Papers by Robert Boessenecker
The extinct edentulous mysticete family Cetotheriidae historically has been viewed as a notorious... more The extinct edentulous mysticete family Cetotheriidae historically has been viewed as a notoriously paraphyletic group, and only recently have rigorous studies been executed to rectify this issue. These problems do not necessarily just stem from lack of phylogenetic analyses, but are in part because of a general lack of complete specimens, poor descriptions of taxa, and long-lived taxonomic instability issues. The fossil mysticete genus Herpetocetus is a poster child of these problems as it is primarily only known from a few relatively incomplete and poorly described specimens. A new species of Herpetocetus from the upper Pliocene of California, Herpetocetus morrowi sp. nov., provides an archetypal model for the genus based on a multitude of well-preserved specimens. These specimens reveal a diminutive mysticete characterized by an elongate rostrum and roughly quadrate cranium. A mosaic of primitive and derived features preserved in this new species underscores its potential value in helping to resolve a number of taxonomic and phylogenetic problems. The occurrence of specimens assignable to juvenile through to mature adult individuals provides a basis for investigating ontogenetic changes. Functional analysis of the unusual craniomandibular anatomy of H. morrowi suggests a limited degree of mandibular gape and an enhanced capacity for longitudinal rotation of the dentary, features that support a hypothesis of suction feeding convergent with that of living grey whales. A phylogenetic analysis provides support for recognition of a redefined and monophyletic Cetotheriidae and Herpetocetinae, and also serves as a basis for evaluating the recent proposal that the pygmy right whale (Caperea marginata) is a living cetothere. Morphological features of Herpetocetus morrowi, including features of the cranium and petrosal, suggest that a number of the purported synapomorphies supporting a Caperea−cetothere grouping are either symplesiomorphies, nonhomologous features, or are highly variable. © 2014 The Linnean Society of London
The newly discovered Upper Miocene to Upper Pliocene San Gregorio assemblage of the Purisima Form... more The newly discovered Upper Miocene to Upper Pliocene San Gregorio assemblage of the Purisima Formation in Central California has yielded a diverse collection of 34 marine vertebrate taxa, including eight sharks, two bony fish, three marine birds (described in a previous study), and 21 marine mammals. Pinnipeds include the walrus Dusignathus sp., cf. D. seftoni, the fur seal Cal lorhinus sp., cf. C. gilmorei, and indeterminate otariid bones. Baleen whales include dwarf mysticetes (Herpetocetus bramblei Whitmore & Barnes, 2008, Herpetocetus sp.), two right whales (cf. Eubalaena sp. 1, cf. Eubalaena sp. 2), at least three balaenopterids ("Balaenoptera" cortesi "var." portisi Sacco, 1890, cf. Balaenoptera, Balaenopteridae gen. et sp. indet.) and a new species of rorqual (Balaenoptera bertae n. sp.) that exhibits a number of derived features that place it within the genus Balaenoptera. This new species of Balaenoptera is relatively small (estimated 61 cm bizygomatic width) and exhibits a comparatively narrow vertex, an obliquely (but precipitously) sloping frontal adjacent to vertex, anteriorly directed and short zygomatic processes, and squamosal creases. Fossil odontocetes include the lipotid "river dolphin" Parapontoporia sternbergi (Gregory & Kellogg, 1927), four true porpoises including a bizarre new genus also known from other strata (Phocoenidae indet., Phocoenidae unnamed genera 1 and 2, and cf. Phocoena), an indeterminate delphinid (Delphinidae indet.) a pilot whale-like delphinid (cf. Globicephalinae indet.), an undetermined sperm whale (cf. Physeteroidea indet.), and an indeterminate odontocete. The new record of Parapontoporia sternbergi is noteworthy as it represents the first association of any earbones (petrosal, tympanic bulla, malleus, and incus) for the extinct genus. Discovery and description of a complete marine mammal assemblage permits faunal comparisons with other published Pliocene marine mammal assemblages from around the globe. The aggregate Pliocene marine mammal assemblage from GEODIVERSITAS • 2013 • 35 (4) Boessenecker R. W.
