The Cymatoceratidae is a family of Mesozoic

Mesozoic

The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

 and early Cenozoic

Cenozoic

The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

 nautiloid cephalopods and the most abundant of this kind in the Cretaceous. They are characterized by ribbed, generally involute shells of varied form - coiled such that the outer whorl envelops the previous, as with Nautilus, and sutures that are variably sinuous.

Cymatoceratids first appear in the Middle Jurassic, derived from the Lower Jurassic Cenoceras

Cenoceras

The genus Cenoceras is a member of the Nautilidae, which in turn makes up part of the superfamily Nautilaceae.Cenoceras is variable in form, depending on species; ranges from evolute to involute, compressed lenticular to globose with rounded to flattened venter and flanks. The suture generally has...

(Nautilidae) and extend as far as the Oligocene

Oligocene

The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

.

Taxonomy

The Cymatoceratidae includes ten known genera, beginning with the Middle Jurassic Procymatoceras and Middle and Upper Jurassic Cymatonautilus. Procymatoceras is described as tightly involute, large, rapidly expanding; with early whorl sections more rounded (than latter). Cymatonautilus has a subquadrate whorl section and wide umbilcus. Procymatoceras and Cymatonautilus are followed by Cymatoceras and the similar Paracymatoceras.

Cymatoceras has the longest temporal range of any of the Cymatoceratidae, from the Upper Jurassic to the Oligocene. Paracymatoceras' is much shorter, extending only to the Lower Cretaceous. Cymatoceras has a generally subglobular, variably involutie shell with a rounded whorl section, conspicuous ribs, and suture that is only slightly sinuous. Paracymatoceras differs primarily in that the suture in more sinuous with deep, broadly rounded lateral lobes.

Genera from the Lower Cretaceous are the subglobular involute Eucymatoceras with prominent ribs that form a deep v-shaped ventral sinus, the involute and compressed Heminautilus

Heminautilus

Heminautilus is an extinct Nautiloid genus from the nautilacean family Cymatoceratidae that lived during the early Cretaceous....

with sides converging on a narrow flattened venter. and Anglonautilus with large transverse foldlike undulations, which is also known from the Upper Cretaceous.

Upper Cretaceous genera, in addition to Anglonautilus, are Deltocymatoceras, a broadly arched, involute form with a keel-like ridge running along the venter, i.e. outer, rim; the discoidal and widely evolute Epicymatocers with a narrow flattened venter, and the subglobular Syrionautilus with acute ribs that are widely spaced.