The foetal remains consist of one slightly disarticulated individual located in the area corresponding to the right uterine cornu, in the anterior portion of the maternal trunk. Much of the cranium, a large part of the postcranial axial skeleton and elements of the appendicular skeleton are preserved. The cranial material lies far forward, directly behind the maternal pectoral girdle, while the foetal pelvic girdle, hindlimb and haemal arches are grouped further back. Preflexural vertebrae are scattered between the maternal ribs and gastralia, but a clear anterior-to-posterior gradient remains visible: caudal vertebrae are the only vertebral type clustered more posteriorly. The absence of obvious mirrored elements from the left side, combined with this gradient, led the researchers to the most parsimonious conclusion that all the material belongs to a single foetus rather than several.

The description, published in the journal Fossil Record, is technically rich because the foetus preserves bones of every developmental origin. Dermal skull elements — including the lacrimal, nasal, jugal, parietal, squamosal, supratemporal, postfrontal, quadratojugal, pterygoid and palatine — are all well ossified, with surface striations revealing the position of their ossification centres and the direction of growth. Endochondral elements such as the stapes, exoccipital and opisthotic show a rougher, ridged texture indicating cancellous bone, confirming a well-documented pattern among diapsid reptiles in which the dermatocranium ossifies ahead of the chondrocranium. The quadrate is a notable exception, matching the dermal elements in maturity. Lower-jaw bones — the dentary, angular and surangular — are also well developed, with the preglenoid process already prominent while the paracoronoid process remains small or absent.

Staging a fossil foetus is notoriously difficult, since the material is a snapshot of prenatal development. To gauge the Besanosaurus baby’s maturity, the team applied a recently published framework developed for the Jurassic ichthyosaur Stenopterygius quadriscissus, which defines relative developmental stages on the basis of cranial ossification. The overall well-ossified cranium of the Besanosaurus foetus, together with the fact that the parietal and pterygoid do not lag behind other dermal elements — a delay seen throughout Stenopterygius development — indicates a stage comparable to stage 4, the latest prenatal phase in Stenopterygius. The researchers also measured the width of the notochordal canal relative to the centrum in seven vertebrae, calculating a mean Notochordal Canal Index of 0.09. That value is significantly lower than in perinatal Stenopterygius, echoing a pattern previously seen in the large-bodied Cretaceous ichthyosaur Platypterygius australis, and suggesting that terminal foetal body size influences the index. Taken together, the evidence places the Besanosaurus foetus in the final stages of prenatal osteological development, immediately prior to birth.

The orientation of the baby is equally telling. The cranium lies far forward in the maternal body cavity, at roughly 20 percent of the maternal coelomic length measured between the pectoral and pelvic girdles, implying an anterior carrying position and a future caudal — tail-first — presentation at birth. Tail-first delivery is the preferred, though not exclusive, mode of parturition in Merriamosauria, the group that includes Besanosaurus and all later ichthyosaurs. The far anterior placement suggests the foetus had not yet begun the posterior migration that precedes birth, although the team acknowledges a taphonomic caveat: the mother is preserved with her ventral side facing away from the sediment, which would have allowed more time for the foetus to shift before burial. Comparisons with pregnant Stenopterygius specimens show that foetal position is best preserved in mothers that came to rest laterally on the seafloor.

Perhaps the most peculiar feature of the foetus is the presence of teeth on the palatine, a bone in the roof of the mouth. The element bears a tooth row with two teeth on its ventral surface, separated by a bulge and followed by a gap that may represent an empty alveolus or incomplete ossification. Palatal teeth are largely absent in ichthyosaurs, having been reported previously only in early-diverging ichthyosauriforms such as Chaohusaurus brevifemoralis from the Lower Triassic of South China, on the pterygoid of a primitive ichthyopterygian from Japan, and in Wimanius odontopalatus, a poorly known small-bodied ichthyosaur that co-occurs with Besanosaurus at Monte San Giorgio. The single-row arrangement in the foetus matches that seen in Wimanius and Chaohusaurus, and the team considers the presence of palatal dentition in the foetus very likely.

