Post by Life on Sept 8, 2022 6:01:22 GMT 5
Zygophyseter varolai
Hypothetical reconstructions of Zygophyseter varolai gen. et sp. nov. A, head in lateral view with a parasagittal section of the nasal area based on Physeter macrocephalus (Heyning, 1989: 36); B, head in dorsal view with evidence for the circular supracranial basin of the skull; C, body in lateral view.
Figure 1. Reconstruction of the Zygophyseter varolai skull and mandibles in lateral view, based on the cast of the holotype (MSNUP I-16828), and schematic reconstruction of the temporalis (red) and masseter (including the pars superficialis and pars profunda) (yellow)
New York Institute of Technology: www.nyit.edu/medicine/zygophyseter_varolai/
Figure 15 in (Bianucci & Landini, 2006) for reference:
Hypothetical reconstruction of a Late Miocene marine scenario showing the killer sperm whale Zygophyseter attacking a kentriodontid (delphinoid). Painting by Giovanni Bianucci.
Classification
| Kingdom | Animalia | |
| Phylum | Chordata | |
| Subphylum | Vertebrata | |
| Class | Mammalia | |
| Order | Cetacea | (Bianucci & Landini, 2006) |
| Suborder | Odontoceti | (Bianucci & Landini, 2006) |
| Superfamily | Physeteroidea | (Bianucci & Landini, 2006) |
| Family | Incertae Sedis | (Bianucci & Landini, 2006) |
| Genus | Zygophyseter | (Bianucci & Landini, 2006) |
| Species | Zygophyseter voralai | (Bianucci & Landini, 2006) |
English Common Name: killer sperm whale (Bianucci & Landini, 2006)
Timeline of existence
| Era | Period | Epoch | Age |
| Cenozoic | Neogene | Miocene | Tortonian |
| The Geological Timescale | The Geological Timescale | (Bianucci & Landini, 2006) | (Bianucci & Landini, 2006; Peri et al., 2021) |
Holotype specimen
The holotype specimen Zygophyseter varolai gen. et sp. nov. (MAUL 229/1) is discussed at length in (Bianucci & Landini, 2006). This partially preserved specimen consist of a skull, mandible with 22 teeth in place, 25 loose teeth (some as fragments), incomplete left periotic and left tympanic bulla, atlas, nine thoracic vertebrae, ten lumbar vertebrae and nine caudal vertebrae, 23 complete or fragmentary ribs; almost complete left scapula and fragment of right scapula, a small fragment of right radius, and one phalanx (Bianucci & Landini, 2006).
Figure 2 in (Bianucci & Landini, 2006) for reference:
Sketch showing in plain view the relative positions of the skeletal elements of MAUL 229/1, holotype of Zygophyseter varolai gen. et sp. nov. as they were preserved. CV, caudal vertebrae; LV, lumbar vertebrae; MD, mandible; RB, ribs; SC(l), left scapula; SC(r), right scapula; SK, skull; TV, thoracic vertebrae.
Size estimation
The holotype specimen Zygophyseter varolai gen. et sp. nov. (MAUL 229/1) was about 7 m in total length (Bianucci & Landini, 2006; Peri et al., 2021).
Dentition and skull reconstruction
Zygophyseter varolai had a total of 26 teeth in its upper jaw structure and a total of 28 teeth in its lower jaw structure (Bianucci & Landini, 2006). Teeth of Zygophyseter varolai have crenulated enamel on the crown whereas teeth of the extant species Physeter macrocephalus lack enamel (Bianucci & Landini, 2006). The posterior teeth of Zygophyseter varolai are mediolaterally compressed whereas all teeth of Physeter macrocephalus are circle-shaped in cross-section (Bianucci & Landini, 2006). Teeth of Zygophyseter varolai are also relatively larger than those of Physeter macrocephalus (Bianucci & Landini, 2006).
Figure 9 in (Bianucci & Landini, 2006) for reference:
Zygophyseter varolai gen. et sp. nov. Mandible (A-E) and isolated upper teeth (F, G) of the holotype (MAUL 229/1). A, anterior view; B, ventral view of the anterior portion of the symphysis; C, dorsal view; D, lateral view; E, medial view of left posterior portion of right dentary; F, lateral or medial views; G, posterior views.
Figure 4 in (Toscano et al., 2013) for reference:
Figure 4 in (Bianucci, 2014) for reference:
Figure 14 in (Bianucci & Landini, 2006) for reference:
Bite force estimation
Zygophyseter varolai was able to generate the same bite force as a great white shark individual well beyond sexual maturity (Peri et al., 2021).
