Transitional Nature of Archaeopteryx and Other Maniraptorans: Archaeopteryx (what differentiates it from other maniraptors) Part 2/2
Introduction
Hello again r/Debatevolution, previously we showed 5 lesser-known aspects that highlight the transitional nature of maniraptoran dinosaurs and demonstrated that they are not simply birds, as creationist organizations have tried to portray them:
Now we will present characteristics that make the iconic Archaeopteryx transitional, one of the first examples of transitional forms to be discovered and consequently one of the first to be attacked by creationists.
For example: Richard Owen considered it an unusual bird; creationists later considered it a theropod with artificially created feathers; and finally, relatively recently, it has been presented as a "fully formed" bird.
It is important to mention that Archaeopteryx, in addition to being an avian, is a dinosaur because it fulfills all the characteristics that dinosaurs possess, which we already mentioned in the previous section.
Before we begin, I should mention that many of the characteristics discussed in the previous section are also present in Archaeopteryx. Without further ado, we will now discuss the characteristics that make Archaeopteryx transitional or that differentiate it from other maniraptors, and therefore quite significantly distinguish the Avialae clade.
This word will be repeated a lot, and it refers to a specific group of maniraptoran dinosaurs that is defined as:
>"The largest clade containing Vultur gryphus Linnaeus, 1758 (Aves sensu Clarke et al., 2020 or Neornithes) but not Dromaeosaurus albertensis Matthew & Brown, 1922 (Dromaeosauridae) and Saurornithoides mongoliensis Osborn, 1924 (Troodontidae)."
https://pmc.ncbi.nlm.nih.gov/articles/PMC9762251/#abstract1
Without further ado, let's begin.
1. Teeth
As we all know, unlike modern birds, Archaeopteryx possesses teeth. I have occasionally encountered a very specific group of creationists who claim that modern birds (such as certain hummingbirds and geese) have teeth, and that Archaeopteryx teeth are made of keratin. This is based on a confusion between teeth and tomial serrations (teeth made of cartilage and keratin, respectively), pseudo-teeth (bone teeth), and true teeth (those that possess enamel and dentin).
https://a-z-animals.com/animals/goose/do-geese-have-teeth/
Not to mention that Answer in genesis claimed that perhaps in the past (referring to a few thousand years) birds had teeth:
>"Now you may have heard of the old saying “as rare as hen's teeth” being used to describe something so atypical or unlikely that it is almost surely not to occur or to be found (as modern chickens don't have teeth).
>"And yet, the saying itself lends a clue to the fact that perhaps at some time in the past someone may have observed chickens with teeth. Otherwise, the saying makes less sense."
This has absolutely no support (at least not within the Neornithes groups and the timescales they propose). But getting back to the topic, Archaeopteryx is a mosaic in terms of its teeth.
Theropods had teeth in individual sockets and were generally serrated; this is also noticeable in Maniraptors, even those most closely related to birds, such as Troodontids and Dromaeosaurs. Modern birds have no teeth at all and possess oral papillae.
Therefore, there must be something in between, and this is the Avialae lineage. In the earliest members (Anchiornithidae), true teeth are observed in individual sockets, but the serrations disappear. Later, Archaeopteryx and many toothed birds that preceded it also lacked serrations and possessed oral papillae.
Further on, in the group called Ornithuria (close relatives of modern birds), teeth were reduced and disappeared from the jaw, and were located in a canal instead of individual sockets. Finally, they disappeared completely in Neornithes.
In this scenario, Archaeopteryx is situated between non-avian theropods and modern birds.
2. Foot, Ankle, and Tibia
In general, the feet of birds and dinosaurs are very similar; however, they also possess several differences, for example:
Birds have their metatarsals fused into a structure called the tarsometatarsus. Similarly, the fibula, tibia, and some ankle bones are fused into a structure called the tibiotarsus. Furthermore, birds possess a hallux capable of turning backward, known as a perching foot.
Meanwhile, theropods lack both the tarsometatarsus and the tibiotarsus; all these elements are free.
From an evolutionary perspective, a transition to the modern avian foot is expected to be found in the fossil record, and this is indeed the case.
