RESEARCH ARTICLE Petrosal morphology and cochlear function in Mesozoic stem therians 1 2 Tony HarperID , Guillermo W. RougierID * 1 Center for Functional Anatomy and Evolution, Johns Hopkins University, Baltimore, Maryland, United States of America, 2 Department of Anatomical Sciences and Neurobiology, University of Louisville, Louisville, Kentucky, United States of America * [email protected] a1111111111 a1111111111 Abstract a1111111111 a1111111111 Here we describe the bony anatomy of the inner ear and surrounding structures seen in a1111111111 three plesiomorphic crown mammalian petrosal specimens. Our study sample includes the triconodont Priacodon fruitaensis from the Upper Jurassic of North America, and two iso- lated stem therian petrosal specimens colloquially known as the HoÈoÈvoÈr petrosals, recov- ered from Aptian-Albian sediments in Mongolia. The second HoÈoÈvoÈr petrosal is here OPEN ACCESS described at length for the first time. All three of these petrosals and a comparative sample Citation: Harper T, Rougier GW (2019) Petrosal of extant mammalian taxa have been imaged using micro-CT, allowing for detailed anatomi- morphology and cochlear function in Mesozoic cal descriptions of the osteological correlates of functionally significant neurovascular fea- stem therians. PLoS ONE 14(8): e0209457. https:// tures, especially along the abneural wall of the cochlear canal. The high resolution imaging doi.org/10.1371/journal.pone.0209457 provided here clarifies several hypotheses regarding the mosaic evolution of features of the Editor: Alistair Robert Evans, Monash University, cochlear endocast in early mammals. In particular, these images demonstrate that the AUSTRALIA membranous cochlear duct adhered to the bony cochlear canal abneurally to a secondary Received: December 4, 2018 bony lamina before the appearance of an opposing primary bony lamina or tractus foramino- Accepted: June 7, 2019 sus. Additionally, while corroborating the general trend of reduction of venous sinuses and Published: August 14, 2019 plexuses within the pars cochlearis seen in crownward mammaliaforms generally, the HoÈoÈ- Copyright: © 2019 Harper, Rougier. This is an open voÈr petrosals show the localized enlargement of a portion of the intrapetrosal venous plexus. access article distributed under the terms of the This new vascular feature is here interpreted as the bony accommodation for the vein of Creative Commons Attribution License, which cochlear aqueduct, a structure that is solely, or predominantly, responsible for the venous permits unrestricted use, distribution, and drainage of the cochlear apparatus in extant therians. Given that our fossil stem therian reproduction in any medium, provided the original author and source are credited. inner ear specimens appear to have very limited high-frequency capabilities, the develop- ment of these modern vascular features of the cochlear endocast suggest that neither the Data Availability Statement: Surface files of labyrinthine endocasts have been deposited to initiation or enlargement of the stria vascularis (a unique mammalian organ) was originally MorphoSource (https://www.morphosource.org/ associated with the capacity for high-frequency hearing or precise sound-source Detail/ProjectDetail/Show/project_id/796). localization. Funding: Specimen collecting and data analysis was funded in part by PICT-2016-3682 from CONICET-Agencia de PromocioÂn CientifõÂca y TeÂcnica, Argentina; and NSF grants DEB 0946430, and DEB 1068089 to GWR and the Department of Anatomical Sciences, University of Louisville. Introduction Competing interests: The authors have declared Therian mammals (the last common ancestor of marsupials and placentals and its descendants that no competing interests exist. [1]) today display an impressive variety of auditory characteristics facilitating the adept PLOS ONE | https://doi.org/10.1371/journal.pone.0209457 August 14, 2019 1 / 62 Stem therian petrosals Abbreviations: ?ptv, possible location of post- detection of airborne sound, making this group arguably the most acoustically sophisticated trigeminal vein; ac, aqueductus cochleae (for and diverse clade of terrestrial vertebrates [2]. The widespread capacity for high-acuity hearing perilymphatic duct); acf, aperture of cochlear (in terms of sensitivity, specificity, and highest detectable frequency) among the majority of fossula (external to fenestra cochleae); adm, arteria diploeÈtica magna; al, anterior lamina; amp-a, extant therian mammals has reinforced the assumption among neontologists that the Meso- anterior semicircular canal ampulla; amp-h, zoic members of the therian stem linage were stereotypically nocturnal forms that leveraged horizontal semicircular canal ampulla; amp-p, their auditory faculties to