Discovery and historiography of USNM PAL V 11357
With basic image enhancement tools (e.g., Adobe Photoshop), we were able to better resolve the original but faded specimen card included with USNM PAL V 11357 (Figure 1). Specifically, we were able to read now-faded handwritten notes (Figure 1A-B), revealing critical information about the specimen. The widespread availability of image enhancement for faded fieldnotes and labels provides a new source of information for uncovering legacy issues in museum collections (e.g. 21–23), especially in cases where locality data or collecting information cannot be well resolved.
Accession files with this specimen (Supplementary 4) show that it was gifted from Arthur M. Ames to the United States National Museum (now the National Museum of Natural History, Smithsonian Institution) on 15 October 1925, and approved by George P. Merrill, head curator of geology from 1917-1929. Prior to its accession to the museum, an anonymous individual identified the tooth as belonging to Desmostylus hesperus by anonymous. Forty years later, on 17 November 1965, Charles A. Repenning reidentified this specimen as Paleoparadoxia sp (Figure 1A-B), an assertion that was incorporated into its catalog information. According to the label, USNM PAL V 11367 was collected in the city of Corona, Riverside County, California, yet no precise information of its geological provenance was recorded. On the backside of the label, there are notes (Figure 1B) referring to the US Geologic Survey Corona South 7.5' quadrangle map for Riverside and Orange counties, California24. However, no exact horizon or lithology was stated, and the specimen’s collector, A. M. Ames, lived in Santa Barbara, California but who died on 25 August 193921–23
In a century after its discovery, the only mention of USNM PAL V 11367 was by Panofsky25, who listed it in a catalog of desmostylian tooth specimens used as a comparative basis for a mandible restoration of the Stanford specimen N. repenningi. Panofsky25 identified USNM PAL V 11367 as a left m2 with six main cusps, with no additional cusps (Table 1 in 25), while also stating that this specimen has “an open lake in the center of each of the seven cusps” (25: p. 103). The inconsistency of this description differs from our own, which we attribute to differences in morphological criteria during the identification process or a typing error.
Geological horizon and age of USNM PAL V 11367
In this paper, we refer to the “Topanga” Formation following recent recent studies20,26,27. The Topanga Formation was originally based on a sequence of marine sandstones exposed in an anticline just west of Old Topanga Canyon in the central Santa Monica Mountains, California28. It has yielded abundant early to middle Miocene marine invertebrates and a few land mammals that are as old as early late Hemingfordian in age29. After the initial naming, the name of the formation was applied to a much thicker and heterogeneous sequence of sedimentary and volcanic rocks30. Cambell et al. 31 compiled the history and chronology of changes in usage of “Topanga” in the Miocene stratigraphic nomenclature in Southern California,showing that continuous deposition and shared provenance were not demonstrated in every instance. Here we follow i recent studies20,26,27 and use the name of “Topanga” Formation for the early to middle Miocene rocks bearing fossil marine mammals.
According to the original museum records (Figure 1), USNM PAL V 11367 was collected in the city of Corona, Riverside County, California, USA. This city is in the western part of Riverside County, comprising an approximate area of 100 km2 32. Previously, Panofsky25 suggested that USNM PAL V 11367 would have derived from the Temblor Formation (14.8 to 15.8 Ma; 33), likely as a guess based on the prevalence of desmostylian teeth recovered from this unit. . However, there are no Temblor Formation outcrops mapped near Corona24,34; the closest outcrops are in Fresno and Kern counties35, approximately 200 km away.
Geologic maps of Riverside County24,34,36 indicates that the city limits of Corona encompass a wide variety of sedimentary rocks from the Jurassic to the Holocene in age, although there are no previous fossil records from the “Topanga” Formation in Corona, California. In Corona, only a few marine deposits such as the Jurassic Bedford Canyon Formation and the middle Miocene “Topanga” Formation are exposed24. Specifically, the marine sandstones of the “Topanga” Formation occur within the fault zone at the southeast and northwest of Corona. In assessing the age of the “Topanga” Formation, Boessenecker and Churchill26,37 argued that the lands mammals (late Hemingfordian North American Land Mammal Age, represented by Aepycamelus, Copemys and Merychippus; 17.5–15.9 Ma; 29,33), benthic foraminifera, fossil mollusks, and K/Ar dating all placed the age range between 17.5–15 Ma for this geological unit29. More recently, Velez-Juarbe20 revised the age of “Topanga” formation to 16.5–14.5 Ma based on Ogg et al. 38.
Outside of Riverside County, the “Topanga” Formation has yielded several remains of fossil marine vertebrates in Southern California, including desmostylians referred to Desmostylus hesperus, Paleoparadoxia sp., and Desmostylia indet. in Orange County (see 20,26,37,39; Supplemental 2). USNM PAL V 11367 also represents the second reported fossil marine mammal from Riverside County. Previously, an isolated record of Cetacea indet. was described from the Imperial Formation (40 and Supplementary 1). The invertebrate assemblage from this unit indicates a late Miocene age (8–6 Ma41) , which is younger than the type occurrence of Neoparadoxia cecilialina3. Furthermore, no outcrops of the Imperial Formation are recognized in or near the city limits of Corona, ruling out a potential origin from Imperial Formation for USNM PAL V 11367.
