Antarcticeras
Enigmatic Eocene cephalopod from Antarctica, disputed as orthocone or squid.
Antarcticeras is an extinct genus of cephalopod from the Eocene of Antarctica, known only from two fossil shell fragments. It has a single species, *A. nordenskjoeldi*, named after Swedish geologist and Antarctic explorer Otto Nordenskjöld. The genus is highly debated: it has been interpreted either as the last orthocone-type cephalopod, the sole member of the subclass Paracoleoidea and a descendant of orthoceratids that independently evolved coleoid-like features, or as an oegopsid squid representing a transitional stage in the evolution of the modern squid gladius.
The two fossil fragments come from the Early Eocene La Meseta Formation on Seymour Island, Antarctica. They are straight shells resembling those of orthocones, but are thought to have been internal, similar to the cuttlebone of cuttlefish or the shell of spirulids. The shells are weakly mineralized and contain traces of soft tissue, likely preserved only because of a unique depositional environment. Other cephalopods found in the same area include the coiled nautiloid *Euciphoceras*.
Antarcticeras was first described in 2017 by Doguzhaeva and colleagues, who placed it as the only member of a ghost lineage that had split from all other cephalopods during the Paleozoic. Its internal shell was seen as convergent with coleoids. The siphuncle placement resembled that of Actinocerida and Orthocerida, which were proposed as its ancestors, leading to the creation of a new subclass, Paracoleoidea, along with the order Antarcticerida and family Antarcticeratidae. This classification implied a previously unknown, ancient cephalopod lineage that survived multiple mass extinctions into the early Cenozoic in the far south, and that orthocerid descendants persisted into the Cenozoic. Later that year, *The Guardian* ranked it among the top fossil discoveries of 2017.
In 2018, Fuchs and colleagues published a study rejecting the new subclass. They argued that the poorly mineralized phragmocone was a transitional form between the mineralized shells of some coleoids and the fully demineralized gladius of oegopsid squid. They proposed Antarcticeras as the first fossilized squid shell, suggesting that modern squid gladii evolved much later than previously thought, supporting a Late Cretaceous origin for most decapodiform groups and a Cenozoic origin for modern squid.
Quick Facts
- Taxon
- Antarcticeras nordenskjoeldi
Facts from the source article.
Lore & Background
Antarcticeras is represented by two fossil shell fragments from the Early Eocene La Meseta Formation from Seymour Island, Antarctica. These are straight shells that closely resemble those of the orthocones, although they are thought to have been internal akin to those of certain coleoids such as the cuttlebone of cuttlefish and the 'rams horn' shell of spirulids. The shells are weakly mineralized with the presence of soft tissues and were likely only preserved due to a unique depositional environment. Sympatric cephalopod taxa include the coiled nautiloid Euciphoceras.
Antarcticeras was described in 2017 by Doguzhaeva et al., who recovered it as the only member of a ghost lineage of cephalopods that had diverged from all others during the Paleozoic. Its coleoid-esque internal shell was recovered as an instance of convergent evolution with coleoids. The coleoids are thought to have originated within the Bactritida during the Late Paleozoic and share similar anatomical features with one another, but Antarcticeras was found to differ from both groups, with its siphuncle placement more closely resembling that of the Actinocerida and especially the Orthocerida, which were recovered as its potential ancestors. For this reason, it was classified within the new subclass Paracoleoidea, order Antarcticerida, and family Antarcticeratidae.
In 2018, Fuchs et al published a study refuting the previous classification of Antarcticeras as a new subclass of cephalopods. The poorly mineralized phragmocone was recovered as a transitional form between the mineralized, functional phragomocones seen in certain coleoids and the fully demineralized gladius seen in oegopsid squid. For this reason, Antarcticeras was proposed as the first instance of a fossilized squid shell. Later in 2018, Doguzhaeva published a rebuttal, noting the presence of siphuncles and septa in Antarcticeras and the lack of these traits in modern oegopsids, and found the shell morphology of Antarcticeras to be unique among cephalopods, concluding that it was impossible of Antarcticeras to be a squid.
Reader's Guide
Antarcticeras holds a disputed but significant place in cephalopod evolutionary history. If classified as the sole member of the subclass Paracoleoidea, it represents a ghost lineage that diverged from all other cephalopods during the Paleozoic and survived multiple mass extinctions into the early Cenozoic in the far southern latitudes. This would also indicate the survival of orthocerid descendants into the Cenozoic. Alternatively, if interpreted as an oegopsid squid, it provides the first fossilized example of a squid shell and suggests that the modern squid gladius developed remarkably late, supporting a Late Cretaceous origin for most extant decapodiform groups and a Cenozoic origin for modern squid. The debate remains unresolved, with both sides presenting evidence from shell morphology, siphuncle placement, and the depositional environment. The potential for reworking from older geologic layers has been considered but deemed unlikely by Doguzhaeva, though not impossible. A Late Devonian orthocone from Timan Ridge in Russia was cited as a potential Paleozoic representative of the Paracoleoidea, further complicating the picture. Antarcticeras was ranked by The Guardian as one of the top fossil discoveries of 2017.
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