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Schematic showing morphology of internal acoustic meatus in stem therians. Illustrations and renderings of left-sided internal acoustic meatus from endocranial view. Anterior is toward the right dorsal is toward top of page. a, condition in the docodont Borealestes and hypothetically all early mammaliaforms; b, c condition in Priacodon and hypothetically many early stem therians; d, e condition in Höövör petrosal 1; f, g condition in Höövör petrosal 2; h, i condition in extant therian mammals (e.g. Erinaceus). Character states in d-i are more morphologically derived than in Priacodon.
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Here we describe the bony anatomy of the inner ear and surrounding structures seen in three plesiomorphic crown mammalian petrosal specimens. Our study sample includes the triconodont Priacodon fruitaensis from the Upper Jurassic of North America, and two isolated stem therian petrosal specimens colloquially known as the Höövör petrosals, recovered...
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... Several smaller canals cross the ventral surface of the cochlear canal anterior to the perilymphatic foramen. Harper and Rougier (2019) described those canals as the 'hypocochlear sinus', which we follow here. While there are numerous smaller canals for the hypocochlear sinus that anastomose in MCZ 19974 (Fig. 2e), there appear to be fewer but larger canals for the hypocochlear sinus in MCZ 19973 (Fig. 2b). ...
... The comparison focuses on the well described and illustrated petrosals of Priacodon (Rougier et al. 1996;Harper and Rougier 2019;Fig. 4d), Trioracodon (Kermack 1963;Fig. ...
... Three-dimensional reconstructions and or detailed descriptions of the venous system surrounding the cochlear canal are limited in Eutriconodonta. Astroconodon represents only the second virtual rendering of these vessels after Priacodon (Harper and Rougier 2019). ...
Although several well-preserved crania are known for the Mesozoic Eutriconodonta, three-dimensional reconstructions of the character-rich inner ear and basicranial region based on high-resolution computed tomography scans have previously only been published for the Late Jurassic Priacodon. Here we present a description of the petrosal and inner ear morphology of a triconodontid eutriconodontan from the Lower Cretaceous Cloverly Formation, which we provisionally assign to Astroconodon. The bony labyrinth of Astroconodon is plesiomorphic for mammaliaforms in lacking a primary osseous lamina, cribriform plate, and osseous cochlear ganglion canal. However, as in Priacodon and the zhangheotheriid Origolestes, Astroconodon has a secondary osseous lamina base that extends nearly the complete length of the cochlear canal. The cochlear canal is straighter in Astroconodon and other eutriconodontans compared to several basal mammaliaform clades (e.g., morganucodontans, docodontans), that exhibit varying degrees of cochlear canal curvature. The pars cochlearis of the petrosal was well vascularized in Astroconodon, exhibiting a network of venous canals that crossed the cochlea transversely on its ventral and dorsal aspects. Of particular note are several canals that passed along the base of the secondary osseous lamina. As in Priacodon and Origolestes, those canals do not show the extensive connections to the cochlear labyrinth as seen in the basal mammaliaforms Morganucodon and Borealestes. The inner ear of Astroconodon thus highlights the complex history of the mammaliaform cochlear canal, in which different clades appear to follow independent evolutionary trajectories and various key morphological features (e.g., cochlear canal length, curvature, vascularization and osseous supports for the basilar membrane) exhibit considerable homoplasy.
... The hyomandibular bone of piscine sarcopterygians, including Latimeria, is transformed into the stapes in tetrapods (Clack and Ahlberg, 2016;Harper and Rougier, 2019) The insertion of the stapes into the oval window is known in most amphibians and all amniotes (Carr, 2020;Maier and Ruf, 2016), and is driven by sound pressure. ...
... Unfortunately, the resolution in fossils shows just the two or three semicircular canals, the utricle, saccule and occasionally the lagena, but not any basilar papillae, nor the amphibian papilla in amphibians (Clack and Ahlberg, 2016;Zhu et al., 2021a,b). Most recent data have expanded our fossil understanding of tetrapods (Harper and Rougier, 2019). Still, the obvious difference in resolution of fossil ears can be compared to much older ear images, including the pattern of innervation, provided by Retzius (Retzius, 1881;Retzius, 1884). ...
... Still, the obvious difference in resolution of fossil ears can be compared to much older ear images, including the pattern of innervation, provided by Retzius (Retzius, 1881;Retzius, 1884). The details of the hair cells and their innervation, tectorial membrane, and cochlear aqueduct can be seen in Latimeria (Fritzsch, 1987) compared to the best images of whole skulls and fossil ears (Harper and Rougier, 2019;Zhu et al., 2021a,b). ...
Sarcopterygians evolved around 415 Ma and have developed a unique set of features, including the basilar papilla and the cochlear aqueduct of the inner ear. We provide an overview that shows the morphological integration of the various parts needed for hearing, e.g., basilar papilla, tectorial membrane, cochlear aqueduct, lungs, and tympanic membranes. The lagena of the inner ear evolved from a common macula of the saccule several times. It is near this lagena where the basilar papilla forms in Latimeria and tetrapods. The basilar papilla is lost in lungfish, certain caecilians and salamanders, but is transformed into the cochlea of mammals. Hearing in bony fish and tetrapods involves particle motion to improve sound pressure reception within the ear but also works without air. Lungs evolved after the chondrichthyans diverged and are present in sarcopterygians and actinopterygians. Lungs open to the outside in tetraposomorph sarcopterygians but are transformed from a lung into a swim bladder in ray-finned fishes. Elasmobranchs, polypterids, and many fossil fishes have open spiracles. In Latimeria, most frogs, and all amniotes, a tympanic membrane covering the spiracle evolved independently. The tympanic membrane is displaced by pressure changes and enabled tetrapods to perceive airborne sound pressure waves. The hyomandibular bone is associated with the spiracle/tympanic membrane in actinopterygians and piscine sarcopterygians. In tetrapods, it transforms into the stapes that connects the oval window of the inner ear with the tympanic membrane and allows hearing at higher frequencies by providing an impedance matching and amplification mechanism. The three characters-basilar papilla, cochlear aqueduct, and tympanic membrane-are fluid related elements in sarcopterygians, which interact with a set of unique features in Latimeria. Finally, we explore the possible interaction between the unique intracranial joint, basicranial muscle, and an enlarged notochord that allows fluid flow to the foramen magnum and the cochlear aqueduct which houses a comparatively small brain.