The origin of the pygmy right whale (Caperea marginata) has long been one of the most vexing conu... more The origin of the pygmy right whale (Caperea marginata) has long been one of the most vexing conundrums of marine mammal evolution. The extremely disparate skeletal structure of Caperea and a patchy fossil record have left morphology and molecules at odds: whereas most morphological analyses ally Caperea with right whales (Balaenidae), most molecular studies instead suggest a close relationship with rorquals (Balaenopteridae) and grey whales (Eschrichtiidae). The morphological evidence supporting a Caperea-balaenid clade consists of several shared features of the skull and mandible, as traditionally observed in adult individuals. Here, we show that at least two of these features, the ascending process of the maxilla and the coronoid process, arise from substantially different precursors early during ontogeny and therefore likely do not represent genuine synapomorphies. Both of these juvenile morphologies have adult counterparts in the fossil record, thus indicating that the ontogenetic variation in the living species may be a genuine reflection of differing ancestral states. This new evidence contradicts previous morphological hypotheses on the origins of Caperea and may help to reconcile morphological and molecular evidence.
PLOS One, Jan 16, 2013
Background
A number of aberrant walruses (Odobenidae) have been described from the Neogene of ... more Background
A number of aberrant walruses (Odobenidae) have been described from the Neogene of the North Pacific, including specialized suction-feeding and generalist fish-eating taxa. At least one of these fossil walruses has been hypothesized to have been a specialized predator of other marine mammals, the middle Miocene walrus Pelagiarctos thomasi from the Sharktooth Hill Bonebed of California (16.1–14.5 Ma).
Methodology/Principal Findings
A new specimen of Pelagiarctos from the middle Miocene “Topanga” Formation of southern California (17.5–15 Ma) allows a reassessment of the morphology and feeding ecology of this extinct walrus. The mandibles of this new specimen are robust with large canines, bulbous premolars with prominent paraconid, metaconid, hypoconid cusps, crenulated lingual cingula with small talonid basins, M2 present, double-rooted P3–M1, single-rooted P1 and M2, and a P2 with a bilobate root. Because this specimen lacks a fused mandibular symphysis like Pelagiarctos thomasi, it is instead referred to Pelagiarctos sp. This specimen is more informative than the fragmentary holotype of Pelagiarctos thomasi, permitting Pelagiarctos to be included within a phylogenetic analysis for the first time. Analysis of a matrix composed of 90 cranial, dental, mandibular and postcranial characters indicates that Pelagiarctos is an early diverging walrus and sister to the late Miocene walrus Imagotaria downsi. We reevaluate the evidence for a macropredatory lifestyle for Pelagiarctos, and we find no evidence of specialization towards a macrophagous diet, suggesting that Pelagiarctos was a generalist feeder with the ability to feed on large prey.
Conclusions/Significance
This new specimen of Pelagiarctos adds to the knowledge of this problematic taxon. The phylogenetic analysis conclusively demonstrates that Pelagiarctos is an early diverging walrus. Pelagiarctos does not show morphological specializations associated with macrophagy, and was likely a generalist predator, feeding on fish, invertebrates, and the occasional warm-blooded prey item.
"The Miocene to Pliocene Purisima Formation crops out in multiple transform fault bounded structu... more "The Miocene to Pliocene Purisima Formation crops out in multiple transform fault bounded structural blocks in central California. As a result of poor exposure, strike slip fault offset, and uncertain intraformational correlations, some exposures of the Purisima Formation are not well dated. The San Gregorio section of the Purisima Formation occurs in the Pigeon Point
Block, west of the San Gregorio Fault, along the coast of southern Halfmoon Bay. Ages based on invertebrate and diatom biostratigraphy support a Late Miocene to Early Pliocene age,
while ash correlations indicate a much younger Middle to Late Pliocene (3.3-2.5 Ma) age. Abundant remains of marine vertebrates occur in the Purisima Formation. Recent fieldwork
in the San Gregorio section identified a modest assemblage of 26 taxa, including sharks (Carcharodon carcharias, Carcharodon sp., Cetorhinus maximus, cf. Hexanchus, Isurus oxyrinchus,
Pristiophorus sp., Squatina sp., and Sphyrna sp.), skates (Raja sp., cf. R. binoculata), bony fish (Paralichthys sp., Thunnus sp.), birds (Mancalla diegensis, Morus sp.), and 13 marine
mammal taxa, including several new records for the Purisima Formation. The nonmammalian vertebrates of this assemblage are described herein. The vertebrate assemblage is utilized to evaluate previous biostratigraphic and tephrochronologic age determinations for the San Gregorio section. The stratigraphic range of Carcharodon carcharias, Raja sp., cf. R. binoculata, Mancalla diegensis, and some of the marine mammals strongly indicate a Middle to Late Pliocene age for the upper and middle parts of the section, while a Late Miocene or Early Pliocene age is probable for the base of the section.
