This rare and genuine ammonite fossil is identified as Australiceras jacki, a distinctive heteromorph ammonite historically associated with the name Erioceras. It originates from Queensland, Australia, and dates from the Aptian Stage of the Lower Cretaceous Period.
Unlike the tightly coiled, involute shells commonly associated with ammonites, Australiceras developed a more open, evolute shell in which the earlier whorls remained clearly exposed. This unusual growth form places it among the heteromorph ammonites, an exceptionally varied group of extinct marine cephalopods known for shells that departed from the conventional compact spiral.
The fossil has been carefully chosen for its rarity, recognisable form and palaeontological interest. The listing photographs show the actual specimen you will receive, allowing its individual preservation, natural colour, matrix and geological characteristics to be examined before purchase. Full sizing and scale information are provided in the photographs.
Australiceras jacki is the type species of the genus Australiceras. The species was originally described by Robert Etheridge Junior in 1880 under the name Crioceras jackii. Frederick William Whitehouse later established the genus Australiceras in 1926 and reassigned the species to it.
The spelling Australiceras jacki is widely accepted in modern taxonomic databases, although the alternative historical spelling jackii frequently appears in older scientific literature and fossil collections. The genus has also been discussed in relation to Tropaeum, reflecting continuing scientific study of the relationships between closely allied Aptian heteromorph ammonites.
Scientific classification:
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
Order: Ammonitida
Suborder: Ancyloceratina
Superfamily: Ancyloceratoidea
Family: Ancyloceratidae
Subfamily: Ancyloceratinae
Genus: Australiceras
Species: Australiceras jacki
Species authority: Etheridge Junior, 1880
Genus authority: Whitehouse, 1926
The shell of Australiceras is characterised by open, evolute coiling, with successive whorls remaining visible rather than being deeply enclosed by later shell growth. Depending on the individual and its stage of development, the whorls may be separated or positioned close together without forming the tightly overlapping spiral seen in many conventional ammonites.
The shell ornament generally consists of strong transverse ribs extending across the flanks and outer edge. On the earlier juvenile whorls, smooth ribs may alternate with more strongly ornamented ribs bearing stout, blunt or conical tubercles. These tubercles tend to become less prominent as the animal grows and may disappear from the mature body chamber, leaving comparatively smoother ribs on the outer whorls.
This changing ornamentation through growth is an important characteristic of the genus. Rib spacing, tubercle development, whorl shape and the degree of coiling are among the features palaeontologists use when comparing Australiceras specimens.
The original shell was internally divided by curved walls called septa, producing a series of chambers. These chambers were connected by a narrow organic tube known as the siphuncle. The living ammonite occupied the final and largest body chamber, while the earlier chambers assisted with buoyancy regulation.
Heteromorph ammonites developed a remarkable variety of shell forms during the Cretaceous. Some possessed open spirals, while others produced hooked, helical, straight or irregularly coiled shells. The open coiling of Australiceras would have affected its balance, buoyancy and movement through the water.
Although the precise lifestyle of every heteromorph group remains a subject of scientific interpretation, these ammonites were fully marine animals. Their unusual shells may have been suited to relatively slow movement or vertical positioning within the water column rather than the faster horizontal swimming often proposed for more streamlined ammonites.
The animal itself would have resembled other cephalopods in possessing a soft body, head, eyes and arms extending from the shell opening. Like living squid, cuttlefish, octopuses and nautiluses, it belonged to the molluscan class Cephalopoda.
This fossil dates from the Aptian Stage of the Lower Cretaceous, an interval extending from approximately 125 to 113 million years ago. The Aptian was a period of changing sea levels, widespread marine sedimentation and major diversification among ammonites.
Ancyloceratid heteromorph ammonites were an important component of Aptian marine faunas. Because ammonite lineages evolved relatively quickly and many species had restricted geological ranges, their fossils are valuable for dating and correlating marine sedimentary rocks.
Australiceras jacki is especially associated with the Lower Cretaceous deposits of eastern Australia. Its occurrence helps palaeontologists compare Australian marine successions with similarly aged strata elsewhere in the former southern continents and across the broader Cretaceous world.
During the Aptian, large areas of Queensland were influenced by marine conditions associated with the ancient Eromanga Sea and connected sedimentary basins. This extensive inland sea covered substantial parts of eastern and central Australia at different times during the Early Cretaceous.
The surrounding waters supported ammonites, belemnites, bivalves, gastropods, marine reptiles, fish and numerous smaller organisms. Fine mud, silt and carbonate-rich sediment accumulated on the seabed, creating conditions in which shells could become buried and preserved.
After this ammonite died, its shell settled onto the sea floor and was covered by sediment. The soft tissues decayed, while sediment entered or surrounded the shell. During burial and compaction, mineral-rich groundwater altered the remains, and the original shell could be replaced, dissolved or preserved as an internal and external mould.
Natural fractures, missing shell sections, areas of matrix, mineral deposits, weathering and variations in rib preservation are authentic features of the fossilisation process. They document the specimen’s long geological history rather than representing manufacturing defects.
This is a genuine fossil specimen and not a cast, replica or artificial reproduction. The photographs show the exact Australiceras jacki heteromorph ammonite that will be supplied. Each fossil is unique, and its natural shape, ribbing, colour, matrix and state of preservation will differ from every other example.
A generic Certificate of Authenticity card carrying a lifetime guarantee is included with the fossil. Full sizing is shown in the listing photographs.
With its unusual open shell, changing pattern of ribs and tubercles, documented Aptian age and Queensland provenance, this rare Australian ammonite is an excellent addition to a specialist heteromorph ammonite collection, Cretaceous fossil display, natural-history cabinet or educational geological collection.