Two unusually bulky fossil rostra from Wyoming belong to a previously unknown belemnite lineage that survived into the Oxfordian, millions of years after most of its family had vanished.
The fossils came from the upper part of Wyoming’s Sundance Formation, where belemnite remains are common but their diversity has remained poorly understood. One specimen was found loose on a slope near Thermopolis, while the other was discovered in place near Hyattville in a sandstone layer containing smaller belemnites and shells. Both came from the upper Redwater Shale, dated to the late early to early middle Oxfordian of the Late Jurassic.
What immediately set the two specimens apart was their unusual bulk. Their rostra, the hard mineralized structures at the rear of belemnites, reach roughly 60 to 70 millimeters in diameter at the front. The researchers describe them as the thickest belemnite rostra known so far.
The fossils have now been named Wyoteuthis linsterorum, a new genus and species. More importantly, their detailed anatomy places them within the Megateuthididae, a belemnite family whose main diversity was concentrated much earlier in the Jurassic.
That makes the fossils unusual not simply because they were chunky, but because they appear to represent a surviving branch of a lineage thought to have largely disappeared millions of years earlier.
Two fossils preserved an unexpected survivor
Belemnites were extinct marine cephalopods related to modern squids and cuttlefish. Much of their fossil record comes from their robust, mineralized rostra, which could survive long after the soft body had disappeared.
In Wyoming’s Sundance Sea, belemnite rostra are found repeatedly through the upper part of the formation, sometimes in dense accumulations. Earlier work had assigned the region’s fossils to several different names, but the authors note that the taxonomy of these belemnites is still in need of modern revision.
The two specimens described in the new study are different. The holotype, discovered by Cliff Linster, consists of a complete rostrum assembled from two pieces. Its external surface is exceptionally well preserved. The second specimen, the paratype, was assembled from eight fragments and had enough internal preservation to allow higher-resolution micro-CT scanning.
Those scans allowed the researchers to examine structures that cannot be reliably seen from the outside, including the alveolus, the cavity extending into the rostrum where the phragmocone was housed. The two specimens were scanned with CT, while the better-preserved paratype also received micro-CT scanning for greater detail.
The result was a combination of features that did not fit the better-known belemnites from the same deposits.
The “chunky” rostrum was unlike the local norm
The new belemnite had a robust, roughly conical rostrum with a very short solid portion behind the alveolus. Its preserved length exceeds 130 millimeters, while the diameter near the anterior end is about 60 to 70 millimeters.
The cross-section was broadly oval, becoming more rounded toward the rear. The sides also showed conspicuous flattening. At the back, the apex was displaced toward the underside and ended in a small point, or mucro.
The rear surface carried another important clue. Both specimens have apical striation, including paired groups of grooves or depressions toward the upper sides and incised striae on the underside. In the better-preserved holotype, some of these striae branch as they extend away from the apex.
Inside the rostrum, the alveolus extends through most of its length. Its cross-section is also compressed, and its tip is displaced toward the underside. The measured alveolar angle is about 29 to 30 degrees.
Those details mattered because outward shape alone could have pointed the researchers toward several unrelated groups.
Anatomy tied the fossils to a much older family
The authors considered several possibilities, including the Cylindroteuthididae, which were abundant in the Sundance Sea and include belemnites with shortened rostra.
But the internal and external anatomy pointed elsewhere.
The researchers compared the Wyoming fossils with members of the Passaloteuthididae and Megateuthididae, among other groups. Their measured alveolar angle is typical of short members of the latter two groups but generally larger than that of short cylindroteuthidids. The Wyoming fossils also have a particular pattern of apical striation and a thickened ventral wall of the alveolus that the authors regard as characteristic of the evolutionary stock containing the megateuthidids.
The apical structures provided another line of evidence. The researchers examined the fine striation of numerous comparative specimens, including the Bajocian megateuthidid Brevibelus. In some Brevibelus specimens that would normally be described as lacking apical grooves, careful examination revealed striation and shallow scores in positions that resemble those of Wyoteuthis.
Taken together, these features led the authors to interpret Wyoteuthis as a late offshoot of the passaloteuthidid-megateuthidid evolutionary stock.
The closest proposed ancestor is Brevibelus, particularly the species Brevibelus gingensis. That species lived in the Aalenian to early Bajocian, and it resembles Wyoteuthis in several important features, including its shape, pointed apex and somewhat thickened ventral wall of the alveolus. A specimen of B. gingensis has also been reported from the lower Bajocian of Alaska, making a North American ancestral connection plausible. The authors describe this as a hypothetical ancestral lineage, not a demonstrated direct ancestry.