"
"Two fossil baleen whale (Mysticeti) dentaries from the Upper Miocene (10–12 Ma) Santa Margarita ... more "Two fossil baleen whale (Mysticeti) dentaries from the Upper Miocene (10–12 Ma) Santa Margarita Sandstone of Central California preserve several distinct features similar to the enigmatic herpetocetine whale Herpetocetus. These features include an elongate coronoid process, a mandibular condyle with a planar articular surface, and a posteriorly extended angular process. The dentary is unknown for several Herpetocetinae (and the more inclusive clade Cetotheriidae),
including the coeval Nannocetus eremus. This occurrence would extend the known record of Herpetocetus by 6 Ma. Given the currently poor knowledge of Pacific Cetotheriidae during the Miocene, these specimens are identified
to the subfamily Herpetocetinae, despite the similarity of these specimens to Herpetocetus. As the morphology of the supposedly distinctive lectotype dentary of Herpetocetus scaldiensis (the type species of Herpetocetus) may not be unique to Herpetocetus, this study suggests that the mandibular morphology of fossil mysticetes may be more homoplastic (or conservative) than previously assumed. Mysticete taxonomy should employ autapomorphic characters beyond the morphology of the dentary alone."
Journal of Vertebrate Paleontology, Jan 1, 2011
New fossils representing two species of the fur seal Callorhinus are reported from the uppermost ... more New fossils representing two species of the fur seal Callorhinus are reported from the uppermost Pliocene to lower Pleistocene Rio Dell Formation of northern California. The finds include latest Pliocene–earliest Pleistocene dentaries and postcrania of Callorhinus gilmorei, and a partial dentary of early Pleistocene age identified as Callorhinus sp. The aforementioned material is ascribed to C. gilmorei due to the incipient single-rooted condition of the p1–2, retention of double-rooted p3–m1, and overall small size. The dentary identified as Callorhinus sp. exhibits a more derived pattern of tooth morphology, including single-rooted p1–p4 (and double-rooted m1), larger size than C. gilmorei, and in the size range of extant Callorhinus ursinus (which typically exhibit fused roots on all postcanine teeth). Fusion of postcanine roots began with the p2 and continued posteriorly, and is likely an adaptation to accommodate crowded teeth anteriorly in the jaws. Callorhinus gilmorei has previously been reported from the upper Pliocene of southern California and Japan, and this new record extends the range of this taxon further north in the Northeast Pacific. Callorhinus sp. is the most complete pinniped fossil to be described from the early Pleistocene of the Northeast Pacific. The wide biogeographic range of Callorhinus during the Pliocene and Pleistocene documents the persistence of this taxon, potentially as a Pliocene-Holocene anagenetic lineage. This highlights the antiquity of the Callorhinus lineage, which has persisted in the Northeast Pacific since the Pliocene, establishing it as the oldest and earliest diverging crown otariid.
2009 Portland GSA …, Jan 1, 2009
Fossils represent important non-renewable resources. Construction projects present both a threat ... more Fossils represent important non-renewable resources. Construction projects present both a threat to these resources and an opportunity to preserve them for future study. Legislative requirements and professional standards are required to prevent fossil resources from being lost. In ...
