Halisaurinae are an extinct subfamily of small- to medium-sized mosasaurids that diversified in the latest Cretaceous. Halisaurines were particularly abundant and diverse in the Tethys Sea and Atlantic Ocean surrounding North Africa. Here we report a new species of Halisaurus from the lower Maastrichtian of the Dakhla Shale from the Dakhla Oasis, in the Western Desert of Egypt. The new species is characterized by very long and slender maxillae and dentaries, a high tooth count, with 20+ maxillary and dentary teeth, a long and narrow frontal, and a highly modified quadrate in which the stapedial notch is expanded and the suprastapedial and infrastapedial processes form a thin ring of bone surrounding it. Phylogenetic analysis suggests that the new species is sister to a clade including the late Maastrichtian Halisaurus platyspondylus and Halisaurus arambourgi. The long, slender jaws and posteriorly placed coronoid process suggest a fast but relatively weak bite compared to other species of Halisaurus; along with the specialized ears they suggest a niche distinct from the contemporary Halisaurus hebae or other Halisaurinae. The existence of two morphologically distinct species inthe Dakhla Shale suggests that halisaurines engaged in niche partitioning.

Key words: Mosasauridae, Halisaurinae, Late Cretaceous, Maastrichtian, Tethys, Egypt

Ahmed Ali Shaker [ahmedshaker@sci.asu.edu.eg; a7medshaker48@yahoo.com; ORCID: https://orcid.org/0009-0003- 8654-7831 ], Amin Strougo [aminstrougo@gmail.com], and Anhar Asan [anhar_hasan@yahoo.com; ORCID: https://orcid.org/ 0000-0003-2710-3256], Department of Geology, Faculty of Science, Ain Shams University, 11566 Cairo, Egypt. Nicholas Roy Longrich [nrl22@bath.ac.uk; ORCID: https://orcid.org/0000-0002-9586-8907] (corresponding author), Department of Biology and Biochemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK. Mohamed Kamel Mousa [Mohamedkamel.sci@sci.nvu.edu.eg; ORCID: https://orcid.org/0000-0002-9951-1905] and Gebely Abdel-Maksoud Abu El-Kheir [gebely2006@sci.nvu.edu.eg; ORCID: https://orcid.org/0000-0001-5905-8790], New Valley Vertebrate Paleontology Centre, New Valley University, New Valley, Egypt.