PaleoBios

This checklist is a sequel to the one published by A. Myra Keen and Herdis Bentson in 1944 and is an alphabetical listing of California marine Paleogene–Neogene mollusk species or subspecies described and/or figured in the published literature spanning the interval of 1944 through 2020. The original data are given for each species and subspecies of bivalves, gastropods, chitons (polyplacophorans), scaphopods, and cephalopods. Where detected, formation names, ages, taxonomy, systematics, and specimen disposition were corrected. A total of 559 genera and 1,698 species/subspecies were tabulated, with the bivalves and gastropods being the most abundant taxa. Bivalve and gastropod diversity steadily built up during the Paleocene, nearly doubled during the Eocene warm time, declined greatly during the cool time of the Oligocene, rebounded to its highest peak in the Miocene and then declined slightly during the Pliocene. The other classes represented only minor faunal components. Chitons were only reported from the Pliocene, and cephalopods had their highest diversity during the Paleocene and Eocene. Bivalve genera having the highest diversity are: Glycymeris, Macoma, Mactromeris, Nuculana, and Tellina. Gastropod genera having the highest diversity are Cancellaria and Turritella. It is very likely that the Turritella species/subspecies complex has been significantly overnamed. The high point of research productivity of published reports on California Paleogene-Neogene mollusks was in 1990–1994, and a significant decline occurred in 1995–1996, following the closure of the Menlo Park USGS Paleontology and Stratigraphy facility. An overall decline in productivity has continued, with noticeable upticks in 2003 and in recent years.

New material of three equids is described from the middle Miocene Cajon Valley Formation in San Bernardino National Forest, San Bernardino County, California. The material includes teeth of Archaeohippus mourningi, Scaphohippus sumani, and Parahippus brevidens. Scaphohippus intermontanus is considered a junior subjective synonym of S. sumani. Parahippus brevidens is identified from an upper molar that closely resembles the morphology of the holotype as well as referred specimens of Pa. brevidens from the Mascall Formation in Oregon and the Temblor Formation in California. The presence of Pa. brevidens in the Cajon Valley Formation represents a geographic range extension for the taxon of over 400 km. Interesting ecological implications emerge for the Cajon Valley Formation when compared to the nearby Barstow Formation, including the presence of chalicotheres and apparent lack of Hypohippus affinis and Megahippus mckennai.

Califrapana n. gen., is proposed for California late Oligocene to early Miocene muricids attributed previously to the possibly Paleocene to modern western Pacific and Indian oceans genus Rapana. Four fossil species have been assigned to Rapana in the eastern Pacific. One of these species, R. perrini Clark and Arnold (1923), should be placed in another genus, the other species Purpura vaquerosensis Arnold (1907), R. imperialis Hertlein and Jordan (1927), and R. serrai Wiedey (1928) are synonymized here with the morphologically variable species Califrapana vaquerosensis n. comb. We confirm C. vaquerosensis is an index fossil for the lower and middle “Vaqueros” California provincial molluscan stage of late Oligocene to early Miocene age in southern California and Bája California, México, although the lack of numerical dating and the misuse of lithostratigraphic and biostratigraphic names had made that difficult to determine. Califrapana is distinguishable from similar genera by 1) its heavier shell, 2) an aperture that is pointed at its anterior and posterior ends, 3) more numerous fine to coarse spiral cords on the ultimate whorl, 4) less numerous nodes at the top of the ultimate whorl, which are commonly larger and more pronounced, and 5) the siphonal fasciole, which is large and broad, with a large, open channel.

Isolated equid phalanges are relatively common finds in the Barstow Formation (Miocene, 19 to 13 million years ago, southern California), but anecdotal observations suggested that not all positions (proximal, middle, and distal/ungual) of the primary digit (digit III) are recovered with equal frequency. Our sample includes primarily surface-collected phalanges from the Barstow Formation, which we compare with phalanges of Pleistocene horses from Pit 3 and Pit 77 from Rancho La Brea (Los Angeles, California). The null hypothesis is that the three positions of phalanges should be equally common. Our Barstow sample includes 228 proximal, 151 middle, and 36 distal phalanges. A chi-square test (p<0.001) is consistent with preservation bias in phalangeal frequency for the full Barstow Formation sample, and this pattern generally holds within sub-samples by locality or depositional environment. Pit 3 of La Brea produced 163 proximal, 144 middle, and 103 distal phalanges. A chi-square test with correction for multiple comparisons suggests that proximal and middle phalanges are preserved with statistically equal frequency (p=0.278) whereas distal phalanges are less common (p<0.001). For Pit 77, there are 54 proximal, 55 middle, and 51 distal phalanges; the chi-square test finds that all three types are equally common (p=0.922). Overall, differences in physical properties between phalangeal positions, such as surface area, density, shape, and size, could influence preservation within each environment. The observed differences between Barstow and La Brea might be caused by variations in depositional environment that influence the surface exposure time of fossils and disarticulation pre-burial, as well as by differences in the size of the horses at each locality. We suggest that when permitted by sample size, it is desirable to distinguish unguals from other phalanges when analyzing taphonomic patterns in the fossil record.

Apatemyidae are a rare and enigmatic group of small insectivorous mammals that lived in North America and Europe in the Paleogene. The last known apatemyids in North America are two species in the genus Sinclairella, known from sites in the Great Plains and Florida. Here, I formally describe an upper second molar and lower incisor of the apatemyid, Sinclairella dakotensis, from the incredibly well-studied Turtle Cove Member of the John Day Formation in Oregon. These early Arikareean age specimens represent the first records of the family west of the Rocky Mountains. Sinclairella dakotensis filled a ‘woodpecking’ niche unlike any other mammal known from the region, and its co-occurrence with a number of forest-adapted mammal species is consistent with previous interpretations of environments at the time having been dominated by woodlands.

