Cranial Asymmetry in Eocene Archaeocete Whales and the Evolution of Directional Hearing in Water
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Cranial asymmetry in Eocene archaeocete whales and the evolution of directional hearing in water Julia M. Fahlkea,1, Philip D. Gingericha,b, Robert C. Welshc, and Aaron R. Wooda,d aMuseum of Paleontology, bDepartment of Geological Sciences, and cDepartment of Radiology, University of Michigan, Ann Arbor, MI 48109; and dDepartment of Geology and Geological Engineering, Paleontology Research Laboratory, South Dakota School of Mines and Technology, Rapid City, SD 57701 Edited by David B. Wake, University of California, Berkeley, CA, and approved July 28, 2011 (received for review June 2, 2011) Eocene archaeocete whales gave rise to all modern toothed and basilosaurid archaeocetes (two Basilosaurus isis and two Dorudon baleen whales (Odontoceti and Mysticeti) during or near the atrox)(Fig. S1). Eocene-Oligocene transition. Odontocetes have asymmetrical skulls, The oldest archaeocete in our study that shows cranial asym- with asymmetry linked to high-frequency sound production and metry is A. clavis, a protocetid from the early middle Eocene echolocation. Mysticetes are generally assumed to have symmetrical (47 Ma) of Pakistan (18) (Fig. 1). The holotype skull (GSP-UM skulls and lack high-frequency hearing. Here we show that proto- 3458) (Fig. 1B) was found palate-up in marl with no evident cetid and basilosaurid archaeocete skulls are distinctly and direc- compression or deformation (18), and yet the dorsal midline su- tionally asymmetrical. Archaeocete asymmetry involves curvature ture deviates notably from RC. and axial torsion of the cranium, but no telescoping. Cranial asym- Archaeocete asymmetry, like that of odontocetes (4), is variable metry evolved in Eocene archaeocetes as part of a complex of traits in amount but consistent in direction. To quantify the asymmetry linked to directional hearing (such as pan-bone thinning of the we calculated mean relative deviations (mean δx/RC) of un- lower jaws, mandibular fat pads, and isolation of the ear region), deformed archaeocete crania and compared these to a model probably enabling them to hear the higher sonic frequencies of distribution based on measurements of 24 symmetrical artiodactyls sound-producing fish on which they preyed. Ultrasonic echolocation (Fig. 2). The model distribution is normally distributed with evolved in Oligocene odontocetes, enabling them to find silent prey. a mean of zero and with the empirical variance estimated from the Asymmetry and much of the sonic-frequency range of directional artiodactyl sample (standard deviation s = 0.0045 mm/mm) (yel- hearing were lost in Oligocene mysticetes during the shift to low- low reference bands in Fig. 2). All six archaeocete skulls studied frequency hearing and bulk-straining predation. here have mean deviations that are positive in sign, indicating consistent and significant midcranial deviation to the right of RC. Cetacea | land-to-sea transition The six archaeocete crania, taken together, have midline suture deviations significantly greater in amount than expected for sym- ost mammals have bilaterally symmetrical skulls. Symmetri- metrical skulls. Four of the six archaeocetes, considered in- fi Mcal crania characterize the artiodactyls closely related to dividually, have midline suture deviations signi cantly greater than whales, and symmetrical crania characterize mysticetes within expected (for statistics, see Tables S1 and S2). A The probability of having all six archaeocete crania right- Cetacea (1) (Fig. 1 ). Odontocetes are exceptional because most 6 odontocete crania are asymmetrical, with dorsal cranial bones deviated if sampled from a symmetrical population is (0.5) = – 0.016. This probability is substantially less than 0.05, and the shifted posteriorly and to the left side (1 8). Living odontocetes fi have a hypertrophied melon, nasal sacs, and phonic lips used to archaeocete sample in Fig. 2 is signi cantly directional. The dorsal produce high-frequency sound (> 20 kHz) (9–11). Mysticetes lack midline suture is a curved rather than straight line (Fig. S1), and these specializations of the nasal apparatus, use low-frequency a midline suture lying to the right of RC means that the studied crania bend to the left. This curvature is similar to that seen in sound (11, 12), and may use the larynx (13) to produce low- odontocetes. However, the posterior-leftward shift of cranial frequency sound. Coupling of high-frequency echolocation with bones near the cranial vertex associated with telescoping (2) has facial and cranial asymmetry in living odontocetes, and the absence moved the odontocete midline suture to the left of RC near the of both in living artiodactyls and living mysticetes, make it reason- – back of the skull (4). able to expect that asymmetry originated in odontocetes (5 7). In archaeocetes, the midline suture seems to deviate most However, it is unresolved how the cranial asymmetry of odonto- strongly from RC along the nasal, frontal, and parietal bones, cetes evolved in the transition from archaeocetes to