locate small prey and escape gigantic predators (i.e. [3]). However, posterior semicircular canal ampulla; av, this supposition has not been supported by the known fossil record of stem therians, with aqueductus vestibuli (for endolymphatic duct); prior descriptions [4±9] demonstrating that these forms lacked the level of petrosal organiza- bpsal, border of periosteal surface of anterior tion characterizing plesiomorphic marsupials [10], afrotheres [11], eulipotyphlans, and prima- lamina; bs, basisphenoid sutural surface; cas, cavum supracochleare; cc-p, primary common tomorphs [12]. Additionally, many of these older reports lacked CT imaging, reliying heavily crus; cc-s, secondary common crus; cdh, on the description of the external morphology of the ear region. Therefore, much of the ana- centripetal diverticulum of horizontal semicircular tomical detail informing the performance and/or physiological evolution of the inner ear canal; ce, cavum epiptericum; ci, crista remained understandably inaccessible. interfenestralis; coc, cochlear canal; cVIII, cochlear Conversely, several biomechanical/experimental studies on auditory anatomy and physiol- branch of vestibulocochlear nerve; dag, dorsal ascending groove; eapc, endocranial aperture of ogy across extant tetrapods [2, 13±16] highlight the unique nature of the therian cochlear prootic canal; ect, ectotympanic bone; eps-p, apparatus (with its well-ordered acoustic hair-cell populations arrayed along the organ of posterior epicochlear sinus (ªtrans-cochlear sinus Corti, high endolymphatic potential, and absence of the lagenar macula), as well as the plesio- pº in [31]); fac, facial canal (aqueductus Fallopii in morphic nature of the monotreme cochlea with respect to many modern and fossil therians [32]); fc, fenestra cochleae (for secondary [17,18]. The complete loss of the lagenar macula in particular has been posited as a adaptive tympanic membrane); fcn, foramen for cochlear breakthrough that permitted later elongation and sophistication of the cochlear apparatus for nerve; FHS, functional head size (in microseconds); fs, fenestra semilunaris; fv, fenestra vestibuli non-linear amplification of high-frequency stimuli [19]. The plesiomorphic retention of a (fenestra ovalis); gpn, greater petrosal nerve; H1, functional lagenar macula, along with its accompanying otoconial and neurovascular struc- HoÈoÈvoÈr petrosal specimen 1 (PSS-MAE-104); H2, tures, in the monotreme and sauropsid lineages is also physiologically incompatible with sev- HoÈoÈvoÈr petrosal specimen 2 (PSS-MAE-119); hF, eral soft tissue adaptations seen in modern therians. These ªtherianº features include: 1) the hiatus Fallopii (for greater petrosal nerve); hfn, exclusive reliance on the stria vascularis as the major endolymph producing organ, 2) the well- hyomandibular branch of facial nerve; hps, hypocochlear sinus; hs, half-pipe shaped sulcus; developed electromotility of prestins and other molecular components of the ªcochlear ampli- iam, internal acoustic meatus; IHC, inner hair cell; fierº, and 3) the radical elongation and geometrical reorganization of the cochlear sensory ijv, internal jugular vein; ILD, interaural level epithelium. The accumulation of these characteristics among therian ancestors points to a difference; ips, inferior petrosal sinus; ir, inferior fundamental transformation of the cochlear apparatus somewhere near the origin of Theria ramus of stapedial artery; ITD, interaural time [19]. difference; jn, jugular notch; lf, lateral flange; lhv, The lack of detailed cochlear reconstructions for Mesozoic fossils is understandable given lateral head vein; li, lagenar inflation; mm, manubrium of malleus; ntr-a, notch for temporal the general lack of high-fidelity bony correlates for key soft tissue structures such as the ramus a; ntr-b, notch for temporal ramus b; OHC, cochlear duct, lagenar macula, and stria vascularis. This report uses high-resolution micro-CT outer hair cell; pcs, sinus around prootic canal; pfc, information to update descriptions of petrosal anatomy provided in the representative sample prefacial commissure (suprafacial commisure); pf, of stem therians focused on by [4] and [5], two large-scale studies characterizing the fossil perilymphatic foramen; pop, paroccipital process; record of early mammalian petrosal evolution (Fig 1). These images and reconstructions are pos, paroccipital sinus; pov, prootic vein; pptc, petrosal contribution to post-temporal canal; re, framed within a broader comparative and functional setting of modern mammalian auditory recessus ellipticus (for utricle); rpm, rostral physiology, and morphology with the hope of bridging both fields which, due to the steep (anterior) process of malleus (= goniale/ learning curves involved, have