Ungulata

In traditional classification, "Ungulata" is a general term used to refer to mammals with hooves, including Perissodactyla and Artiodactyla. However, with the progress of molecular phylogenetic research, the traditional "ungulates" or "Ungulata" is not a strict monophyletic concept. In modern classification, whales (Cetacea) are included in even-toed ungulates and merged with even-toed ungulates (Artiodactyla) into "Cetartiodactyla". Therefore, when discussing ungulates today, they are usually broken down into two independent major living groups:

1. Perissodactyla: including horses, rhinos, tapirs, etc., with an odd number of toes (the middle toe bears weight).

2. Cetartiodactyla: integrates traditional even-toed ungulates (such as pigs, cattle, deer, camels) and cetaceans (whales, dolphins, porpoises) into a natural monophyletic group. Even-toed ungulates are characterized by an even number of toes on each foot (usually 2 or 4), while the evolution of whales shows that they are closely related to hippos.

Historically, "Ungulates" also included some extinct primitive groups (such as Callostomidae and "Obtsodontia" in a broad sense), but here we mainly focus on the two major living groups: Perissodactyla and Cetiformes.

I. Introduction to Hoofed Mammals

1. Traditional vs. Modern Classification:
Traditionally, “Ungulata” was used as a broad term to include all hoofed mammals. This encompassed horses, rhinos, tapirs (odd-toed) as well as pigs, camels, deer, cattle, and others (even-toed). However, with advances in molecular phylogenetics, it became clear that whales (Cetacea) share a close evolutionary relationship with hippopotamuses (Hippopotamidae), which are even-toed ungulates. As a result, whales have been placed within the same clade as even-toed ungulates, creating the order Cetartiodactyla. This modern grouping better reflects their evolutionary origins.

2. The Two Main Orders of “Ungulates”:

• Perissodactyla (Odd-toed Ungulates): Horses, rhinos, and tapirs—characterized by having an odd number of toes (usually one or three) and bearing weight mainly on the middle toe.

• Cetartiodactyla (Even-toed Ungulates + Whales): This group combines traditional Artiodactyla (even-toed ungulates like pigs, camels, deer, bovids) with Cetacea (whales, dolphins, porpoises). Even-toed ungulates typically have two or four toes, and whales are highly specialized aquatic descendants of ancient terrestrial ungulate ancestors.

II. Evolutionary History

• In the Paleogene and Neogene periods (roughly 20–50 million years ago), hoofed mammals underwent extensive adaptive radiations, spreading across the globe.

• Perissodactyls once included a much greater diversity, but only three families survive today. They dominated certain habitats in the past but declined due to changing climates and the rise of grasslands.

• Artiodactyls diversified more extensively, giving rise to a broad range of large herbivores on land. Within the even-toed lineage, one group returned to the water, evolving into modern whales and dolphins. Molecular data confirm that whales are nested within the even-toed ungulates, closely related to hippos, leading to the modern concept of Cetartiodactyla.

III. Classification Tables

A. Perissodactyla (Odd-Toed Ungulates)

B. Cetartiodactyla (Even-Toed Ungulates + Whales)

Even-Toed Ungulates (Artiodactyla)

Whales (Cetacea, included in Cetartiodactyla)

(There are many other whale families; the above are representative examples.)

IV. Summary

The concept of “hoofed mammals” as a single order is outdated. Modern evolutionary and molecular studies have clarified their relationships, placing whales within the lineage of even-toed ungulates. Thus, today we speak of:

• Perissodactyla (Odd-Toed Ungulates): Horses, rhinos, tapirs.

• Cetartiodactyla (Even-Toed Ungulates + Cetaceans): Pigs, hippos, camels, deer, cattle, antelopes, and whales/dolphins.

This modern classification highlights the incredible evolutionary journey of some terrestrial herbivores that eventually took to the oceans and became fully aquatic cetaceans. By understanding these relationships, users can appreciate how modern taxonomy accurately reflects the deep evolutionary history and adaptation of hoofed mammals.