modern whales, when viewed dorsally. However, Heyning (7) stated that shapes of and the history becomes even more complex when archaeocetes paired left and right cranial bones of odontocetes can differ sub- themselves are considered. stantially without greatly influencing the dorsal midline of a skull. Eocene archaeocete whales gave rise to all modern toothed and If true, then the full extent of cranial asymmetry cannot be cap- baleen whales during or near the Eocene-Oligocene transition tured by study of the midline suture in the dorsal view alone. To – (14 16). Archaeocetes were previously thought to have symmet- test this idea we developed a more general measure of asymmetry rical skulls (3, 5, 7). Asymmetry observed in fossil crania has often that compares dorsal and ventral midline sutures of the cranium been assumed to be an artifact of deformation following burial, simultaneously. This measure was applied to a digital surface and it has been ignored or even removed in published drawings [as was done initially for three of the skulls we studied (17–19)]. EVOLUTION Results Author contributions: J.M.F., P.D.G., and A.R.W. designed research; J.M.F., P.D.G., and R.C. W. performed research; A.R.W. contributed new analytic tools; J.M.F., P.D.G., R.C.W., and A. Here we document and quantify asymmetry in archaeocete crania. R.W. analyzed data; and J.M.F. and P.D.G. wrote the paper. Further observations on exceptionally well-preserved archaeocete The authors declare no conflict of interest. crania and dentaries suggest a link between cranial asymmetry and This article is a PNAS Direct Submission. the ability to locate sound sources in water. Freely available online through the PNAS open access option. We quantified midline suture deviation, δx, from a straight 1To whom correspondence should be addressed. E-mail: [email protected]. rostrocaudal axis [RC, after Ness (4)] for undeformed crania of This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. two protocetid (Artiocetus clavis and Qaisracetus arifi) and four 1073/pnas.1108927108/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1108927108 PNAS | August 30, 2011 | vol. 108 | no. 35 | 14545e14548 Downloaded by guest on September 28, 2021 Fig. 1. Asymmetry in the evolution of whales. (A) Evolutionary relationships between terrestrial Artiodactyla, Eocene archaeocetes, and modern Mysti- ceti and Odontoceti. Cranial asymmetry is present in archaeocetes and Odontoceti, but absent in artiodactyls and Mysticeti. Archaeocetes are rep- resented by Artiocetus, Mysticeti are represented by Balaenoptera, Odon- toceti are represented by Tursiops, and terrestrial artiodactyls are represented by Elomeryx.(B) Skull of the protocetid archaeocete A. clavis (GSP-UM 3458) in dorsal view showing maxillary and frontal sinuses visible in Fig. 3. Torsion of the cranium of the middle Eocene protocetid Artiocetus a 3D micro-CT reconstruction. Note the rightward deviation of the mid- clavis (GSP-UM 3458). Absolute deviations of dorsal and ventral midline cranium. fc, caudal frontal sinus; fr, rostral frontal sinus; ml, lateral maxillary sutures (black lines) are shown in dorsal and ventral view, respectively. sinus; mm, medial maxillary sinus. Torsion is documented by successive deflection angles, where each is the angle between a line connecting dorsal and ventral midline points, mea- sured from the xz-plane, for 1,000 successive values of x. Note that torsion is model of the cranium of A. clavis computed from CT scans. The clockwise and greatest in the rostrum. red line in Fig. 3 represents successive deflection angles of the dorsal and ventral midline sutures from the vertical sagittal mid- cranial surface with an axially torted midline plane, and “axial plane in consecutive transverse sections. This representation torsion” is a more inclusive descriptor of the asymmetry involved. Artiocetus shows that cranial asymmetry in involves 3D torsion Axial torsion has not yet been quantified in odontocetes. about an anteroposterior axis, affecting the whole cranium. Dentaries of protocetid and basilosaurid archaeocetes have Viewed from the braincase forward, the torsion is clockwise and large odontocete-like mandibular foramina on their lingual surfa- increases anteriorly, being greatest in the rostrum. Rightward ces and large mandibular canals (20, 21). Furthermore, excep- deviation of the dorsal midline suture reflects intersection of the tionally preserved dentaries of B. isis (WH-74) (Fig. 4) show distinct thinning of the outer wall of the dentary indicative of the “pan bone” found in modern Odontoceti (22). The large mandibular canals in protocetids and basilosaurids indicate the presence of a mandibular fat body similar to that of modern odontocetes. The fat body of odontocetes has an acoustic impedance close to that of seawater (22) and functions as a wave guide, conducting un- derwater sound received via vibration of the pan bone and through the mandibular foramen to the auditory bulla (10, 22–24). The pan bone in B. isis is located in front of the mandibular foramen and not, as in odontocetes (22), directly opposite it. The position of the pan bone in B.