Seldsienean megistorhynchus is a long-snouted teleosauroid from Bathonian rocks of France and England, roughly 168–165 million years old. It was first named in the nineteenth century as Steneosaurus megistorhynchus. A 2020 revision transferred it to a new genus and designated a neotype because the original skull and lower jaw were presumed lost in the bombing of Caen in 1944. Its history is one strand in the ancient crocodylomorph catalogue.
The genus name is published and used in later research, but a proposal to change the type species of Steneosaurus has kept the broader nomenclature under discussion. That uncertainty concerns how names are applied; it does not erase the diagnostic fossils or their Bathonian age.
Quick facts
| Species | Seldsienean megistorhynchus |
|---|---|
| Age | Bathonian, Middle Jurassic; about 168–165 million years ago |
| Regions | Normandy, France; Oxfordshire, England |
| Original name | Steneosaurus megistorhynchus |
| Neotype | MMT P28-1, partial skull and mandible with associated elements |
| Original type | Partial skull and complete mandible, presumed destroyed in 1944 |
| Diagnostic anatomy | Long rostrum, lower-jaw symphysis, cranial ornament and osteoderms |
| Nomenclatural note | A proposal to change the type of Steneosaurus remains unresolved in the cited literature |
What can the fossils tell us?
Georges Cuvier mentioned the skull and lower jaw from Caen in 1824. Jacques-Amand Eudes-Deslongchamps described the material in detail in 1866–1867 as Steneosaurus megistorhynchus. In 2020, Johnson, Young and Brusatte established Seldsienean for the species after restricting the old, poorly diagnosed genus Steneosaurus. The species name records a taxonomic combination; the bones and their nineteenth-century history are the underlying evidence.
The name-bearing material was held in Caen and is presumed to have been destroyed in the 1944 bombing. To make the species objectively identifiable, the 2020 revision designated MMT P28-1 as a neotype. It includes a partial skull and mandible, isolated vertebrae and fragments, teeth and three osteoderms. The locality is not known, but the specimen is from Bathonian France, matching the original type’s country and age. The designation is a formal taxonomic act, not a claim that MMT P28-1 is the original fossil.
The rostrum is slender and narrows toward the front. The mandibular symphysis – the joined front section of the lower jaws – extends for more than half the jaw length, and each dentary carries more than 30 tooth sockets. Cranial ornament differs across the frontal, prefrontal and lacrimal bones; the palatal margins form a complex interlocking outline. Osteoderms bear irregular pits separated by raised bone and a small but distinct keel. The combination, rather than the silhouette alone, supports recognition of the species.
Two English dentaries have been referred to the species: OUMNH J.1414 is nearly complete, whereas LPP.T.1 is partial. The material comes from the Enslow Bridge area of Oxfordshire, from Great Oolite deposits near the Forest Marble–Cornbrash boundary. Those jaws let researchers compare the lower-jaw proportions with the neotype rather than relying only on fossils from Normandy. They do not turn the English and French pieces into one skeleton, and their association with the neotype is a taxonomic referral.
The 2020 analysis recovered Seldsienean among Machimosauridae. A later analysis placed it nearer Teleosauridae, as sister to Aeolodontini. These are phylogenetic hypotheses based on coded characters and sampled taxa, not competing observations of different fossils. The 2024 historical and systematic treatment also notes that an application was submitted to the International Commission on Zoological Nomenclature to change the type species of Steneosaurus. A 2023 published nomenclatural discussion states that the authors awaited the Commission’s decision. The safe account is that Seldsienean is the published 2020 genus name while the proposed change remains unresolved in the literature consulted here.
The long, narrow jaws and numerous teeth fit the general longirostrine teleosauroid feeding design. They are compatible with rapid capture of smaller aquatic prey, but no stomach contents identify a meal. The armour and vertebrae provide only a partial postcranial record, so artists use other teleosauroids to complete the body. The Bathonian deposits place the animal in a marine setting; they do not record how often it entered shallow water, how fast it swam or what it ate on a particular day.
The genus is uncommon in collections compared with some other Bathonian teleosauroids. Several occurrence records can refer to localities, historical identifications or fragments rather than separate articulated individuals. The neotype contains a useful mix of cranial and armour material, but the preserved skeleton remains incomplete. A thin fossil record makes it harder to assess size variation, growth, sex differences and the species’ full geographic range.
From a Caen fossil to a revised combination
Georges Cuvier mentioned the skull and lower jaw from Caen in 1824. Jacques-Amand Eudes-Deslongchamps later redescribed the material and named it Steneosaurus megistorhynchus. The species epithet refers to its large snout. At the time, Steneosaurus was used for a wide assortment of long-snouted marine crocodylomorphs. Subsequent work showed that this collection of species did not provide a reliable portrait of one natural genus.