This discovery raises two provocative possibilities. First, the palatal teeth may represent a developmental atavism — a feature inherited from early ichthyosauriform ancestors that was lost in postnatal stages of Besanosaurus and completely eliminated around the origin of Parvipelvia, the derived group to which Stenopterygius belongs, whose foetuses clearly lack palatal dentition. Second, and more controversially, the presence of palatine teeth in the foetus raises the possibility that Wimanius odontopalatus, known only from a single partial cranium exposed in ventral view, is a junior synonym of Besanosaurus. The Wimanius holotype, with a mandible length of just over 20 centimetres, falls neatly between the estimated foetal mandible length of 10 to 12 centimetres and the smallest recognised postnatal Besanosaurus specimen at 41 centimetres. Previous authors have already questioned the validity of Wimanius, noting morphological similarities between the two genera, and the team argues that a detailed revision of the Wimanius holotype is now warranted.

The new description also settles a more recent debate about the foetal material’s identity. After the discovery of a megapredatory Guizhouichthyosaurus with thalattosaurian remains in its abdominal region, it was proposed that Besanosaurus might likewise have been a megapredator and that the small vertebrae in its body cavity could represent mixosaurid stomach contents. The team rejects this interpretation on four grounds: the remains are associated and arranged in an anteroposterior sequence; the bones are in pristine condition with no signs of digestive alteration; the ossification differences between elements match patterns seen in other ichthyosaur foetuses; and the vertebral centra retain a small notochordal pit, a feature inconsistent with ingested mixosaurid material. The foetus is clearly attributable to Besanosaurus based on character states unique to the genus among Besano Formation ichthyosaurs, including a triangular process on the quadrate, a large preglenoid process on the surangular and a quadrangular proximal femur. Its slender rostrum and small, narrow teeth — and stomach contents consisting of coleoid hooklets in a referred specimen — further argue against a megapredatory ecology.

Beyond the single foetus, the study offers a broader survey of ichthyosaur reproduction. Litter size in the Besanosaurus holotype is one, which is not uncommon within Ichthyosauria and is the most frequent litter size in the best-studied genus Stenopterygius. Across the three genera with multiple known pregnant females — Stenopterygius, Ichthyosaurus and Mixosaurus — between two and four foetuses appears to be the norm, with six or more occurring only exceptionally. In females carrying multiple foetuses, the young lie in partial anterior-posterior sequence within both uterine cornua but also stack substantially dorsoventrally, overlapping and underlapping their siblings. The team hypothesises that after foetuses transition from a coiled to a straightened posture early in gestation, they lose relative positional freedom, causing them to stack while continuing to develop along the body axis. This pattern suggests the ichthyosaur uterus was not tightly compartmentalised around each foetus, as in modern viviparous squamates and mosasauroids, but was instead more tube-like and flexible, with both uterine cornua well developed.

The anterior carrying position of the foetus also lends support to the so-called trim control hypothesis, which proposes that ichthyosaur mothers carry their foetuses with the heaviest portion — the head — as close as possible to the maternal centre of buoyancy near the lungs, reducing energy expenditure during pregnancy. The observation does not exclude the alternative mechanical hypothesis, which holds that anterior cranial placement is advantageous during the peristaltic contractions of birth, and both factors may have contributed to the tail-first preference in Euichthyosauria. For a lineage often described as superficially convergent with cetaceans, the Besanosaurus foetus adds a rare developmental datapoint from a stage of ichthyosaur evolution from which foetuses remain poorly known, illuminating how the first giant marine reptiles managed pregnancy in the open ocean some 240 million years ago.

Subject of Research: A foetal specimen of the Middle Triassic ichthyosaur Besanosaurus leptorhynchus from Monte San Giorgio, Italy

Article Title: An in utero foetus of the Middle Triassic ichthyosaur Besanosaurus leptorhynchus from Monte San Giorgio, Italy

Article References: Miedema, F., Bindellini, G., Wolniewicz, A. S., Maxwell, E. E., & Dal Sasso, C. (2026). An in utero foetus of the Middle Triassic ichthyosaur Besanosaurus leptorhynchus from Monte San Giorgio, Italy. Fossil Record, 29(1), 299-315. https://doi.org/10.3897/fr.29.183128

Image Credits: AI Generated

DOI: 10.3897/fr.29.183128

Keywords: Besanosaurus leptorhynchus, ichthyosaur, foetus, viviparity, Monte San Giorgio, Middle Triassic, palatal teeth, tail-first birth, Wimanius, Fossil Record, prenatal development, marine reptiles