Although the metatarsals of Archaeopteryx are certainly independent, it is possible to observe that they are proximally fused, which is the ancestral condition of Avialae.
https://pmc.ncbi.nlm.nih.gov/articles/PMC5788062/#sec4
Furthermore, unlike other theropods (with the exception of paraves), it possesses semi-fused distal tarsal 3 and distal tarsal 4.
https://onlinelibrary.wiley.com/doi/abs/10.1002/jez.b.23022
While the tibia, fibula, and ankle bones were virtually identical to any other theropod dinosaur, and contrary to what many creationists have claimed, its femur was positioned almost vertically, rather than the horizontal position of the femur in modern birds.
"It retains the almost vertical orientation of the elongated femur, indicative of hip-driven locomotion, as opposed to the knee-driven locomotion present in derived ornithuromorphic birds."
https://link.springer.com/article/10.1007/s44396-026-00026-z#Sec2
The hallux of Archaeopteryx was probably capable of greater retroversion than other theropods, meaning it was at least partially mobile, but it also differed from modern birds, and it is possible to see the transition between Archaeopteryx and ornithuromorphs.
>"The straight, untwisted shape of Mt1 in dinosaurs and the earliest long-tailed birds like Archaeopteryx supports the conclusion that these taxa had not yet evolved an opposable hallux."
Furthermore, the hallux of Archaeopteryx is itself transitional between theropod dinosaurs and modern birds.
>"This morphology is arguably an evolutionary intermediate between the straight Mt1 of dinosaurs and the twisted Mt1 of modern birds and conceivably allowed greater retroversion of Mt1 than Archaeopteryx."
https://www.nature.com/articles/srep09840
https://www.nature.com/articles/srep09840/figures/7
This condition is closer to that of basal pygostylan birds, for example, Confucius ornis, being more advanced than that of anchiornithids but less so than that of more advanced birds.
https://www.nature.com/articles/s41586-025-08912-4
https://peerj.com/articles/4191/
As a result, although most of the features of Archaeopteryx's hind limbs are dinosaurian, it also possesses many transitional features and others that are clearly avian.
3. Skull
The skull of the organism we have explored throughout this text is strange. On the one hand, it possesses clear dinosaurian features and features typical of other archosaurs: two temporal fenestrae; absence of the postfrontal bone; Ectopterygoid bone lateral to the pterygoid; quadrate head exposed laterally, etc. But even so, it certainly has an avian appearance.
Something that clearly had a major impact on avian evolution is heterochrony, which refers to the change in the timing of changes during ontogeny, that is, the life history of an organism.
In the case of birds, their cranial characteristics are a result of neoteny, which refers to the retention of juvenile characteristics in adults.
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/neoteny
Some examples of these characteristics are:
Larger eyes
Rounded skull
Larger brain
Shorter snout
Etc.
And in fact, Archaeopteryx already exhibits these traits:
https://www.eurekalert.org/news-releases/858103#:
https://www.nature.com/articles/s41559-017-0288-2
On the other hand, the skull of Avialae members possesses some synapomorphies (shared ancestral characteristics) that are probably not so noticeable to the naked eye:
- The nasal and lacrimal bones form the dorsal border of the antorbital fossa in lateral view.
- The long axis of the external nares is approximately equal in length to the long axis of the antorbital fenestra.
Cau (2018), for topology with scansoriopterygids as avialans:
- Relatively shortened nasal bones.
https://pmc.ncbi.nlm.nih.gov/articles/PMC9762251/#sec13
Therefore, on the one hand, we have a skull with all the dinosaurian synapomorphies, with similarities to birds and synapomorphies that define a group: Avialae
4. Arms
As we discussed in Part 1, the forelimbs of Archaeopteryx and other dromaeosaurs, specifically the digits, are virtually identical. However, their proportions are constructed differently.
During avian evolution, it is possible to observe how the forelimbs increase in size. This trend began in the Paraves group, but it really became noticeable in the Avialae clade. This trend becomes more pronounced in Archaeopteryx, although to a lesser extent than in later birds.
https://onlinelibrary.wiley.com/doi/full/10.1111/evo.12150
One proposed way to differentiate avialans from other maniraptorans is by arm length. It is possible to observe how these bones are significantly longer; for example, the humerus is longer than the scapula, and from the Archaeopterygidae onward, it is longer than the femur, as is the ulna. The hands are also unusually large.
https://www.geol.umd.edu/~tholtz/G104FC/lectures/104FCmani.html
This is important for the next topic, as it indicates their ability to fly.
https://www.nature.com/articles/s41467-018-03296-8
5. Locomotion and Flight Ability
As we have explored in the previous section, Archaeopteryx has profoundly transitional characteristics, to the point that it not only has mosaic characteristics, but all of its mosaic characteristics are a mosaic in themselves, from its teeth to its limbs. This is most evident in its locomotion and flight ability.
