Supraorbital salt glands in Spinosaurinae provide new evidence for high-salinity adaptation in non-avian dinosaurs

Modern non-mammalian amniotes, such as reptiles and birds, have relatively limited kidney function. As a result, species living in high-salinity environments have evolved salt glands that remove excess salt from the body. These glands can occur in different parts of the head, including the nasal cavity, orbit, and oral cavity.

Sea turtles excrete excess salt through their salt glands, making them appear to be crying.（Image source：Matthias Scholz， CC BY 2.5 。）

In some birds and the marine iguana, the salt glands are located above the orbit. To accommodate these glands, distinct depressions are formed on the corresponding area of the skull. The degree of salt gland development is closely related to environmental factors such as salinity and temperature. Studies suggest that these salt glands evolved independently multiple times during bird evolution, enabling birds to adapt to marine and other high-salinity environments.

In contrast, salt glands are much more difficult to identify in extinct animals. Some researchers have previously proposed that certain non-avian dinosaurs possessed supraorbital salt glands, but the evidence has remained controversial. At present, reliable fossil evidence is mainly limited to more derived birds that are closely related to modern crown birds, whose skulls preserve traces similar to the salt gland impressions seen in living birds.

A study published in 2026 reported, for the first time, a complete set of skeletal features resembling the supraorbital salt glands of modern birds in the skulls of some spinosaurids (Spinosauridae). The study further investigated whether these salt glands were related to adaptation to high-salinity environments and how this supraorbital salt gland evolved.

After examining the skulls of several spinosaurid dinosaurs, the researchers found that archosaurs in general, as well as most non-avian dinosaurs, possessed shallow grooves on the upper lateral surface of the snout that may have housed salt glands. However, skulls of spinosaurids from Morocco exhibit a pair of distinct depressions on the frontal bones directly above the orbits. These depressions closely resemble those that accommodate supraorbital salt glands in some modern seabirds. In addition, the surrounding area contains large vascular foramina and grooves, indicating an extensive blood supply, which supports the interpretation that large supraorbital salt glands were present.

Spinosaurus（Image source：Connor Ashbridge， CC BY 4.0 。）

The study also found that these features are more common among members of Spinosaurinae, including Spinosaurus and Irritator, whereas they are absent in other spinosaurid taxa.

The researchers then reconstructed the evolution of habitat preference within Spinosaurinae. Their results suggest that the group evolved from ancestors that preferred freshwater environments toward descendants capable of inhabiting brackish and marine environments. By the time more derived members such as Irritator and Spinosaurus appeared, they may already have been adapted to high-salinity environments, which could explain the evolution of supraorbital salt glands in Spinosaurinae.

Because spinosaurids relied heavily on fish and other aquatic prey, and because fossils of more derived spinosaurines are more commonly found in brackish or marine deposits, this pattern resembles that seen in modern birds. Species living in environments with higher salinity are more likely to evolve well-developed supraorbital salt glands. The researchers therefore suggest that the relocation of the salt glands in Spinosaurinae may have been associated with adaptation to high-salinity environments.

Irritator（Image source：Fred Wierum， CC BY 3.0 。）

During spinosaur evolution, the external nostrils gradually shifted toward the rear of the skull while the premaxilla became proportionally larger, reducing the distance between the orbit and the nostril. The study proposes that these changes in skull morphology may have reduced the available space for the original nasal salt glands, ultimately leading to the relocation of the glands above the orbit.

Whether spinosaurids were highly adapted to an aquatic lifestyle has long been a subject of debate in paleontology. The authors argue that if Spinosaurinae indeed possessed supraorbital salt glands, this would provide new evidence for adaptation to high-salinity environments. However, well-developed salt glands indicate adaptation to saline conditions rather than a fully aquatic lifestyle. Therefore, this discovery alone does not demonstrate that spinosaurids were aquatic animals.

Nevertheless, spinosaurids may represent the first step toward a marine adaptation among non-avian dinosaurs. Although this transition may only have been in its early stages, it remains evolutionarily significant.

（Author: Bai Leng）

Reference：

Cau, A., Gostling, N. J., Lacerda, M. B. S., Falasca, M., Paterna, A. (2026). Avian-like salt glands in Spinosauridae. Historical Biology.