Journal of Vertebrate Paleontology, Jan 1, 2011
Fossils of pelagornithids (bony-toothed birds) have been reported from strata of Paleocene to Pli... more Fossils of pelagornithids (bony-toothed birds) have been reported from strata of Paleocene to Pliocene age, and from every continent. The extreme fragility of pelagornithid bones has no doubt contributed to their geographically and temporally sporadic record, and thus it has been difficult to appreciate any long-term phylogenetic trends through geologic time for this group. We report a well-preserved partial humerus of the gigantic bird Pelagornis from the late Neogene Purisima Formation of central California. Due to its incompleteness, we refrain from naming a new species. This fossil is fortuitously bracketed by two ash beds, which have been correlated with volcanic rocks at 3.35 ± 0.05 Ma and 2.5 ± 0.2 Ma, indicating a middle to late Pliocene age for this fossil. This fossil extends the record of the pelagornithids in the Northeast Pacific, previously only known up until the early late Miocene (10–12 Ma). This fossil is the latest record of a pelagornithid for the Pacific Basin, and additionally represents the latest reliably dated pelagornithid record worldwide. This record suggests that the pelagornithids survived until the end of the Pliocene, and became extinct during the ocean restructuring and climatic upheavals that caused the demise of many other groups of marine vertebrates at that time.
Palaios, Jan 1, 2011
"Fossils of extinct fur seals and walruses (Carnivora: Pinnipedia) occur within rich vertebrate f... more "Fossils of extinct fur seals and walruses (Carnivora: Pinnipedia) occur within rich vertebrate fossil assemblages recovered from the shallow marine Mio-Pliocene Purisima Formation, central California. Two isolated postcranial bones—a humerus and a radius—belonging to a juvenile fur seal (Pinnipedia: Otariidae) exhibit circular depressions. These bone modifications are associated with radial and circular fractures, and are characterized by inward displacement of the cortex. These depressions lack features typical of erosive invertebrate borings,
trampling damage from media (5substrate) interaction, puncturing by another object during diagenetic compaction, such as a clast embedded or associated with the modification, or pathologic bone modification. These features are best interpreted as tooth marks. These tooth marks lack certain characteristics of commonly reported marks inflicted by shark
teeth, such as linear gouges and subparallel scrapes formed by xiphodont and serrated teeth. These bone modifications instead exhibit a circular shape and inward displacement of the cortex, consistent with puncturing by a conical mammal tooth. The size and distribution of the tooth marks, in concert with the known vertebrate assemblage from the Purisima Formation, indicate several possible producers of the bone modifications: a pilot whale or beluga-like cetacean, a terrestrial carnivore, a dusignathine or odobenine walrus, or a case of infanticide by a conspecific otariid."
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Papers by Robert Boessenecker
A number of aberrant walruses (Odobenidae) have been described from the Neogene of the North Pacific, including specialized suction-feeding and generalist fish-eating taxa. At least one of these fossil walruses has been hypothesized to have been a specialized predator of other marine mammals, the middle Miocene walrus Pelagiarctos thomasi from the Sharktooth Hill Bonebed of California (16.1–14.5 Ma).
Methodology/Principal Findings
A new specimen of Pelagiarctos from the middle Miocene “Topanga” Formation of southern California (17.5–15 Ma) allows a reassessment of the morphology and feeding ecology of this extinct walrus. The mandibles of this new specimen are robust with large canines, bulbous premolars with prominent paraconid, metaconid, hypoconid cusps, crenulated lingual cingula with small talonid basins, M2 present, double-rooted P3–M1, single-rooted P1 and M2, and a P2 with a bilobate root. Because this specimen lacks a fused mandibular symphysis like Pelagiarctos thomasi, it is instead referred to Pelagiarctos sp. This specimen is more informative than the fragmentary holotype of Pelagiarctos thomasi, permitting Pelagiarctos to be included within a phylogenetic analysis for the first time. Analysis of a matrix composed of 90 cranial, dental, mandibular and postcranial characters indicates that Pelagiarctos is an early diverging walrus and sister to the late Miocene walrus Imagotaria downsi. We reevaluate the evidence for a macropredatory lifestyle for Pelagiarctos, and we find no evidence of specialization towards a macrophagous diet, suggesting that Pelagiarctos was a generalist feeder with the ability to feed on large prey.
