Those Crocodile-Like Creatures of the Past — Side Story: Choristoderes

In the previous article, we introduced phytosaurs—those crocodile-like reptiles that belonged to the early archosaur lineage. Their relationship to crocodiles was somewhat comparable to that between rabbits and guinea pigs: not closely related, but at least still within the same broad evolutionary branch.

The animals introduced today are far more puzzling. They resemble crocodiles in appearance, yet they are not closely related to crocodiles at all. Their evolutionary origin remains uncertain, and even their disappearance from the fossil record is not fully understood. These mysterious reptiles are known as Choristodera.

Fig. 1 Reconstruction of Champsosaurus.（圖片來源：Nobu Tamura，採用 CC BY 2.5 授權）

What Are Choristoderes?

Choristoderes, represented by species such as Champsosaurus, were a highly unusual group of aquatic reptiles. Compared with crocodiles and phytosaurs, the choristodere lineage shows remarkable morphological diversity.

Some members, such as the champsosaurs (family Champsosauridae), closely resembled crocodiles. Others, such as the hyphalosaurids, had long necks reminiscent of nothosaurs or plesiosaurs. Still others, including members of the family Monjurosuchidae, looked more like large lizards.

With such dramatically different appearances, it is easy to see why their classification has long confused both readers and paleontologists alike.

Fig. 2 Reconstruction of Hyphalosaurus lingyuanensis.（圖片來源：Matt Martyniuk，採用 CC BY 3.0 授權）

Classification and Major Lineages of Choristoderes

For many decades, the classification of choristoderes puzzled researchers. The dramatic differences between species made it difficult to recognize them as members of a single evolutionary group. In fact, the discoverer of Champsosaurus, Edward Drinker Cope (1840–1897), originally placed the animal within the order Rhynchocephalia.

Today, paleontologists identify choristoderes primarily by distinctive features of the skull and vertebrae. One notable characteristic is the closed lower temporal fenestra, a trait that distinguishes them from most other reptiles.

Fig. 3 Abdominal fossil of Ikechosaurus, showing the distinctive vertebral structure of choristoderes, housed in the Beijing Museum of Natural History.（圖片來源：Tiouraren，採用 CC BY-SA 4.0 授權）

Phylogenetic analyses published in 2020 divided Choristodera into several evolutionary branches. Basal members include genera such as Cteniogenys, Heishanosaurus, and Coeruleodraco. Two major derived clades are also recognized: Allochoristodera and Neochoristodera.

The neochoristoderes include two well-known families: Champsosauridae and Simoedosauridae. Animals in this group strongly resemble crocodiles. They possess elongated snouts, short limbs, streamlined bodies, and flattened paddle-like tails. However, unlike crocodiles, their skin lacked heavy osteoderm armor and was instead covered in relatively small scales.

Fig. 4 Complete skeletal fossil of Champsosaurus.（圖片來源：D. Gordon E. Robertson，採用 CC BY-SA 3.0 授權）

The skull structure of choristoderes also differs greatly from that of crocodiles. In Champsosaurus, for example, the skull is somewhat bulbous when viewed from above and bears large cranial arches along the sides. These arches provided attachment surfaces for strong jaw muscles, helping the animal grip prey.

Another difference involves the nostrils. In choristoderes such as Champsosaurus, the external nostrils are located at the very tip of the snout. Paleontologists suggest this adaptation may relate to their ambush-hunting lifestyle beneath the water's surface. With nostrils at the tip of the snout, the animal could remain almost completely submerged while still breathing, raising only the end of its snout above the water. This position also helped conceal the body from potential prey.

Fig. 5 Skull fossil of Champsosaurus, showing its characteristic large cranial arches.（圖片來源：Ghedoghedo，CC0 1.0 公共領域）

Fig. 6 Detailed anatomical diagram of the skull of Champsosaurus, showing the external nostrils located at the tip of the snout.（圖片來源：Hemiauchenia，採用 CC BY-SA 4.0 授權）

The Other Major Branch: Allochoristoderes

The second major branch, Allochoristodera, contains more morphologically diverse forms. Members of this group include the long-necked Hyphalosauridae, as well as more primitive genera such as Lazarussuchus and the family Monjurosuchidae.

The hyphalosaurids are particularly striking. Their necks were extremely long and their heads small, giving them an appearance reminiscent of plesiosaurs. However, unlike plesiosaurs, their limbs did not evolve into flippers, and their tails were large and powerful. Paleontologists believe these features reflect adaptation to deep freshwater lakes. The large tail could generate propulsion during dives, while the long neck allowed them to ambush prey near the water surface.

Fig. 7 Size reconstruction of Hyphalosaurus lingyuanensis（圖片來源：Matt Martyniuk，採用 CC BY 3.0 授權）

Another lineage, represented by Lazarussuchus, shows a more lizard-like appearance. These animals retained the characteristic choristodere cranial arches and snout-tip nostrils, but their skulls were shorter, their tails less specialized, and their limbs more slender. Overall, they resembled modern monitor lizards or small crocodilians rather than highly specialized aquatic predators.

Fig. 8 Anatomical diagram of the skull of Lazarussuchus, showing the absence of the lower temporal fenestra, a typical feature of choristoderes.（圖片來源：Hemiauchenia，採用 CC BY-SA 4.0 授權）

When Did Choristoderes Appear and Why Did They Disappear?

The evolutionary history of choristoderes is as confusing as their classification. Because different species share features with both lepidosaurs and archosaurs, paleontologists have long debated their exact position within reptile evolution. The prevailing view today places them within the archosauromorph lineage, though their precise ancestry remains uncertain.

