The Rhiphaeoceratidae are a small family of nautilids

Nautilida

The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, Nautilus and Allonautilus, with six species...

 included in the superfamily Tainocerataceae

Tainocerataceae

The Tainocerataceae is a superfamily in the cephalopod order Nautilida characterized by straight to loosely coiled shells, generally with a degressed, such that the width is greater than the height, to quadrate whorl section. Many bore spines, ribs, frills, wings, or nodes...

 that comprises four very similar genera. These genera are characterized by a perforate umbilicus and little more than a single evolute coil. Whorl sections are oval, subquadrate, or subtrapezoidal. Sutures bend forward on the outer rim, forming wide shallow ventral saddles and dip strongly to the rear on the inner rim, forming deep dorsal lobes.

Rhiphaeoceratidae was named by Ruzhencev and Shimansky in 1954 are, according to Kümmel (1964), derived from the Tainoceratidae

Tainoceratidae

The Tainoceratidae is a family of late Paleozoic and Triassic nautiloids that are a part of the order Nautilida, characterized by large, generally evolute shells with quadrate to rectangular whorl sections...

, although no genus stands out as possible ancestor. They date from the Early Permian

Permian

The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

 in age and are found in parts of Eurasia, south Urals. Azerbaijan

Azerbaijan

Azerbaijan , officially the Republic of Azerbaijan is the largest country in the Caucasus region of Eurasia. Located at the crossroads of Western Asia and Eastern Europe, it is bounded by the Caspian Sea to the east, Russia to the north, Georgia to the northwest, Armenia to the west, and Iran to...

, Tajikistan

Tajikistan

Tajikistan , officially the Republic of Tajikistan , is a mountainous landlocked country in Central Asia. Afghanistan borders it to the south, Uzbekistan to the west, Kyrgyzstan to the north, and China to the east....

Genera included, also named by Ruzhencev and Shimansky, 1954 are:

• Rhiphaeoceras: a riphaeoceritid first found the south Urals (L Perm) with an oval whorl section; flanks with faint, narrow radial ribs; suture with broad ventral saddle and deep dorsal lobe.

• Pararhiphaeoceras: a riphaeoceratid with a subtrapezoidal whorl section, widest across the venter, narrowing to the dorsum; flanks with short oblique ribs; suture as for Rhiphaeoceras. Found in L Permian in Azerbaijan and Tajikistan.

• Rhiphaeonautilus: a riphaeoceratid like Pararhiphaeoceras, with a subtrapezoidal whorl section, but with short inflated ribs on the lateral areas and with a shallow lobe modifying the ventral saddle. Also first found in the south Urals, as with Rhiphaeoceras.

• Skolakoceras: a riphaeoceratid with a subquadrate whorl section, venter and sides flattened, lateral areas bearing slightly oblique ribs. The suture is similar to that of Rhiphaeonautilus, with a broad ventral saddle modified by a shallow lobe.