Leptostomia (lit. 'slim mouth') is an extinct genus of pterosaur that lived during the Cenomanian and possibly Albian stages of the ?Early-Late Cretaceous period in what is now Morocco, North Africa. Leptostomia is known from only two isolated rostrum (beak) fragments. In 2021, paleontologist Roy E. Smith and colleagues named the type and only known species, Leptostomia begaaensis, based on these fossils. Leptostomia is a small pterosaur, with the complete skull length estimated between 6 and 20 centimeters (2.4 and 7.9 in), making it much smaller than many contemporary pterosaurs. The beak of Leptostomia is remarkably long, narrow, and compressed from the top down, a morphology unseen in any other known pterosaur. Due to a lack of remains, the exact classification of Leptostomia is uncertain; it has been recovered in various positions within Azhdarchoidea, a group of edentulous (toothless) pterodactyloid pterosaurs. Leptostomia has been proposed to have a similar lifestyle to probe sensing birds like sandpipers and sanderlings due to its beak anatomy and paleoenvironment. The Leptostomia fossils were found in rock layers of the Ifezouane Formation, which is part of the Kem Kem beds. This environment was made up of river systems, tidal flats, and mangroves, which would have supported a diverse fauna of invertebrates like crabs and worms. If it was a probe feeder, Leptostomia likely would have preyed on these invertebrates. The Kem Kem beds preserve a variety of other fossils, including several other genera of pterosaurs, as well as fossils of dinosaurs, crocodylomorphs, mammals, lizards, turtles, fish, and more.
Discovery and naming
For decades, southern Morocco has produced pterosaur fossils in large numbers. Fossils of Leptostomia were first unearthed in an outcrop near the town of Hassi El Begaa in Errachidia Province, southern Morocco. The remains found are from two specimens, both of which come from the same site and were accessioned at the Faculté des Sciences Aïn Chock. The holotype (name-bearing) specimen, FSAC-KK 5075, is an incomplete rostrum (upper beak), and the paratype (supporting defining) specimen, FSAC-KK 5076, is an incomplete dentary (lower beak). The preserved portions of these elements are missing their anterior (toward the front) ends; an estimated 7 centimetres (2.8 in) is missing from the rostrum and 3.5 cm (1.4 in) from the dentary, assuming these bones tapered evenly, like in other azhdarchoids. These fossils were found at an outcrop corresponding to the upper horizon of the Ifezouane Formation of the Kem Kem Beds, which dates to the Cenomanian and perhaps Albian stages of the Cretaceous. In 2021, paleontologists Roy E. Smith, David Michael Martill, Alexander Kao, Samir Zouhri, and Nicholas R. Longrich described these remains as belonging to a new genus and species of pterosaur, which they named Leptostomia begaaensis. The generic name, Leptostomia, is a combination of the Ancient Greek words leptos, meaning "slender", and stoma, meaning "mouth". The specific name, begaaensis, refers to the oasis village Hassi El Begaa, near which both specimens were found.
Description
Leptostomia was a small, long-beaked pterosaur. The beak fragments of Leptostomia are dorsoventrally (from top to bottom) flattened with thick walls, as in other azhdarchoids. While many pterosaur beaks bear dorsal or ventral crests, Leptostomia lacks either in the preserved anterior section. This gives it a straight, featureless beak. The holotype upper jaw fragment of Leptostomia measures 48 millimeters (1.9 in) in length, indicating a skull length between 6 and 20 centimeters (2.4 and 7.9 in) long. The rostrum of Leptostomia is extremely thin, measuring only 2.2 millimeters (0.087 in) deep at the front. The jaws are toothless, slightly upcurved, and anteriorly (toward the beak tip) tapered. The upper jaw had a narrow midline ridge on the bottom surface, similar to that seen in many other pterosaurs, which would have occluded with a groove in the upper surface of the lower jaw. The ridge on the occlusal (inside) surface of the rostrum extends across the length of the rostrum. However, it is more pronounced towards the posterior (back) end. Many small, slit-like foraminae can be found on the outside and occlusal surfaces of the mandible, similar to those of other azhdarchoids. The ventral margin of the mandible is gently rounded in cross-section, in contrast to the U-shaped or sharp margins present in the other pterosaurs Alanqa and Xericeps. Leptostomia is distinguished from other pterosaurs by a combination of diagnostic traits. The cross-sections of the anterior rostrum and mandibular symphysis (the portion of the mandible where both halves meet) are semi-circular, in contrast to triangular or U-shaped cross-sections seen in contemporary pterosaurs. The lateral (side) and dorsal (top) rostral angles (angles of the beak's height) are less than or equal to 3 degrees, a number much lower than that of beaked pterosaurs like Quetzalcoatlus and Pteranodon. Distinctly, this number is closer to those of toothed pterosaurs rather than other edentulous pterosaurs; the only pterosaur with a lower rostral angle is the toothed Ctenochasma. The beak of Leptostomia has thick cortices (bone walls) and a small central cavity (hollow space in the middle of the jaw). These cortices measure 2 millimetres (0.079 in) thick and surround the central cavity.
Classification
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