"From the FEA simulation, we obtained an estimation of 4312 N (20° gape angle) and 4812 N (35° gape angle) for the anterior bite. We also estimated that Z. varolai generated 10103 N (20° gape angle) and 10823 N (35° gape angle) at the posterior bite." - (Peri et al., 2021)
The bite force values of Zygophyseter varolai are in good agreement with its several osteomorphological and dental characters including a wide temporal fossa, a well-developed zygomatic process of the squamosal, and the presence of deep occlusal facets on teeth suggesting a strong degree of occlusion during bite (Peri et al., 2021).
Figure 1 in (Peri et al., 2021) for reference:
Figure 1. Reconstruction of the Zygophyseter varolai skull and mandibles in lateral view, based on the cast of the holotype (MSNUP I-16828), and schematic reconstruction of the temporalis (red) and masseter (including the pars superficialis and pars profunda) (yellow)Considering the significant difference observed between the anterior and posterior bite forces, Zygophyseter varolai conforms to "grip-and-shear" feeding technique, snapping a prey item with an anterior bite and then cutting it with a powerful posterior bite (Peri et al., 2016). Other macroraptorial sperm whales such as members of genus Acrophyseter also employed this feeding technique (Peri et al., 2016).
Figure 3 in (Peri et al., 2021) for reference:
Figure 3. Feeding strategies of marine mammal predators. Modified from Berta and Lanzetti (2020), with the addition of the grip-and-shear feeding (illustration of Zygophyseter varolai modified from Bianucci and Landini 2006)
Extant macroraptorial dolphins such as Orcinus orca do not conform to "grip-and-shear" feeding technique (Peri et al., 2016); these dolphins are known to hold and shake their prey with their jaws to tear off large pieces of flesh without a proper cutting action and conform to "grip-and-tear" feeding technique instead (Peri et al., 2016). These dolphins have a blunt and robust rostrum as well as cheek teeth that are not laterally compressed (Peri et al., 2016), thus differing from the putative "grip-and-shear" feeders like Zygophyseter varolai and Basilosaurus isis (Peri et al., 2016). Nevertheless, the presence of well-developed occlusal facets is indicative of a strong dental occlusion and an extensive use of the biting action during feeding (Peri et al., 2016).
Ecology and diet
Zygophyseter varolai, together with Naganocetus shigensis, belongs to a Middle-Late Miocene lineage of basal sperm whales adapted for feeding on large prey, similarly to the extant killer whale (Orcinus orca) (Bianucci & Landini, 2006).
Zygophyseter varolai is assumed to have fed upon small and medium-sized marine vertebrates (Peri et al., 2016). Fossilized remains of Zygophyseter varolai were retrieved from the Tortonian strata of the Cisterna quarry in Lecce (Italy). This strata also provided fossilized remains of marine vertebrates such as Messapicetus longirostris (Ziphiidae, Cetacea), Metaxytherium medium (Dugongidae, Sirenia), Makaira cf. M. nigricans (Istiophoriade, Perciformes) and Acanthocibius cf. A. solandri (Scombridae, Perciformes) (Peri et al., 2016); these marine vertebrates are assumed to be prey to Zygophyseter varolai (Peri et al., 2016).
Extinction
Global cooling in the late Miocene timeline (7 – 5.4 Ma) might have reduced geographical range of macroraptorial sperm whales (Peri et al., 2021), and one of the factors behind decline of medium-sized baleen whales (Peri et al., 2021). Late Neogene establishment of gigantism in baleen whales compounded this crisis yet further (Peri et al., 2021).
The ecological void of macroraptorial sperm whales (acting as apex consumers in the Miocene epoch) was filled by large-bodied dolphins in the Plio-Pleistocene timeline (Peri et al., 2021).
General References
Fossilworks.org: www.fossilworks.org/cgi-bin/bridge.pl?a=taxonInfo&taxon_no=85977
New York Institute of Technology: www.nyit.edu/medicine/zygophyseter_varolai/
Mindat.org: www.mindat.org/taxon-4971115.html
References
Bianucci, G., & Landini, W. (2006). Killer sperm whale: a new basal physeteroid (Mammalia, Cetacea) from the Late Miocene of Italy. Zoological Journal of the Linnean Society, 148(1), 103-131.
Bianucci, G. (2014). I cetacei fossili nei musei italiani. Museologia Scientifica Memorie, 13, 7-17.
Peri, E., Falkingham, P. L., Collareta, A., & Bianucci, G. (2021). Biting in the Miocene seas: estimation of the bite force of the macroraptorial sperm whale Zygophyseter varolai using finite element analysis. Historical Biology, 1-12. [Full article access: www.tandfonline.com/doi/full/10.1080/08912963.2021.1986814?scroll=top&needAccess=true]
Toscano, A., Abad, M., Ruiz, F., Muñiz, F., Álvarez, G., García, E. X. M., & Caro, J. A. (2013). Nuevos restos de Scaldicetus (Cetacea, Odontoceti, Physeteridae) del Mioceno superior, sector occidental de la Cuenca del Guadalquivir (sur de España). Revista mexicana de ciencias geológicas, 30(2), 436-445.