The Debate in Cladistics: Birds, Mammals, and Their Evolutionary Lineages

In cladistics, the classification of organisms is based on their evolutionary relationships and common ancestry, rather than traditional Linnaean taxonomy which often relied on morphological characteristics. This approach is crucial for a more accurate understanding of the evolutionary tree. This article delves into the discussion of Aves under Reptilia and why Mammalia are not included in the same group, highlighting the importance of modern evolutionary methods such as cladistics.

Aves and Reptilia: A Shared Ancestral Lineage

In cladistics, birds (Aves) are considered part of the Reptilia clade because they share a common ancestor with reptiles. The modern understanding of reptiles includes not only traditional reptiles like lizards and snakes but also birds, which are descended from theropod dinosaurs. This grouping is supported by a variety of evidence, including morphological traits and genetic evidence.

Morphological Traits

Both birds and reptiles share many anatomical features, such as scales, feathers (which are modified scales), similar bone structures, and egg-laying methods. These shared traits indicate a close evolutionary relationship, supporting the classification of birds within the Reptilia clade.

Genetic Evidence

Molecular studies have shown that birds are more closely related to certain groups of reptiles than to mammals, reinforcing the idea that they belong within the Reptilia clade. This genetic evidence further supports the assertion that birds are not a separate class but a subgroup of reptiles.

Mammalia: A Separate Lineage

On the other hand, mammals (Mammalia) are placed in a separate clade because they diverged from their common ancestor with reptiles earlier in evolutionary history. Key reasons for this separation include:

Distinct Characteristics

Mammals have unique features such as mammary glands, fur, and three middle ear bones which are not found in reptiles or birds. These distinct traits are indicative of a separate evolutionary lineage.

Common Ancestry

The clade that includes mammals, known as Synapsida, diverged from the lineage leading to reptiles (Diapsida) long before the emergence of birds. This divergence is significant enough to warrant a separate classification.

Revisiting Traditional Taxonomic Terms: Sauropsida and Synapsida

A more accepted term in cladistics is Sauropsida. Reptilia is no longer a particularly accepted taxon because it traditionally excludes birds, which are reptiles. Animals like pterosaurs, Ichthyosaurs, dinosaurs, and plesiosaurs are commonly considered reptiles, yet they have various characteristics that traditionally separate them from other taxa within Reptilia.

If we accept these creatures as reptiles, then birds are indeed reptiles. The term Sauropsida helps to clarify this by grouping all reptiles, including birds, under a single clade. This term was created after researchers adopted cladistics and determined that synapsids, although reptile-like, were not cladistically part of any reptilian lineage. Synapsids are now more appropriately referred to as “stem-mammals,” “proto-mammals,” or “paramammals.”

Evolutionary Ancestry and Basal Characteristics

Many ancient relatives of mammals, known as synapsids, were very different in appearance from living mammals and were often mistaken for reptiles such as dinosaurs. These animals were frequently referred to as “mammal-like reptiles,” a term reflecting outdated classification methods. In reality, these ancient mammal-relatives are more closely related to living mammals than they are to any reptiles.

It is improper to classify synapsids as reptiles because they represent a separate lineage. However, it is important to note that synapsids were still very reptile-like and became even more reptile-like the further back in time you go. Similarly, the ancestors of all modern Amniotes were very amphibian-like and became more amphibian-like the deeper back in time you go. This is due to the fact that evolution is not linear, and members of a lineage can inherit basal characteristics and retain them for a long period, even when most of their ancestors went extinct.

Modern Examples: Sea Horses and Virology

Modern sea horse relatives, each showing a transitional state in sea horse evolution, still exist today, demonstrating how ancestral characteristics can persist long after the extinction of their ancestors. Similarly, the Omicron variant of COVID-19 is not closely related to Delta or any other variant that has evolved after March 2020. Omicron split from all other variants back in March 2020.

Remember, a lineage can retain ancestral characteristics even well after the ancestors died out. This is why modern reptiles have characteristics basal to Amniotes, or why modern amphibians retain characteristics basal to tetrapods.