Paleontologists have described a new species of mosasaur, one of the marine reptiles that patrolled Cretaceous seas alongside dinosaurs on land, based on fossils recovered from Egypt’s Western Desert. Named Halisaurus auriculatus, the animal lived roughly 70 million years ago and stands out for an unusually modified ear paired with long, slender, tooth-lined jaws. The specimen adds a new entry to one of the rarest and least understood branches of the mosasaur family tree. Its discovery also helps explain how two closely related predators divided up the same hunting grounds without competing head-to-head for food.
A New Member of the Halisaurinae Subfamily
Halisaurus auriculatus belongs to Halisaurinae, a subfamily of generally small- to medium-sized mosasaurs. Researchers led by Ain Shams University paleontologist Ahmed Ali Shaker describe halisaurines as characterized by relatively small size and primitive jaw and tooth anatomy compared with other mosasaur lineages, and note the group has historically been one of the rarest and least documented among mosasaurs. The subfamily currently comprises eleven named species spread across the genera Eonatator, Halisaurus, Phosphorosaurus, and Pluridens, ranging stratigraphically from the Santonian age through the uppermost Maastrichtian, the very end of the Cretaceous period.
Halisaurine fossils are now known from North and South America, Africa, Asia, and Europe, with a particular concentration in the low-latitude seas that once surrounded Africa. Each new discovery in the group has helped researchers piece together how mosasaurs diversified and adapted to different ocean environments during the final tens of millions of years before the end-Cretaceous extinction.
Fossils Recovered From the Dakhla Shale
The remains of Halisaurus auriculatus were found in 2024 in the Dakhla Shale, a rock unit near the village of Teneida in the Dakhla Oasis in Egypt’s Western Desert. The fossil material includes a partial skull, jaw bones, and assorted vertebrae and limb bones, all preserved in three dimensions rather than crushed flat, which is unusual for marine reptile fossils of this age. The specimen was found alongside scattered remains of bony fishes, fragments of turtle shell, and plesiosaur vertebrae, painting a picture of a diverse marine community sharing the same seafloor.
Long Jaws Built for a Fast, Weak Bite
According to the study, Halisaurus auriculatus is distinguished by unusually long, slender jaws lined with more than 20 teeth on each side. That jaw shape points to an animal capable of snapping its mouth shut quickly but without generating especially strong bite force, a combination researchers associate with catching fast, slippery prey rather than crushing armored or heavily built animals.
An Ear Structure Unlike Any Known Mosasaur
The fossil also preserves a highly modified ear structure that researchers say is unlike anything previously documented in this family of reptiles. Combined with specialized features of the quadrate, the bone that anchors the lower jaw and plays a role in hearing in reptiles, the anatomy suggests Halisaurus auriculatus may have had heightened underwater hearing. Researchers say that adaptation could have helped the animal track small prey such as fish and squid in dim conditions, whether at night or in deeper, darker water where vision alone would have been less useful.
Sharing the Seafloor With Halisaurus hebae
Notably, Halisaurus auriculatus was recovered from the same rock layer and general area as another mosasaur, Halisaurus hebae, described by some of the same research team in 2023. The two species differ substantially in skull and jaw anatomy despite living in the same place at the same time, which led the study authors to propose that the animals divided up available food resources rather than competing directly for the same prey, a pattern known as niche partitioning. Shaker and colleagues wrote that Halisaurus auriculatus is set apart from the coeval Halisaurus hebae by its longer, more slender jaws, higher tooth count, and unique specialization of the quadrate, features suggesting a fast bite paired with low bite force and, potentially, ear adaptations tuned for underwater hearing.
What the Discovery Adds to Mosasaur Evolution
Researchers describe halisaurines as a diverse component of the Tethys and Atlantic ocean basins during the Maastrichtian age, underscoring their ecological importance within Late Cretaceous marine ecosystems even though the group remains poorly known compared with larger, more famous mosasaurs. The full description appears in the journal Acta Palaeontologica Polonica, and was first reported this month. By adding a twelfth species to a subfamily long considered under-sampled, the find gives paleontologists another data point for understanding how marine reptiles partitioned Cretaceous seas among multiple predators occupying similar body sizes and habitats.
During the Maastrichtian age, much of what is now Egypt’s arid Western Desert sat beneath a shallow arm of the Tethys Sea, the vast tropical ocean that once separated the northern and southern continents. Sediments such as the Dakhla Shale accumulated on that ancient seafloor, burying the remains of mosasaurs, plesiosaurs, turtles, and fish in fine-grained rock that has since proven well suited to preserving delicate three-dimensional bone. That geological history is why a region now defined by sand seas and oases continues to yield marine reptile fossils of this quality more than 70 million years after the animals died.
Researchers note that continued fieldwork in the Dakhla Oasis area is likely to turn up further halisaurine material, since Halisaurus auriculatus and Halisaurus hebae were recovered from overlapping rock layers within the same general locality. Additional specimens could help clarify how widely the ear and jaw adaptations seen in Halisaurus auriculatus were shared among other halisaurines living in the same shallow, low-latitude seas during the final years of the Cretaceous.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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