Read Part 1 here for an introduction to whale and dolphin earbones (recommended before diving right in).
This is perhaps the part that most readers are looking for – how to identify different dolphin periotic bones. For each family of cetaceans, I’ll briefly list the approximate size reported as complete periotic length (not specific measurements, this is from memory), some of the main attributes, along with any unique features of the family or higher level features within Odontoceti, similar periotic morphs they can be confused with, age, known distribution, and established localities (e.g. localities, including rock unit, where these are confirmed to have been found or are reliably known from).
This guide is not exhaustive, nor is it perfect: identifying isolated periotics has long been called a 'black art' even by seasoned whaleontologists. The attributes listed below should not be interpreted by professionals as synapomorphies - they are generalizations, which can help someone identify many isolated periotics to the family level. Some families are very obvious, but others range quite a bit in anatomy, overlapping with other families - this is particularly a problem within the Delphinoidea. Regardless, I have almost certainly glossed over some important details and missed some things - so if you want to help me improve this, let me know what I've screwed up! Also, I will likely expand this in the future. The specimens in this post are mostly in collections of the Mace Brown Museum of Natural History; genus-level identifications (upper right) for some of these should be taken with a grain of salt. All photographs are by me, unless otherwise stated.
Though many features and conditions vary considerably and have evolved and lost again and again, some major groups are united by some periotic features. Archaic dolphins generally have long anterior and posterior processes and some remnant of the suprameatal fossa, which is large and deep in basilosaurid whales. Many archaic dolphins, as well as platanistoids, have a small spur lateral to the posterior process called the articular process - in Platanista (Ganges river dolphin) it is long and hooked, and generally needs to be broken in order for the periotic to be removed from the skull. The anterior bullar facet is primitively shallow in the earliest dolphins, but deeply concave in many long-snouted early to middle Miocene dolphins (Eurhinodelphinidae, Eoplatanistidae), and the facet is lost entirely in Delphinoidea. Delphinoids also tend to have a proportionally huge pars cochlearis.
As with all other fossils, periotic bones have some degree of natural variation. Above are some photos of periotics of Parapontoporia sternbergi, reasonably interpreted by L.G. Barnes (1985) as a single species from the San Diego Formation of southern California (Pliocene). You'll notice that much of the variation is in the length and inflation of the anterior process and the particular shape of the posterior bullar facet. The cochlear morphology - especially ventrally - seems to vary the least, which according to my Ph.D. adviser Ewan Fordyce, is likely because it ossifies the earliest and is associated with the middle ear sinus. The dorsal side varies considerably as this continues to ossify during growth, so the shape and size of the body and the configuration of foramina and crests within the meatus, and the shape and size of the meatus itself, also can change during growth. What this means is that no two periotics of the same species will ever be identical, and I guarantee you will go mad picking out differences between specimens only to find out they represent different edges of the anatomical envelope of variation or juveniles v. adults. As a result, it's better to look for shared similarities and when possible, match a particular periotic morph to periotics found associated with a skull, though this is certainly more typically the realm of activities of a whaleontologist rather than an amateur collector as many such specimens needed for such comparisons are in museum collections. Image from Barnes (1985).