The earliest known choristodere fossils belong to the genus Cteniogenys, dating to the Middle Jurassic (Bathonian stage). These animals were relatively primitive, retaining a lizard-like body form and only partially webbed limbs. However, the sedimentary environment and anatomy suggest that they were already semi-aquatic.

Fig. 9 Mandibular fossil of Cteniogenys.（圖片來源：Yale Peabody Museum，採用 CC BY-SA 4.0 授權）

During the Cretaceous period, choristoderes underwent an evolutionary radiation. The two major clades—Neochoristodera and Allochoristodera—diverged and occupied different ecological niches.

By the Late Cretaceous, choristoderes were widespread in freshwater ecosystems. They thrived in rivers, lakes, and wetlands across Laurasia. Yet their prosperity was abruptly interrupted by one of the most famous events in geological history: the Cretaceous–Paleogene (K–Pg) extinction event.

About 66 million years ago, a large asteroid struck Earth. The impact triggered global environmental collapse. From land to sea, countless organisms disappeared, including non-avian dinosaurs, pterosaurs, marine reptiles, and ammonites. Choristoderes were also severely affected.

Fig. 10 Philydrosaurus proseilus, a choristodere from the Late Cretaceous, specimen housed in the Beijing Museum of Natural History.（圖片來源：Tiouraren，採用 CC BY-SA 4.0 授權）

After the extinction event, choristodere diversity declined sharply. Many long-necked forms disappeared near the boundary itself. The surviving species tended to have more conservative body plans—either crocodile-like or lizard-like.

Despite this setback, choristoderes did not immediately vanish. During the Paleocene, the neochoristoderes—particularly the champsosaurs—reoccupied rivers and wetlands. Warm climates and abundant habitats allowed them to grow large again, with some species exceeding the size of their Mesozoic ancestors.

Fig. 11 Kosmodraco, a member of the family Simoedosauridae that lived in the early Paleocene.（圖片來源：SeismicShrimp，採用 CC BY 4.0 授權）

However, another competitor was quietly rising: true crocodilians, or Eusuchia. Like choristoderes, crocodilians had suffered losses during the K–Pg extinction, but they recovered during the Eocene.

Compared with choristoderes, crocodilians possessed several advantages, including heavy armor and efficient ectothermic metabolism. These traits allowed them to endure long periods without food and survive harsh environmental conditions. Over time, crocodilians gradually displaced choristoderes from tropical habitats, forcing them into cooler temperate regions where ectothermic reptiles struggled to survive.

Fig. 12 Diplocynodontinae, a crocodilian lineage that lived in Eocene Europe.（圖片來源：Ziko van Dijk，採用 CC BY-SA 3.0 授權）

Climate change compounded the problem. Beginning in the Late Eocene, global climates became cooler and drier. Wetlands shrank as grasslands expanded across the continents. Choristoderes retreated to temperate rivers and lakes, and by the Oligocene the fossil record of neochoristoderes disappears.

For a long time, it seemed that the story of choristoderes ended there—but the fossil record had one more surprise.

In 1992, a strange fossil was discovered in a quarry in the Czech Republic. The specimen belonged to a choristodere, yet its anatomy appeared unusually primitive compared with the advanced neochoristoderes of the Eocene. It lacked the elongated snout and robust limbs typical of crocodile-like choristoderes.

Even more surprising was its age: Late Oligocene, nearly ten million years after the last known neochoristoderes had disappeared.

Subsequent discoveries of similar fossils in the Czech Republic and France pushed the extinction date of choristoderes even further forward, into the Miocene. Because this animal seemed to reappear in the fossil record long after its relatives had vanished, paleontologists gave it a fitting name: Lazarussuchus, the “Lazarus crocodile,” referencing the biblical story of resurrection.

Fig. 13 Modern reconstruction of Lazarussuchus.（圖片來源：SeismicShrimp，採用 CC BY 4.0 授權）

A Remarkable Fossil Record

Beyond their mysterious evolutionary history, choristoderes are notable for another reason: the exceptional fossil record of the long-necked species Hyphalosaurus lingyuanensis.

Fossils of Hyphalosaurus lingyuanensis come primarily from the Yixian Formation of the Jehol Group in northeastern China, a region famous for exquisitely preserved fossils such as Sinosauropteryx and Beipiaosaurus. Thousands of Hyphalosaurus specimens have been recovered from this formation.

Among them, two specimens stand out as particularly extraordinary: one fossil preserving a two-headed embryo, and another specimen preserving soft tissues, sometimes referred to as a “mummified” specimen.

Two-headed developmental abnormalities are extremely rare, and fossilization of embryos or soft tissues is rarer still. The presence of both in the same fossil deposit is therefore remarkable.

The exceptional preservation of the Yixian Formation likely results from its volcanic environment. When eruptions occurred, layers of volcanic ash settled at the bottom of nearby lakes, forming sealed deposits that protected organisms from scavengers and microbial decay. These conditions allowed even delicate structures—such as soft tissues and original body postures—to be preserved.

Fig. 14 Numerous hyphalosaur fossils from the Jehol Biota, housed in the Beijing Museum of Natural History.（圖片來源：Tiouraren，採用 CC BY-SA 4.0 授權）

Since Hyphalosaurus lived in and around lakes and was extremely abundant in this ecosystem, these exceptional circumstances produced some of the most remarkable fossils known for freshwater reptiles.

The story of choristoderes ends here—for now. Their evolutionary history remains enigmatic, and many questions about their origins and extinction remain unanswered. Yet with each new fossil discovery, paleontologists move one step closer to unraveling the mysteries of these strange reptiles.

Author: Rodrigo

Reference:

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