What is most wonderful about natural science is that every fossil—every spiral, ridge, and suture—opens a window onto a vanished world.
Take, for instance, this tremendously robust, intricately sutured ammonite: Holcophylloceras mediterraneum (Neumayr, 1871). Collected from Late Jurassic (Oxfordian) deposits near Sokoja, Madagascar, it is a marvel of paleontological sculpture, a testament to evolutionary experimentation that thrived in the tropical Tethyan seas some 160 million years ago.
Madagascar has long been recognized as a treasure trove of beautifully preserved fossils. From its Cretaceous dinosaurs to its Triassic amphibians and its extraordinary Jurassic ammonites, the island offers a richness few regions can rival.
The spiraled shell of Holcophylloceras mediterraneum is no exception—its ornate sutures and lustrous preservation hint at a creature exquisitely adapted to the warm, shallow continental shelf of Gondwana’s eastern margin.
Like all ammonites, Holcophylloceras built its shell in a series of chambers divided by walls known as septa. These septa, when intersecting the outer shell, formed the elaborate suture patterns that make collectors swoon—tangled, fractal-like lines that resemble botanical tracings or rivers on an ancient map.
Running through each chamber was the siphuncle, a biological marvel that allowed the ammonite to adjust the gas and fluid content inside its shell. In effect, ammonites carried a set of built-in ballast tanks, enabling them to rise and sink through the water column almost effortlessly. Their final and largest chamber—the body chamber—housed the soft tissues, including the tentacles, eyes, and muscular arms.
Picture, if you will, a squid or octopus, then surround it with a coiled, beautifully ribbed shell. Now place it in a warm tropical sea filled with predators and prey, reefs and drifting plankton, and a ton upon ton of water pressing down from above. That was the world Holcophylloceras mastered.
The Oxfordian oceans surrounding Madagascar were not quiet waters. They were alive—thrumming with movement, colour, and competition. The ammonite’s elegant spiral belies the reality of its bustling neighbourhood. Some of the many animals that would have swum, crawled, hunted, or drifted around Holcophylloceras mediterraneum include:
Marine Reptiles
- Plesiosaurs – long-necked Cryptoclidus–like forms gliding between shoals of fish.
- Ichthyosaurs – such as Ophthalmosaurus, sleek torpedo-shaped hunters with dinner-plate eyes built for dim, deeper waters.
- Pliosaurs – apex predators like Liopleurodon, whose cavernous jaws could swallow a human whole.
Other Cephalopods
Belemnites – dart-shaped squid-relatives such as Hibolithes, flickering through the water column like living arrows.
Other ammonite genera sharing these seas:
- Perisphinctes
- Asaphoceras
- Physodoceras
- Aspidoceras
- Glochiceras
Each species filled its own ecological niche, from fast-swimming pursuit hunters to slow-drifting plankton feeders.
Fishes and Sharks
- Hybodont sharks – including Hybodus and Asteracanthus, equipped with crushing teeth for shelled prey and formidable dorsal spines.
- Teleost fishes – early ray-finned fishes beginning to diversify.
- Coelacanths – ancient lobe-finned holdovers patrolling calmer waters.
Invertebrates
- Bivalves – oysters, rudists, and inoceramids carpeting the shallow seafloor.
- Gastropods – from turreted turritellids to broad-shelled neritids.
- Crustaceans – shrimp, lobsters, and small crabs scraping algae from reef structures.
- Sea urchins and echinoids – spiny architects of sandy burrows.
Reefs & Drifting Life
- Sponges and corals creating pocket reefs in warm carbonate-rich environments.
- Planktonic foraminifera and radiolarians – the drifting micro-architecture of the Jurassic sea, powering food webs from below.
Ammonites like Holcophylloceras thrived in these diverse ecosystems by filling a mid-level trophic niche. They were both predator and prey—nimble enough to hunt small fish and crustaceans, yet vulnerable to larger hunters. Their greatest evolutionary advantage was their ability to regulate buoyancy, adjusting depth as easily as a modern submarine.
But their most beautiful legacy remains their shells. In death, they fell to the seafloor, where their chambers filled with sediment, minerals, and eventually time itself.
Today, polished by erosion or revealed in limestone, they offer a perfect blend of geometry, biology, and ancient artistry.