It survived after most megateuthidids had disappeared
The timing is what makes the discovery especially striking.
Megateuthidids flourished mainly from the Toarcian through the Middle Jurassic. Most lineages were thought to have disappeared during the Bathonian. Wyoteuthis, by contrast, lived in the Oxfordian, making it a much younger and geographically isolated representative of the group.
The authors identify another unusually late megateuthidid, Chuvashiteuthis, from the late Kimmeridgian of central Russia. They argue that the two fossils represent different surviving branches that crossed the Bathonian-Callovian boundary in separate parts of the world. Chuvashiteuthis appears related to the Megateuthis–Paramegateuthis group, whereas Wyoteuthis is more plausibly associated with Brevibelus.
The Wyoming discovery therefore does not simply extend the known range of one familiar species. It adds evidence for a previously unrecognized refugium of megateuthidids in the northeastern Pacific region.
At the same time, the authors stress that the family’s evolutionary history remains unsettled. Megateuthididae has no single clear anatomical feature that definitively separates it from the older Passaloteuthididae, and the group may itself be polyphyletic. The study therefore follows an existing taxonomic framework while acknowledging that a broader revision could change relationships among several of these genera.
The biggest-looking part may not mean a giant animal
The fossils also raise a more familiar question about extinct animals: how big was the animal that carried such a large structure?
At first glance, the 6-to-7-centimeter width of the rostrum makes Wyoteuthis look like a giant belemnite. But the authors caution against using rostrum thickness as a direct measure of body size.
For comparison, the Bajocian Megateuthis can have a rostrum as long as about 80 centimeters, yet its typical diameter at that size is only around 4 to 5 centimeters, although its anterior trumpet can sometimes reach about 10 centimeters wide. Wyoteuthis instead combined an unusually thick rostrum with a very short overall form.
The researchers considered existing equations for reconstructing megateuthidid body size, but those formulas were developed without taxa possessing short, stout rostra like Wyoteuthis. A key measurement needed for the calculation, the maximum diameter of its phragmocone, is also unknown.
Using available proportions only as a rough guide, the authors estimate that the complete rostrum may have been about 160 to 175 millimeters long. Applying proportions from other megateuthidids would give an animal roughly 55 to 60 centimeters long including the arms, with a mantle about 30 to 35 centimeters long. But comparisons with exceptionally preserved Toarcian belemnites lead them to a more conservative suggestion that the animal probably did not exceed about 35 to 45 centimeters in length excluding its arms.
The researchers emphasize that a reliable reconstruction is not possible until the full phragmocone can be established.
That uncertainty matters because the rostrum was not simply a rigid body-length marker. The authors discuss evidence that its carbonate mass may have helped counterbalance the soft body, whose rear portion was affected by the buoyant, gas-filled phragmocone. Because Wyoteuthis had such a short solid posterior rostrum, its total carbonate volume could have been comparable to that of the much more elongated rostrum of the co-occurring cylindroteuthidid Pachyteuthis densa.
The authors therefore regard Wyoteuthis as a good example of why a particularly large belemnite rostrum does not necessarily mean a particularly large animal.
Its rarity may reflect a narrow way of life
Only two specimens of Wyoteuthis are currently known.
That is notable because the same Redwater Shale contains numerous cylindroteuthidid belemnites, in some places occurring so abundantly that the fossils can cover the exposed rock surface. The scarcity of adult Wyoteuthis therefore appears not simply to reflect a generally poor fossil record for belemnites in the formation.
The authors propose several possible explanations, while emphasizing that they cannot distinguish among them. The animal may have been specialized for particular prey or a particular environment. Alternatively, its main habitat may have been along the oceanic shelf, with only occasional animals entering the gulf-like Sundance Sea.
The stocky shape itself does not provide a simple ecological answer. Similar short, robust rostra evolved at different times in several unrelated or distantly related belemnite groups. The authors note that comparable forms occurred in the Pliensbachian Coeloteuthis, the Toarcian-Bajocian Brevibelus and the younger cylindroteuthidid Simobelus. They suggest that these recurring shapes may reflect local ecological adaptations, but say that this interpretation needs to be tested across different families.
The two Wyoming fossils thus preserve an unusual combination: an exceptionally thick rostrum, a comparatively short body form, and a lineage that appears to have survived in isolation after most of its relatives had disappeared.
The study was published in Papers in Palaeontology.