Based on the correspondence between the estimated age of the “Topanga” Formation and the known range of Neoparadoxia cecilialina, combined with the absence of other Miocene marine units in the vicinity of Corona, we propose that USNM PAL V 11367 is from the “Topanga” Formation, implying a middle Miocene age, 16.5–14.5 Ma20. Further, considering the reduced distribution of outcrops of the “Topanga” Formation24,34 in Corona, we identify two potential localities for USNM PAL V 11367 (Figure 3). These two localities are situated in urbanized areas, less than 21 km apart, in the northwest and the southeast corners of Corona’s city limits (see Figure 3B); they are equally less than 40 km apart from the locality of the holotype of N. cecilialina in Orange County.
Implications for Neoparadoxia age range
The genus Neoparadoxia was described by Barnes3 based on the comparative morphology of two specimens(the holotype of N. repenningi and N. cecilialina). USNM PAL V 11367 becomes the third known record of Neoparadoxia. Fossil remains of Neoparadoxia have been only found from the middle-late Miocene of California (Figure 3). 15342While the estimates of the geochronologic age range of the “Topanga” Formation range from 16.5–14.5 Ma20, these age ranges fit with the age of N. repeninngi or N. cecilialina: N. repenningi is 14 Ma from the Ladera Sandstone Formation (e.g., 15); and Parham et al. 42 recently revised the age of N. cecilialina to 14.9–13.1 Ma. Therefore, we revise the age range for the genus Neoparadoxia to 16.5-13.1 Ma.
Morphological variation and potential diversity of Paleoparadoxiidae
Our comparisons reveal considerable morphological variation in the arrangement and number of dental cusps across Paleoparadoxiidae (Figure 4). The cusps arrangement for the m2-3 of Archaeoparadoxia and Paleoparadoxia were previously reported by Inuzuka et al. (1994: Figure 4B), but the addition of another specimen (USNM PAL V11367) shows more morphological variability than previously suspected for the genus Neoparadoxia (Figure 4C). Specifically, the holotype of N. cecilialina displays slightly different configurations between its right and left m2, driven mainly by the position of the hypoconulid in occlusal view (Figure 4C). USNM PAL V 11367, the second known Neoparadoxia m2 (or the first m3) is comparable in size and shape with the same teeth in the type specimen of N. cecilialina, especially the right m2. Both the Smithsonian and LACM specimens display a horizontal alignment of the extra cusp, the hypoconulid, and the entoconid; nevertheless, USNM PAL V 11367 shows a tighter configuration, lacking a wide internal spacing between cusps characteristic of the type specimen of N. cecilialina (Figure 4C).
The identification of USNM PAL V 11367 from “Topanga” Formation of Corona also suggests that at least members of three genera of Desmostylia co-existed in this geological unit. The “Topanga” Formation holds the greatest richness of desmostylian taxonomic diversity for any geological unit in the world. Previously, Desmostylus spp. and paleoparadoxiids rarely co-occurred from the same formation43,44 based on taphonomic and faunal assemblage comparisons. There are some localities in California and Japan where Desmostylus and paleoparadoxiids co-occurred (e.g., Santa Margarita Formation45,46, Rosarito Beach Formation47, Tortugas Formation47, Temblor Formation3,4, and Rawan conglomerate sandstone member48,49), but such specimens were found from the horizons where fossils were likely reworked or concentrated in high energy depositional settings, such as coquinas and conglomerates, or represent deposits with long-term (105-6 yrs.) non-deposition and subaqueous weathering, such as phosphoritic deposits, bonebeds, or the deposits just above an unconformity43,50,51. In those units, we suspect that co-occurrences of desmostylians may reflect allocthonous assemblages rather than ecological communities52.
It has been hypothesized that Desmostylus spp. only lived in cool marine environments43,53, whereas paleoparadoxiids inhabited a larger environmental range from tropical to cool marine environments as hinted from co-occurring fossil molluscan assemblages in the northwest Pacific7. However, in the “Topanga” Formation, the fossil record indicates that both Desmostylus and paleoparadoxiids co-occurred54,55. Most desmostylians from the “Topanga” Formation are presented by relatively well-preserved remains (i.e., partial skeletons, teeth with well-preserved roots55,56), suggesting some taphonomic control (or depositional specificity), with low transportation from basal conglomerate layers or flushing out from lower formations. “Topanga” Formation does not fit the formations that Chinzei43 mentioned, and this formation might represent a rare environment where desmostylids and paleoparadoxiids coexisted.
Like other marine mammal lineages, desmostylian body sizes reached their maximum body size late in their evolutionary history57. By the middle to late Miocene, desmostylians were the largest herbivorous marine mammals along the North Pacific coastlines57, although they likely competed ecologically with co-occurring sirenians, which later eclipsed desmostylians in body size and survived until historical times in the North Pacific Ocean58. Specifically, in the “Topanga” Formation, desmostylians co-occurred with sirenians such as Metaxytherium arctodites59, an ecological association that likely was repeated elsewhere in the mid-Miocene of California (e.g., coeval deposits of the Round Mountain Silt). Given that the “Topanga” Formation likely represented a warm, shallow marine environment60, the elevated productivity and high availability of seagrasses during the Middle Miocene Climatic Optimum61 probably contributed to Southern California’s favorability as a location for the highest richness of marine mammal herbivores ever known, with three or four desmostylian taxa (Desmostylus sp.: 56; Paleoparadoxia tabatai: 4; Neoparadoxia: this study; Paleoparadoxiidae gen. sp. indet: 55) and one genus of dugongid sirenian. Similar patterns of high taxonomic richness among co-occurring marine mammal guilds existed in “Topanga” Formation and elsewhere during this time (e.g., 20), indicating that marine food webs significantly differed from such observed today in the North Pacific.