Conclusions/Significance
This new specimen of Pelagiarctos adds to the knowledge of this problematic taxon. The phylogenetic analysis conclusively demonstrates that Pelagiarctos is an early diverging walrus. Pelagiarctos does not show morphological specializations associated with macrophagy, and was likely a generalist predator, feeding on fish, invertebrates, and the occasional warm-blooded prey item.
Block, west of the San Gregorio Fault, along the coast of southern Halfmoon Bay. Ages based on invertebrate and diatom biostratigraphy support a Late Miocene to Early Pliocene age,
while ash correlations indicate a much younger Middle to Late Pliocene (3.3-2.5 Ma) age. Abundant remains of marine vertebrates occur in the Purisima Formation. Recent fieldwork
in the San Gregorio section identified a modest assemblage of 26 taxa, including sharks (Carcharodon carcharias, Carcharodon sp., Cetorhinus maximus, cf. Hexanchus, Isurus oxyrinchus,
Pristiophorus sp., Squatina sp., and Sphyrna sp.), skates (Raja sp., cf. R. binoculata), bony fish (Paralichthys sp., Thunnus sp.), birds (Mancalla diegensis, Morus sp.), and 13 marine
mammal taxa, including several new records for the Purisima Formation. The nonmammalian vertebrates of this assemblage are described herein. The vertebrate assemblage is utilized to evaluate previous biostratigraphic and tephrochronologic age determinations for the San Gregorio section. The stratigraphic range of Carcharodon carcharias, Raja sp., cf. R. binoculata, Mancalla diegensis, and some of the marine mammals strongly indicate a Middle to Late Pliocene age for the upper and middle parts of the section, while a Late Miocene or Early Pliocene age is probable for the base of the section.
"
including the coeval Nannocetus eremus. This occurrence would extend the known record of Herpetocetus by 6 Ma. Given the currently poor knowledge of Pacific Cetotheriidae during the Miocene, these specimens are identified
to the subfamily Herpetocetinae, despite the similarity of these specimens to Herpetocetus. As the morphology of the supposedly distinctive lectotype dentary of Herpetocetus scaldiensis (the type species of Herpetocetus) may not be unique to Herpetocetus, this study suggests that the mandibular morphology of fossil mysticetes may be more homoplastic (or conservative) than previously assumed. Mysticete taxonomy should employ autapomorphic characters beyond the morphology of the dentary alone."
trampling damage from media (5substrate) interaction, puncturing by another object during diagenetic compaction, such as a clast embedded or associated with the modification, or pathologic bone modification. These features are best interpreted as tooth marks. These tooth marks lack certain characteristics of commonly reported marks inflicted by shark
teeth, such as linear gouges and subparallel scrapes formed by xiphodont and serrated teeth. These bone modifications instead exhibit a circular shape and inward displacement of the cortex, consistent with puncturing by a conical mammal tooth. The size and distribution of the tooth marks, in concert with the known vertebrate assemblage from the Purisima Formation, indicate several possible producers of the bone modifications: a pilot whale or beluga-like cetacean, a terrestrial carnivore, a dusignathine or odobenine walrus, or a case of infanticide by a conspecific otariid."
A number of aberrant walruses (Odobenidae) have been described from the Neogene of the North Pacific, including specialized suction-feeding and generalist fish-eating taxa. At least one of these fossil walruses has been hypothesized to have been a specialized predator of other marine mammals, the middle Miocene walrus Pelagiarctos thomasi from the Sharktooth Hill Bonebed of California (16.1–14.5 Ma).
