A large collection of brachyuran specimens from the middle Eocene Tepetate Formation, Baja California Sur, Mexico, has yielded sufficiently well-preserved specimens to provide revised descriptions and diagnoses for genera and species previously known from the area. Revised, more complete descriptions are given for Eriosachila bajaensis Schweitzer et al., 2002, and Lobonotus mexicanus Rathbun, 1930. A gone-placid specimen is questionably referred to Carcinoplax, which has a well-established Pacific record during the Miocene. All of the specimens of Amydrocarcinus dantei Schweitzer et al., 2002, thus far collected for which gender can be determined are males, suggesting that there may have been environmental or behavioral segregation of males and females of this species. Analysis of benthic and planktonic foraminiferans suggests that the Tepetate Formation was deposited in subtropical conditions, probably within the photic zone but below the seasonal thermocline, perhaps below 100–120 m. The age of the Tepetate Formation, according to larger foraminiferans, is middle Eocene, and according to planktonic foraminiferans is further constrained as upper middle Eocene (Bartonian). This age corresponds to the P14 foraminiferal zone based upon the Berggren et al. (1995) scheme and the E13 zone based upon the work of Berggren and Pearson (2005).

All genera previously referred to the Carcineretidae are herein evaluated, and the family is restricted to three genera, Carcineretes, Cancrixantho, and Mascaranada, for which diagnoses are provided. Ophthalmoplax and Longusorbis are herein removed to the Portunidae, and Longusorbis eutychius new species is described from the Eocene Tepetate Formation of Baja California Sur, Mexico, extending the range of that genus across the K/P boundary. The placement of Ophthalmoplax into the Portunidae marks the first confirmed notice of the family in Cretaceous rocks, a major range extension for the family. Important characteristics of the Portunoidea are discussed in the context of placement of fossil taxa within the superfamily and its constituent families.

Recognition of two different configurations of the abdomen in the Eocene raninid crab, Lophoranina bishopi Squires and Demetrion, 1992, from the Tepetate Formation in Baja California Sur, Mexico, represents the first published notice of sexual dimorphism in fossil representatives of the Ranininae. Examination of extant representatives of the six extant subfamilies of Raninidae documents sexually dimorphic characters of the abdomen throughout the family, although the style and degree of development of dimorphic features varies throughout the subfamilies. Recognition of patterns of sexual dimorphism of abdomina in the Raninidae is particularly useful in the fossil record where conventional means of determining gender are not preserved.

Skulls of two specimens of the Lower Permian seymouriamorph tetrapod Seymouria, referred to Seymouria sanjuanensis Vaughn, 1966, with midline skull lengths of about 19.0 and 20.0 mm, respectively, and probably representing an early juvenile ontogenetic stage, are described from the Lower Permian Tambach Formation, lowermost formation of the Upper Rotliegend Group or Series, of the Bromacker quarry locality in the midregion of the Thuringian Forest near Gotha, central Germany. They represent the smallest known specimens of the genus and are compared with those of larger, previously described specimens of Seymouria that include from the same locality: a late juvenile and two fully adult specimens of S. sanjuanensis with skull lengths of 56.0 mm, and 87.9 and 94.6 mm, respectively, and fully adult specimens of S. sanjuanensis and Seymouria baylorensis Broili, 1904, with skull lengths of 86.0–93.0 and 99.0–124.0 mm, respectively, from the Lower Permian of North America. This partial but widely spaced ontogenetic series of the skull of S. sanjuanensis, which includes an early and late juvenile and fully mature adult stages, is the first to be documented for any Seymouriamorpha. The skulls of the two early juvenile specimens of S. sanjuanensis from the Bromacker quarry are compared with similarly sized supposed larval specimens of the very closely related seymouriamorph species Discosauriscus pulcherrimus and Discosauriscus austriacus (Makowsky 1876), from the Lower Permian of the Boskovice Furrow, Moravia, Czech Republic, in which the quadrate and articular were cartilaginous and the sensory grooves are present. Contrary to conditions in Discosauriscus, in the early juvenile specimens of S. sanjuanensis the quadrate and articular are ossified and the sensory grooves are absent. This indicates that Seymouria underwent metamorphosis at much earlier stage than Discosauriscus. In addition, the early juveniles of S. sanjuanensis can be distinguished from those of both species of Discosauriscus on the basis of several differences in the skull roof and palate. However, at this stage of growth three characters indicate that S. sanjuanensis is more similar to D. pulcherrimus than to D. austriacus: 1) a chevron-shaped postorbital; 2) rows of small denticles that radiate anteriorly from the midwidth of the posterior border of the palatal ramus of the pterygoid; and 3) presence of denticles arranged in rows or randomly scattered denticles on the anterior half of the parasphenoidal plate.