Over the years a diverse assemblage of proboscidean remains has been recovered from the Lower to Upper Siwalik Subgroups of Pakistan and India. This article reports newly discovered dental material of tri- and tetralophodont proboscideans that includes cf. Paratetralophodon hasnotensis and Choerolophodon sp., and a Gomphothere gen. et sp. indet., recently collected from late middle to late Miocene localities of the Pakistani Siwalik Group, with a brief history of these species. The partial premolar of cf. Pa. hasnotensis is described for the first time from the Siwalik Group, recovered from the Dhok Pathan Formation, and the specimens reported herein are the latest to be described after a 38-year gap from previously described material for this species. A preliminary survey of the literature and previously described material of Siwalik species suggests a revision of Siwalik Group proboscideans is much needed.

The upper Maastrichtian Breien Member situated within the lower portion of the Hell Creek Formation in south-central North Dakota records one of the last transgressions of the Western Interior Seaway (WIS) during the terminal Cretaceous. A fragmentary articular-prearticular complex and isolated vertebra belonging to a mosasauroid were recovered in 2016 from sandstones and mudstones deposited in a nearshore marine paleoenvironment within the southern arm of the bisected WIS. The medially-rotated retroarticular process on the articular-prearticular complex, the shape of the glenoid fossa, along with the morphology of the isolated vertebra, facilitate a conservative referral to a large-bodied mosasaurine such as Mosasaurus or Prognathodon. The rocks of the Breien Member provide paleontologists a unique glimpse of intracontinental marine ecosystems immediately prior to the end of the Cretaceous Period. This discovery provides additional evidence that the latest Maastrichtian marine fauna is a continuation of the fauna preserved in the underlying Fox Hills Formation and that the marine faunal turnover that gave rise to the subsequent Cannonball Sea fauna recorded in Paleocene rocks in North Dakota occurred at the Cretaceous-Paleogene boundary.

We culled abundance record data from the NSF-funded TCN, Eastern Pacific Invertebrate Communities of the Cenozoic (EPICC), including all southern California localities that recorded the presence of oysters from the last 3.6 million years to document how oyster communities change through time. In total, over 120,000 specimens from 78 localities throughout southern California (i.e., Los Angeles, Orange, and San Diego counties) were examined. The data were broken down into four-time bins: late Pliocene, middle Pleistocene, late Pleistocene, and Holocene. Using multivariate statistics, several statistically coherent groups based on occurrences and abundances through time were indentified. Results indicate that the late Pliocene coherent groups possessed a loose, facultative, individualistic community structure that allowed taxa to shift their latitudinal gradients as they tracked shifting environments. The dominant oyster—Dendrostrea vespertina—as well as other taxa, became extinct at the Plio-Pleistocene boundary. Afterwards, community structure changed, as did the dominant oyster. We suspect that the onset of northern hemisphere glaciation at the Plio-Pleistocene boundary changed both the magnitude and rate of sea surface temperatures such that local extinction occurred causing changes in dominance within marine communities. During the middle Pleistocene, Ostrea conchaphila (lurida) appeared and remained dominant throughout the Holocene. In addition, distinct spatial groups existed causing reduced migration along the coast of southern California. Perhaps southern California marine communities responded to the water-mass differences associated with the mid-Pleistocene transition from a mild, 41 ka glacial-interglacial cycle to the more variable ~100 ka glacial-interglacial cycle reducing migration along the coast of southern California. The loose, individualistic community structure seen in the late Pliocene returned during the late Pleistocene and continued through the Holocene allowing marine communities the flexibility to track shifting environments.

Response to the Eocene doubthouse interval of global climate cooling (53–33.5 Ma) is explored in arcoid bivalves of the families Parallelodontidae, Cucullaeidae, Arcidae, and Noetiidae. An anomalous biodiversity hotspot in the intracontinental Paris Basin of Northern Europe is contrasted with an equally anomalous coldspot at comparable latitude on the tectonically active Cascadia Margin of western North America. Reevaluation of arcoid shell morphology and an annotated glossary of shell features accompanies illustration and discussion of eight exemplar species, identifying new characters and distinguishing those with a strong phyletic signal from those representing functional convergence or developmental differences specific to size or age. Biodiversity anomalies cannot be attributed to any single factor. However, contributing factors include tectonic setting, correlates of bathymetric and sedimentary setting, sediment geochemistry, ocean gateway events, reorganization of current systems and water masses, deepening of the calcium carbonate compensation depth, patterns in the development of sea ice and polar ice storage, changes in sea level, and changes in atmospheric carbon dioxide and the carbon cycle. Opening of the Tasman Gateway and Drake Passage, thermal isolation of Antarctica, and evolution of a Pacific psychrosphere are correlated with the early appearance of cold-water molluscan taxa on the active Cascadia Margin along with the unrelated onset of arc volcanism, subduction, and geochemical changes associated with methane and sulfide seepage. Persistence of a shallow carbonate platform and proliferation of molluscan diversity in spite of global cooling is more difficult to explain, and understanding biogeographic anomalies requires additional climate proxy records. History of the western margin of North America includes an earlier Mesozoic volcanic arc and forearc basin in central and northern California with abundant basal arcoids, negating the need for westward migration out of the Tethyan region to the Cascadia Margin during the Paleogene.