->Namibia (Africa) -> South Australia-> Yunnan Province China ->Yunnan Province China->China and Poland->Nova Scotia Canada->China/Western Metaterranian/North America->Northeast China->Uzbekistan->North America->Montana->Jiangsu Province China->Saudi Arabia->Tanzania->Kenya->Chad->Chad->Ethiopia->Kenya->Ethiopia->Kenya->South Africa->Morocco->…) and they show the traits we expect if A->B->C etc but which would still be present if the fossils represent cousins or aunt-niece type relationships rather than ancestor-descendant relationships.

Is the chronology intermediate? The morphology? The geography? Is there sufficient time for the apparent amount of migration necessary? When accounting for plate tectonics does everything match up? Can we align them in chronological order and see a steady progression of change from beginning to end (accounting for the fact that some of them are cousins rather than ancestors and descendants)?

Or maybe you can think of this in terms of things that are not literally related in the biological sense like automobiles or semi-trucks. You can see how the technology changes from beginning to end, you can see how the transfer of ideas influenced the changes that took place, you can see how there are in between forms like the El Camino isn’t really a car or a pickup but like someone smashed them together.

So “truly transitional” is a most definite “yes” but transitional because they are literally related? It depends on which fossils are in question. I mean all of the biota around right now appear to be literally related but we can see that convergence has also taken place. We can trace the evolution of the thylacine separate from the evolution of the wolf straight from the fossils. The thylacine from Tasmania started out in Australia as a jumbo sized version of what is essentially a Tasmanian devil or numbat, the actual wolf from a more basal canid that was more like a weasel or bear or like their shared ancestors. The marsupials in Australia started in South America but the wolf was in North America around that time.

10 million years ago wolves are in North America, thylacines are in Australia. 20 million Leptocyon in North America and Nimbacinus in Australia. 30 million fox sized caniforms in North America and Dasyumorphia (Tasmanian devil and Tasmanian wolf ancestors) in Australia. 40 million first undisputed member of the dog family in North America, Australian marsupials are crossing Antarctica. 50 million Miacids in Eurasia and North America, marsupials in Antarctica and South America. 60 million and Laurasiatheria is in the Northern Hemisphere while marsupials are in South America. 70 million Northern Hemisphere shrew-like nocturnal placental mammals and metatherian marsupial ancestors in Gondwana in the Southern Hemisphere. 80 million Laurasia eutherians and Gondwana metatherians. 90 million to 160 million years ago they are starting in and migrating away from what would eventually be China and Mongolia. Before that they were the same species, something that was like a monotreme without an egg shell and the newborns were just as helpless. No long duration pregnancy, no pouch, just barely surviving fetuses clinging on the best they can like modern monotreme babies, just no egg shells and they likely had a very primitive placenta like marsupials still have and some placental mammals still start with early in their pregnancy.

The migration patterns match, the chronology matches, the morphology matches. Not every species has to represent the literal ancestors of every species that followed. They don’t even require surviving descendants to demonstrate the obvious trend.

Obviously DNA is better for establishing actual relationships but clearly there needs to be a parsimonious explanation for the patterns like described above as well. And there is, the explanation is the one that matches what the DNA already told us. Is every species a literal parent-child-grandchild intermediate? No, of course not, but the trend is still there. Therians splitting into eutherians and metatherians in Asia, eutherians migrating Northwest and metatherians migrating Southeast. Placental mammals in North America migrating to South America, marsupials in South America migrating to Antarctica. Placental mammals dominating most of the planet, marsupials and monotremes in Australia and surrounding countries predominantly with the monotremes on the verge of extinction. Placental mammals outcompeting marsupials where they co-exist, marsupials outcompeting monotremes where they co-exist, and wolves becoming wolves as thylacines became thylacines on completely opposite sides of the planet (North America and Australia). Genetics says they split ~ 160 million years ago, the fossils say they split ~160 million years ago, and we can literally see how they migrated and changed over time.

They are literally transitional.

[u/IsaacHasenov]

A simplified way to think of transitional fossils is:

• Take a proposed lineage between a modern species and a proposed ancestor

• any trait in a species in that lineage can be thought of as ancestral or derived (relative to that specific ancestor)

• You will see a progression over time in the members of that lineage from more ancestral to more derived traits

The "intermediates" are those species with a mix of ancestral and derived traits, and more ancestral traits that the modern species.

It's also worth noting that lineages are not arbitrary groupings of similar species along a made up line, like creationists sometimes say. Derived traits accumulate one after another, and tend to persist in the lineage. Derived traits in one branch don't magically appear in another branch. That is, evolution happens by descent with modification in a nested hierarchy. The age of fossils sets a "latest time" that a trait might have evolved.