Methodology/Principal Findings
A new specimen of Pelagiarctos from the middle Miocene “Topanga” Formation of southern California (17.5–15 Ma) allows a reassessment of the morphology and feeding ecology of this extinct walrus. The mandibles of this new specimen are robust with large canines, bulbous premolars with prominent paraconid, metaconid, hypoconid cusps, crenulated lingual cingula with small talonid basins, M2 present, double-rooted P3–M1, single-rooted P1 and M2, and a P2 with a bilobate root. Because this specimen lacks a fused mandibular symphysis like Pelagiarctos thomasi, it is instead referred to Pelagiarctos sp. This specimen is more informative than the fragmentary holotype of Pelagiarctos thomasi, permitting Pelagiarctos to be included within a phylogenetic analysis for the first time. Analysis of a matrix composed of 90 cranial, dental, mandibular and postcranial characters indicates that Pelagiarctos is an early diverging walrus and sister to the late Miocene walrus Imagotaria downsi. We reevaluate the evidence for a macropredatory lifestyle for Pelagiarctos, and we find no evidence of specialization towards a macrophagous diet, suggesting that Pelagiarctos was a generalist feeder with the ability to feed on large prey.
Conclusions/Significance
This new specimen of Pelagiarctos adds to the knowledge of this problematic taxon. The phylogenetic analysis conclusively demonstrates that Pelagiarctos is an early diverging walrus. Pelagiarctos does not show morphological specializations associated with macrophagy, and was likely a generalist predator, feeding on fish, invertebrates, and the occasional warm-blooded prey item.
Block, west of the San Gregorio Fault, along the coast of southern Halfmoon Bay. Ages based on invertebrate and diatom biostratigraphy support a Late Miocene to Early Pliocene age,
while ash correlations indicate a much younger Middle to Late Pliocene (3.3-2.5 Ma) age. Abundant remains of marine vertebrates occur in the Purisima Formation. Recent fieldwork
in the San Gregorio section identified a modest assemblage of 26 taxa, including sharks (Carcharodon carcharias, Carcharodon sp., Cetorhinus maximus, cf. Hexanchus, Isurus oxyrinchus,
Pristiophorus sp., Squatina sp., and Sphyrna sp.), skates (Raja sp., cf. R. binoculata), bony fish (Paralichthys sp., Thunnus sp.), birds (Mancalla diegensis, Morus sp.), and 13 marine
mammal taxa, including several new records for the Purisima Formation. The nonmammalian vertebrates of this assemblage are described herein. The vertebrate assemblage is utilized to evaluate previous biostratigraphic and tephrochronologic age determinations for the San Gregorio section. The stratigraphic range of Carcharodon carcharias, Raja sp., cf. R. binoculata, Mancalla diegensis, and some of the marine mammals strongly indicate a Middle to Late Pliocene age for the upper and middle parts of the section, while a Late Miocene or Early Pliocene age is probable for the base of the section.
"
including the coeval Nannocetus eremus. This occurrence would extend the known record of Herpetocetus by 6 Ma. Given the currently poor knowledge of Pacific Cetotheriidae during the Miocene, these specimens are identified
to the subfamily Herpetocetinae, despite the similarity of these specimens to Herpetocetus. As the morphology of the supposedly distinctive lectotype dentary of Herpetocetus scaldiensis (the type species of Herpetocetus) may not be unique to Herpetocetus, this study suggests that the mandibular morphology of fossil mysticetes may be more homoplastic (or conservative) than previously assumed. Mysticete taxonomy should employ autapomorphic characters beyond the morphology of the dentary alone."
trampling damage from media (5substrate) interaction, puncturing by another object during diagenetic compaction, such as a clast embedded or associated with the modification, or pathologic bone modification. These features are best interpreted as tooth marks. These tooth marks lack certain characteristics of commonly reported marks inflicted by shark
teeth, such as linear gouges and subparallel scrapes formed by xiphodont and serrated teeth. These bone modifications instead exhibit a circular shape and inward displacement of the cortex, consistent with puncturing by a conical mammal tooth. The size and distribution of the tooth marks, in concert with the known vertebrate assemblage from the Purisima Formation, indicate several possible producers of the bone modifications: a pilot whale or beluga-like cetacean, a terrestrial carnivore, a dusignathine or odobenine walrus, or a case of infanticide by a conspecific otariid."