There are characteristics of Archaeopteryx that indicate it likely possessed some degree of flight capability, due to the presence of humeral tertiary feathers and the length and geometry of its wings, which, as mentioned in the previous point, were unusually large.
https://www.nature.com/articles/s41586-025-08912-4
https://www.nature.com/articles/s41467-018-03296-8
However, it should be noted that its flight was more similar to that of gliding birds and short-flight birds like pheasants, which are frequently flying birds, and this aligns with other characteristics we will explore.
On the one hand, we need to review certain characteristics. One recognized feature is that the sternum of Archaeoptryx was not ossified and was flat, similar to that of anchiornithids and the primitive bird Sapeornis. This is important because the muscles that allow wing flapping are anchored to the sternum, so its absence or reduced size would directly impact its flight capabilities.
Unlike modern birds, the wrist of Archaeopteryx could not fold in the same way as in modern birds.
Another important point is that although their wings had a more advanced configuration than those of non-avian maniraptorans and more primitive "birds," it was also more primitive than that of modern birds.
https://www.cell.com/current-biology/fulltext/S0960-9822(12)01194-3
Finally, we can highlight its lack of a tail fan, which, together with its wing configuration, would have made its flight very fast, inefficient, and unstable, with dangerous landings. Therefore, we could conclude that Archaeopteryx had flight capabilities, although more advanced than those of anchiornithids like anchiornis; these were based on short bursts of flight followed by gliding.
Thanks to knowledge of its footpads, it is possible to know that, like *Anchiornis*, it was largely terrestrial, although in a different way, as it was not raptorial and was similar to some terrestrial birds and birds of prey.
https://www.nature.com/articles/s41467-022-35039-1?fromPaywallRec=true
Similarly, although both had sickle-shaped claws on their feet, they had largely lost the ability to hyperextend them, while the ancient avialan *xiaotingia* still retained this ability.
https://link.springer.com/article/10.1007/s44396-026-00026-z#Sec2
https://www.nature.com/articles/nature10288
And as we mentioned in previous points, the hallux is partially inverted, to a greater degree than in theropod dinosaurs but less than in modern birds.
https://peerj.com/articles/4191/
https://www.nature.com/articles/s41586-025-08912-4
Therefore, we have a terrestrial and partially arboreal bird, with some arboreal traits and a primitive capacity for powered flight and gliding, intermediate between terrestrial maniraptorans and the birds that would come later.
During the editing of this text, an article was published that prompted me to write this section, and from which I paraphrase. You can find these and more points here:
https://link.springer.com/article/10.1007/s44396-026-00026-z
Conclusion
Archaeopteryx is remarkably similar to many theropods, especially its close relatives. This was noted from the beginning of the study of avian evolution, which, as we saw in the first part, led Thomas Henry Huxley, "Darwin's bulldog," to describe this relationship.
Although this hypothesis was ignored for much of the 20th century, thanks to the work of John Ostrom with the discovery of Deinonychus anthirropus in 1964 (and its description in 1969), it was reconsidered and, with time, multiple discoveries, and predictions, became the paradigm.
Since then, the BAND (Bird Are Not Dinosaurs) position has existed, in which a group of scientists and associates refused to accept this hypothesis, instead maintaining an inconsistent and non-uniform set of hypotheses that lack real fossil evidence, in addition to some refuted arguments such as the identity of toes II, III, IV (Wagner & Gauthier 1999; Tamura et al 2011; Towers et al. 2011;Carkett & Logan 2011;Gouvêa and Brigandt 2023).or “the time paradox” which is likewise not supported by the current fossil record (Mudroch et al 2011; Hartman et al 2019; Wills, Underwood, Barrett 2023).
Despite everything, many creationists continue to use these and other outdated arguments, which are their main tools for denying avian evolution. In these two texts, we have explored one of these arguments and its derivatives: the claim that Archaeopteryx and other maniraptors were exclusively avian. We have demonstrated that while they possessed multiple derived traits, they also possessed primitive (characteristic of other theropod dinosaurs) and intermediate traits.
For now, we can say that maniraptor dinosaurs and Archaeopteryx are a perfect example of transitional forms, demonstrating the relationship between birds and dinosaurs because, as Huxley himself said:
>"Birds are glorified reptiles"