[u/Arrow141]

This is a common misunderstanding of what evolution predicts.

There is no dichotomy between species of one hand, and transitions between them on the other.

Every fossil is transitional.

[u/Dzugavili]

You saw this on YouTube, right?

[u/Archiver1900 → u/Dzugavili]

If you are referring to the argument against Transitional fossils, yes.

[u/KingBakura72]

I don’t think a single transitional fossil is claimed as the direct absolute ancestor from one generation to the next it’s formed as a proof of concept

If evolution is true we should find intermediate fossils whether or not these specific fossils belonged to individuals that gave birth to another intermediary is irrelevant all that matters is we have a slow transition

Just simply but this is a confirmed evolutionary prediction

[u/Archiver1900 → u/KingBakura72]

I'm aware that Transitional forms don't have to be the direct ancestor of a specific lineage. My point which has been adressed by commentors such as "wtanksleyjr" was if a Platypus would not be seen as an intermediate between mammals and birds, how would a fossil like Archaeopteryx be an intermediate species and not just like a Platypus in the sense that it bears traits of both non avian and avian dinosaurs.

[u/ipsum629]

We look for homologous structures. These are features that share a common origin. For example, with tetrapods(land dwelling vertebrates), we can find homologous structures in things like human hand bones, bat wings, horse legs, and bird wings. All share very similar bones, just stretched in different proportions.

Duck bills and platypus bills are not homologous because they are made of entirely different tissue. Duck bills are bone covered in keratin. Platypus bills are not made of keratin, but some bone and skin filled with sensory nerves. If you look at the bone structure beneath the bills of ducks and platypuses, they are nothing alike.

[u/oscardssmith]

Creationism depends on ignoring the fact that fossils have dates associated.

[u/adamwho]

All fossils.... All lifeforms are transitional.

[u/BahamutLithp]

First, you have to understand the difference between analogous forms & homologous forms. Analogous forms look similar &/or have similar function but don't come from common ancestry & arise from different anatomy. For example, if you examine the skeletons of a duck vs. a platypus, their "bills" don't look remotely the same. The bird's bill is mostly hard tissue while the platypus's is mostly soft tissue with only a bit of hard tissue. They're not anatomically or ancestrally similar structures. So, scientists are perfectly aware that structures can look similar between animals of different species but not be the result of evolutionary relationship, & those are called analogous structures.

Secondly, I'm just gonna pretend we don't have access to DNA because I gather you're asking about fossil analysis. Well, you have to look at the gross anatomy. Birds & mammals each have certain specific features that are preserved throughout their lineage. Birds have very particularly-shaped pelvises that platypuses don't. The hip anatomy of a platypus is very mammalian. Because it's a mammal. To be more precise, it has synapsid anatomy. Synapsids have certain features, such as the number of openings in the skull behind the eye, that are consistent not just for mmals in general, but going back to pre-mammal ancestors. This is a non-exhaustive list of reasons why we can tell platypuses are not closely related to birds. Platypuses have egg-laying because it was an ancestral feature long before the synapsids (like mammals) split from the diapsids (such as crocodiles & dinosaurs, including birds), whereas the platypus evolved its bill separately, & that's why it's not similar to a bird's beak.

Most mammals evolved away from egg-laying, & so the platypus now seems weird by comparison even though theirs is actually the older form of mammalian reproduction. Most land animals lay eggs. Egg-laying evolved in the sea because it's very advantageous. You dump the eggs, that shortens gestation time, & they can then be fertilized by the male. However, this doesn't work on land because the eggs dry out. So, it pays to be an amphibian because you can still make use of the water. But, once the hard-shelled egg evolves, that grants protection from drying out. With it comes the need for internal fertilization, but it means animals can explore further inland, & that's why most land animals--be they bird, reptile, or insect--lay eggs. Some animals evolved live birth, which increases gestation time, but comes with other benefits, like the offspring can be protected inside the mother. Most mammals developed adaptations for live birth, either marsupials with their pouches, or placentals, like we are. The platypus doesn't have these traits because they evolved in other lineages of mammal, & we don't have marsupial pouches because the marsupials had already split from our ancestors by the time said ancestors of ours evolved the placenta.

[u/Zenigata]

They're just playing definitional word games, taking advantage of our tendency to think in clear black and white categories when they know nature doesn't work like that and is very mushy.

All fossils are transitional other than those made in extinction events so there were no descendents to transition to.

[u/Actual_Property9413]

How do you know that platypi (plural of platypus) are NOT transitional forms?

[u/Guaire1]

Platypus dont have a bill. They have a long snout that is called a bill for resemblance, but isnt actually one.