The gastropod Aporrhais, a relict from the Mesozoic

The shallow-marine gastropod (snail) family Aporrhaidae [pronounced "a-poor-rye-id-dee"] has a geologic range of Triassic (210 million years ago) to modern day. During the Jurassic and Cretaceous, this family diversified greatly, but the mass-extinction event at the end of the Late Cretaceous (66 million years ago) greatly decimated the family. There are only a few Cenozoic genera and only a few remaining species in the world today. 

The most characteristic shell feature of aporrhaids is the expanded outer lip with its labral digitations or "wing-like" structure present on the lower half or so of the shell and originating from the right side of the aperture (the opening where the snail extends its body, when it crawls about). The shape of a "wing" is very important in the identification of the various genera and species, but finding specimens with the "wing" intact is not common. Juvenile aporrhaid shells (those without a "wing") tend to have similar looking shells. Naming of new species, therefore, requires having mature specimens available.

Aporrhais pespelecani (Linnaeus, 1758), which lives today in waters extending from Norway, the British Isles, and into the Mediterranean Sea, is one of the most common extant (living) aporrhaids. The species name, pespelecani, is based on this gastropod having a forked "wing," shaped like a pelican's webbed foot. The following two pictures are of the same specimen of this species.

Two views, front and back, of a specimen of Aporrhais pespelecani (Linnaeus).
The specimen is 42 mm in height and 31.5 mm in width.

Aporrhais serresianus (Michaud, 1828) is another apporaid that lives in the Mediterranean Sea. This species is uncommon and is found in relatively deep water. The following two pictures are of a specimen of this species.

Two views, front and back, of a specimen of A. serresianus. The specimen is 41.4 mm in height and 22.3 mm in width.

Aporrhais pesgallinae Barnard, 1963 lives in mud, in moderately deep water in southwest Africa. The following two pictures are of a specimen of this uncommon subspecies.

Two views, front and back, of a specimen of A. pesgallinae. The specimen

 is 33.8 mm in height and 31.8 mm in width.

Geologic Time Scale

A geologic time scale is given here in order to help discussions
that will follow in my future posts.

For a more complete and much more detailed version, as well as one that is to scale, the link is HERE
It is from the Geological Society of America and is their 2012, version 4.0.

It is important to realize that the Precambrian accounts for about 7/8 of all geologic time. A way to visualize this is consider that if your  entire arm (from shoulder to wrist) and your hand represent all of geologic time, the Precambrian would extend from the shoulder to the wrist. The rest of geologic time is represented by your hand. 

A handy way to remember the sequence of the names of the periods of the geologic time scale involves the following mnemonic devices (by using the first letter of each period name in its respective position in the sequence):

For the Paleozoic:  "Certain old stones delight my paleontology professor."

For the Mesozoic and Cenozoic: "The jungle cat prowls nightly, quietly."