Leptostomia

Leptostomia Temporal range: Late Cretaceous (Cenomanian), 100–94 Ma 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
Superfamily:	†Ornithocheiroidea
Clade:	†Azhdarchoidea
Genus:	†Leptostomia Smith et al., 2021
Type species
†Leptostomia begaaensis Smith et al., 2021

Leptostomia (meaning "slim mouth") is a genus of pterosaur, a group of extinct flying reptiles, that lived during the Cenomanian or Albian stages of the Late Cretaceous period in what is now Morocco, North Africa. Leptostomia is known from two isolated fossils: the holotype (name-bearing) specimen, a rostrum (front of upper beak) fragment, and the paratype, a dentary (lower beak) fragment. These fossils were described by paleontologist Roy E. Smith and colleagues in 2021, who named the type and only known species, L. begaaensis, in reference to the Hassi El Begaa village where the holotype was found.

Leptostomia is a small pterosaur, with the holotype rostrum fragment measuring just 48 millimeters (1.9 in) in length. This indicates a complete skull length of 6 and 20 centimeters (2.4 and 7.9 in), making it much smaller than many contemporary pterosaurs. The beak anatomy of Leptostomia is unforeseen in any other known pterosaur as it is remarkably long, narrow, and dorso-ventrally (top-down) compressed. Additionally, the interior of the beak has thick cortical bone with a unique cross-section. Due to a lack of remains, much of its anatomy is unknown and can only be inferred from related taxa.

The classification of Leptostomia is extremely uncertain; it is unclear whether it is an azhdarchid, chaoyangopterid, or member of its own clade in Azhdarchoidea, a group of edentulous (toothless) pterodactyloid pterosaurs. Phylogenetic analyses have recovered Leptostomia as a member of Alanqidae, a proposed clade of azhdarchoids, or Thalassodromidae, a family of large-skulled, crested azhdarchoids, however these results have come under scrutiny. Leptostomia has been proposed to have a similar lifestyle to probe sensing birds like sandpipers and sanderlings due to its beak anatomy and paleoenvironment.

Leptostomia was found in the strata of the Ifezouane Formation, a formation in the Kem Kem Beds. This environment was made up of river systems, tidal flats, and mangroves, indicating it had 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 preserves a variety of other fossils including several other genera of pterosaurs, such as Afrotapejara, Coloborhynchus, Siroccopteryx, and many others. In addition to pterosaurs, the Kem Kem Beds preserves fossils of many dinosaurs, including sauropods and theropods, crocodylomorphs, mammals, lizards, turtles, fish, and more.

Fossils of Leptostomia were first unearthed in an outcrop near the town of Hassi El Begaa in Errachidia Province, southern Morocco. For decades, southern Morocco has produced pterosaur fossils in large numbers. The remains found are from two specimens, both of which come from the same site and were deposited at the Faculté des Sciences Aïn Chock: the holotype, an incomplete rostrum (upper beak) (FSAC-KK 5075), and the paratype, an incomplete dentary (lower beak) (FSAC-KK 5076). The preserved portion of the dentary may have lain in front of the holotype rostrum, with some overlap. 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 or perhaps Albian stages of the Cretaceous, about 112-94 million years ago.^[1][2]

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]

Leptostomia was a small, long-beaked pterosaur with adaptations for sediment probing. The beak fragments of Leptostomia are mostly flat with 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, indicating a skull length between 6 and 20 centimeters (2.4 and 7.9 in) long.^[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 (toward the beak 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 the other pterosaurs 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 jaw). These cortices measure 2 millimetres (0.079 in) thick and surround the central cavity.^[1][2]

Due to its fragmentary nature and unique anatomy, the affinities of Leptostomia are unclear; Smith and colleagues (2020) suggested that the genus was a azhdarchoid pterodactyloid.^[1] As for its classification within the family, it is unclear whether it is an azhdarchid, chaoyangopterid, or a member of a its own family in Azhdarchoidea. Azhdarchoidea is a clade of pterosaurs that existed during the Cretaceous period with a global distribution. The clade is very diverse, including the families Tapejaridae, Azhdarchidae, Chaoyangopteridae, Thalassodromidae,^[3][4][5] and possibly Dsungaripteridae.^[6] However, some studies have proposed the existence of another family, Alanqidae.^[3][7] Azhdarchoids were edentulous and had wing proportions which were adapted to a more terrestrial life, in contrast to their relatives the pteranodontians.^[8]

Subsequent phylogenetic analyses have recovered Leptostomia as a close relative of Xericeps, within either Thalassodromidae^[9] or Alanqidae.^[7] The first of the below cladograms shows the results recovered by Pêgas (2024),^[7] and the second shows the results recovered by Andres (2021).^[9] However, Smith and colleagues (2023) were skeptical of both of the results of these phylogenetic analyses due to the fragmentary nature of Leptostomia's material.^[2]

In their description, Smith and colleagues wrote extensively about the possible paleobiology of Leptostomia. The beak of Leptostomia is unlike that of any previously described pterosaurs. Instead, it closely resembles long-billed 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). Additionally, these birds are specialized to hunt at different degrees of sediment probing.^[1]

Smith and colleagues hypothesized that Leptostomia was a probe feeder, like dunlin and sandpipers. Quetzalcoatlus,^[10] Pterodactylus,^[11] Purbeckopus,^[12] and a Pteraichnus-like trackmaker^[13] have also been suggested to be probe feeders, however Leptostomia is the only known pterosaur to have a beak morphology consistent with a probe feeding lifestyle. Leptostomia's beak displays this, as it is long, narrow, dorso-ventrally compressed, and has a cross-section similar to that of the sanderling, a remote sensing probing shorebird. Leptostomia also bears thick bone walls in its beak, an adaptation for stress-resistance during probing. Although not preserved in any known specimens, the occurrence of foramina in a beak tip would further suggest a probing lifestyle. The environment of the Kem Kem Beds where Leptostomia is known from is further evidence. The Kem Kem Beds are a river system that preserves a diverse freshwater fauna that includes fish, sharks, and semi-aquatic spinosaurids. Additionally, crabs, which are preserved in the Beds, molluscs, fiddler crabs, and worms could have provided a food source.^[1]

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.^[14][15][16] 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.^[17][18] 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.^[19][20] 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.^[2][1][15] These riverine deposits bore large fishes, including the sawskate Onchopristis, coelacanth Mawsonia, and bichir Bawitius.^[21] 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.^[22][15][23]

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.^[2] 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.^[24][15]

• The Kiwi Pterosaur - by Nick Longrich