Johnson, Young and Brusatte’s 2020 teleosauroid revision restricted Steneosaurus and erected Seldsienean for megistorhynchus. The proposed genus name comes from Old English words meaning “rare one”, a reference to the comparatively sparse material. It is a zoological genus name, not a geological stage or a formation.
Why MMT P28-1 became the neotype
The original type material was kept in Caen and is presumed to have been destroyed during the 1944 bombing. When a name-bearing type is lost and the taxon’s identity needs clarification, zoological nomenclature allows a neotype designation under specified conditions. The 2020 authors selected MMT P28-1, a specimen with a partial skull and mandible, isolated vertebrae and fragments, teeth, and three osteoderms.
Its precise locality is unknown. The authors documented that it comes from Bathonian deposits in France, matching the known age and country of the original fossil. The specimen also preserves the anatomy needed to distinguish the species. A neotype stabilises the name by giving later researchers a physical reference; it is not presented as the old holotype rediscovered.
Diagnostic features across the skull and armour
The most conspicuous feature is a long, narrow rostrum. In the lower jaw, the symphysis extends across more than half of the mandibular length and each dentary contains over 30 alveoli. The teeth themselves are not the only relevant evidence. Researchers compare the outline of the eye openings, the sculpturing on the frontal and surrounding bones, the shape of the palate and the detailed contacts between skull elements.
The external bone surface bears different patterns in different regions. The prefrontal and lacrimal carry small, separate rounded pits. The frontal’s ornament is concentrated centrally. On the osteoderms, large irregular pits are separated by raised areas and a low keel runs along the midline. These details form a character combination that can be tested against other Bathonian forms such as Teleosaurus and the more heavily built Machimosaurus. Similar overall snout proportions do not make those genera synonyms.
English material extends the comparison
OUMNH J.1414 is a nearly complete dentary from the Enslow Bridge area in Oxfordshire; LPP.T.1 preserves part of another lower jaw. They have been referred to Seldsienean megistorhynchus and provide a check on the proportions seen in the French neotype. The English fossils come from Great Oolite strata around the Forest Marble–Cornbrash boundary. Their value is comparative: they represent separate jaws, not additional pieces from MMT P28-1.
Keeping the specimens distinct makes the species diagnosis more transparent. Shared mandibular proportions support referral, while each individual fossil retains its own locality and geological context. The small amount of postcranial material also means that a full body outline still requires comparison with other teleosauroids.
A live discussion about Steneosaurus
The 2020 phylogenetic analysis placed Seldsienean within Machimosauridae. A later analysis placed it on the teleosaurid side of the tree, close to Aeolodontini. This movement can result from different character coding, new taxon sampling or revised relationships among long-snouted forms. It should not be mistaken for disagreement over whether the neotype is a real fossil.
A separate nomenclatural question concerns the type species of Steneosaurus. Young and colleagues submitted an application to the International Commission on Zoological Nomenclature proposing a change. The 2024 historical treatment describes the application, while the published 2023 nomenclatural discussion says its authors awaited a decision. The sources consulted for this profile do not establish that the Commission has issued one. Accordingly, Seldsienean megistorhynchus is used here as the valid published combination, with the proposed change clearly identified as unresolved.
What the marine fossils do and do not show
The Bathonian age and marine deposits place the fossils within a Jurassic sea. A long rostrum, many closely spaced tooth sockets and the jaw’s slender proportions are consistent with capturing aquatic prey. They do not establish a fish-only diet, bite force or a particular hunting sequence. No stomach contents, feeding trace or prey item has been directly associated with the species.
The known material is comparatively rare and incomplete. It cannot define a growth series, precise adult length, reproductive biology or the full soft-tissue body plan. “Rare” in the genus name describes its representation in the fossil record, not a measured population density in the Jurassic ocean. The most secure picture is a diagnostic longirostrine teleosauroid whose name is anchored by a replacement specimen and whose exact branch placement remains revisable.
Frequently asked questions
Is Seldsienean a valid animal genus?
Seldsienean was established in a 2020 taxonomic revision and is used here as the published genus name. A separate proposal about the type of Steneosaurus is described in later literature as unresolved.
When did Seldsienean live?
It lived during the Bathonian of the Middle Jurassic, about 168–165 million years ago.
Why does it have a neotype?
The original skull and jaw from Caen were presumed destroyed in 1944. MMT P28-1 was designated as a replacement name-bearing specimen.
Could its scientific name change?
A proposal to change the type species of Steneosaurus could affect how related names are applied. The literature consulted here reports that the proposal was awaiting an ICZN decision.