Leptostomia

Not to be confused with Leptostomias.

Leptostomia is a genus of long-beaked pterosaur from the mid-Cretaceous (Cenomanian) of Morocco, North Africa. The type species is L. begaaensis, which was named and described in 2021 from sediments of the Kem Kem Group in Morocco.^[1] It was a small animal with a long, slender bill which is thought to have been used to probe sediments for worms and other invertebrates, similar to kiwi birds and curlews. Leptostomia is likely a member of the Azhdarchoidea.

Leptostomia Temporal range: Cenomanian PreꞒ Ꞓ O S D C P T J K Pg N
Life restoration
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	†Pterosauria
Suborder:	†Pterodactyloidea
Clade:	†Ornithocheiroidea
Clade:	†Azhdarchoidea
Genus:	†Leptostomia Smith et al., 2021
Type species
†Leptostomia begaaensis Smith et al., 2021

Discovery and naming

For decades, the fossiliferous outcrops of the Kem Kem Group in southern Morocco have produced pterosaur fossils. These rock layers belong to the upper Ifezouane Formation, dating to the Early Cretaceous period. The specific age however is uncertain, but it is believed to date back to the Cenomanian or perhaps Albian stages of the Cretaceous, about 112-94 million years ago. Leptostomia is known from two specimens, both of which were unearthed in the region and deposited at the Faculté des Sciences Aïn Chock; the holotype, an incomplete rostrum (FSAC-KK 5075), and the paratype, an incomplete dentary (FSAC-KK 5076). The preserved portion of the lower jaws may have lain in front of the holotype rostrum according to the authors' interpretation, with some overlap. The specimens of Leptostomia were examined by means of a CT scan.^[1]

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, named Leptostomia begaaensis. The generic name, Leptostomia, is a combination of the Ancient Greek words leptos (meaning "slim") and stoma (meaning "mouth"). The specific name begaaensis refers to the oasis village Hassi El Begaa where the holotype was found.^[1]

Description

Leptostomia was a small, long-beaked pterosaur with adaptations for sediment probing. The holotype consists of two beak fragments. The fragments of Leptostomia were mostly flat, and had thick walls, unusual for a pterosaur. While many pterosaur beaks bear dorsal or ventral crests, Leptostomia lacks either. This gives it a straight, featureless beak. The holotype of Leptostomia measured 48 millimeters (1.9 in) in length, giving an estimated a total skull length measuring between 6 and 20 centimeters (2.4 and 7.9 in).^[1]

The rostrum of Leptostomia is extremely thin, only 2.2 millimeters (0.087 in) deep at the front. The jaws are toothless, slightly upcurved, and taper anteriorly (frontly) toward the snout tip. The upper jaw had a narrow ridge, similar to that seen in many other pterosaurs, which would have occluded with a groove in the lower jaw. The median ridge on the occlusal (inside) surface of the rostrum extends across the length of the rostrum, however 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. As for the mandible, its ventral (bottom) margin is gently rounded; this is in contrast to the U-shaped or sharp margins present in Alanqa and Xericeps. Leptostomia is distinguished by other pterosaurs through a variety of traits, known as diagnostic traits. The lateral (side) and dorsal (top) rostral angles are less than or equal to 3 degress, a number much lower than that of pterosaurs like Alanqa. 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. Notably, the beak of Leptostomia has thick cortices (bone walls) and a small central cavity (hollow space in the middle of the mandible). These cortices measure 2 millimetres (0.079 in) thick and surround the central cavity.^[1]

Classification

Smith et al. suggest that Leptostomia was a pterodactyloid, and likely a member of the clade Azhdarchoidea, but its affinities remain unclear because of the fragmentary nature of the fossils. Because no pterosaurs closely resembling Leptostomia have been described before, it is unclear whether it belongs to a known family such as Azhdarchidae or Chaoyangopteridae, or represents a new azhdarchoid clade.^[1]

Subsequent phylogenetic analyses have recovered Leptostomia as a close relative of Xericeps, within either Thalassodromidae^[2] or Alanqidae.^[3]

Paleobiology

Restoration of Leptostomia probing

The beak of Leptostomia is unlike that of any previously described pterosaurs. Instead, it closely resembles birds that feed by probing in sediment, such as kiwis, ibises, hoopoes and snipes. These birds typically feed on invertebrates such as earthworms (in soil) or polychaete worms, fiddler crabs, or bivalves (in estuarine or marine intertidal settings). Leptostomia's prey could have been sought by a sensitive system of nerve endings capable of detecting movement at a distance.^[1]

Paleoecology

During the Early to Middle Cretaceous, North Africa bordered the Tethys Sea, which transformed the region into a mangrove-dominated coastal environment filled with tidal flats and waterways.^[4][5] The Kem Kem Beds are a sequence of fluvial and lacustrine sediments, though it has some marine sediments. Isotopes from fossils of the dinosaurs Carcharodontosaurus and Spinosaurus suggest that the Kem Kem Beds witnessed a temporary monsoon season rather than constant rainfall, similar to modern conditions present in sub-tropical and tropical environments in Southeast Asia and Sub-Saharan Africa.^[6][7] The Izefouane Formation of the Kem Kem Beds where Leptostomia is known has been interpreted as a braided river system, similar to that found in the Bahariya Formation.^[8][9] This river system was freshwater based on the presence of lungfishes and other freshwater vertebrates. This indicates that the Izefouane Formation had a wide variety of niches, including rivers channels, river banks, sandbars, and more.^[10][1][11] These riverine deposits bore large fishes, including the sawskate Onchopristis, coelacanth Mawsonia, and bichir Bawitius.^[12] This led to an abundance of piscivorous crocodyliformes evolving in response, such as the giant stomatosuchid Stomatosuchus in Egypt and the genera Elosuchus, Laganosuchus, and Aegisuchus from Morocco.^[13][11][14]

Pterosaurs known from the Kem Kem Beds include the azhdarchid Alanqa, the tapejarid Afrotapejara, the possible chaoyangopterid Apatorhamphus, the possible azhdarchoid Xericeps, and the ornithocheirids Anhanguera, Coloborhynchus, Ornithocheirus, and Siroccopteryx. However, all of these pterosaurs are known from fragmentary and/or isolated remains, making their classification difficult to confirm. Many fossils have been found without overlap, such as vertebrae, notaria, or limb bones, that may belong to these taxa.^[10] Dinosaurs are also known from the Kem Kem Beds, including the sauropod Rebbachisaurus, the theropods Deltadromeus, Carcharodontosaurus, and Spinosaurus, and unnamed taxa including ankylosaurs, titanosaurs, and noasaurid theropods.^[15][11]