... Coiling and elongation were also likely related to the loss of the lagenar macula, a vestibular sensory epithelium that sits at the apical end of the cochlear duct in nonmammals and monotremes [7]. Early therian mammals also evolved additional bony support around the cochlear ganglion and adjacent to the organ of Corti [14]. This may have initially contributed to more efficient impedance matching with the middle ear and later on to the detection of high frequency sounds [9]. ...
... The tegmentum vasculosum and the stria vascularis, evolved independently [25] and maintain the high endolymphatic potential in the auditory organs of birds (~10 mV;~30 mV in owls) and mammals (~100 mV), respectively. In mammals, the stria vascularis emerged gradually after the split of the monotreme branch pre-dating high-frequency sound sensitivity [14,25]. ...
After the transition to life on land, tympanic middle ears emerged separately in different groups of tetrapods, facilitating the efficient detection of airborne sounds and paving the way for high frequency sensitivity. The processes that brought about high-frequency hearing in mammals are tightly linked to the accumulation of coding sequence changes in inner ear genes; many of which were selected during evolution. These include proteins involved in hair bundle morphology, mechanotransduction and high endolymphatic potential, somatic electromotility for sound amplification, ribbon synapses for high-fidelity transmission of sound stimuli, and efferent synapses for the modulation of sound amplification. Here, we review the molecular evolutionary processes behind auditory functional innovation. Overall, the evidence to date supports the hypothesis that changes in inner ear proteins were central to the fine tuning of mammalian hearing.
... The cochlear canal of the Teete petrosal shows an unusual co-existence of both the canaliculus for the aquaeductus cochleae and a posteromedial opening of a fully separated aquaeductus cochleae from the fenestra cochleae, and an open sulcus in the medial corner of the fenestra cochleae leading to the jugular notch. A perilymphatic foramen as shared opening of fenestra cochleae and perilymphatic duct and the presence of an open sulcus for the perilymphatic duct are considered to be two ancestral characters of mammaliamorphs (Kermack et al. 1981;Crompton and Luo 1993;Lillegraven and Hahn 1993;Wible andHopson 1993, 1995;Rougier et al. 1996;Luo et al. 2001), but the occurrence of a well separated aquaeductus cochleae and a fenestra cochleae is regarded as a derived character for the cladotherians including living therians (Wible 1990;Wible et al. 2001;Ekdale 2013;Luo et al. 2016;Harper and Rougier 2019;Luo and Manley 2020). The preserved sulcus in the medial corner of the fenestra cochleae of the Teete petrosal might be a pathway for a vein that formerly was the accompanying vein of the perilymphatic duct. ...
... The preserved sulcus in the medial corner of the fenestra cochleae of the Teete petrosal might be a pathway for a vein that formerly was the accompanying vein of the perilymphatic duct. A perilymphatic foramen is also found in the eutriconodontan Priacodon (Rougier et al. 1996;Harper and Rougier 2019), but an additional separated aquaeductus cochleae like in the Teete petrosal is not present in Priacodon. Interestingly, in both a tentatively assigned gobiconodontid and a trechnotherian petrosal (i.e., Höövör 1 and 2 in Harper and Rougier 2019) from the Lower Cretaceous of Mongolia a fully separated aquaeductus cochleae was observed using virtual 3D endocast reconstruction (Harper and Rougier 2019). ...
... A perilymphatic foramen is also found in the eutriconodontan Priacodon (Rougier et al. 1996;Harper and Rougier 2019), but an additional separated aquaeductus cochleae like in the Teete petrosal is not present in Priacodon. Interestingly, in both a tentatively assigned gobiconodontid and a trechnotherian petrosal (i.e., Höövör 1 and 2 in Harper and Rougier 2019) from the Lower Cretaceous of Mongolia a fully separated aquaeductus cochleae was observed using virtual 3D endocast reconstruction (Harper and Rougier 2019). In an earlier work the trechnotherian petrosal was described to lack an aquaeductus cochleae (Rougier et al. 1996), but this was corrected by Harper and Rougier (2019). ...
A mammalian petrosal from the Lower Cretaceous Teete locality in Yakutia (Russia) shows a prominent and complex system of venous channels in the bony wall of the pars cochlearis surrounding the straight cochlear canal. This complex venous system is distinctive and more strongly developed than in other mammalian petrosals. A bony ridge is present on the ventral side of the cochlear canal endocast, continuing from between fenestra vestibuli and fenestra cochleae in anterior direction. This ridge corresponds to the position of the scala tympani, and is similar to the secondary bony lamina of crown therians, but lacks the sharp laminar edge. The fenestra cochleae is separate from the canal for the aquaeductus cochleae (derived), but the fenestra retains a deep sulcus that resembles the perilymphatic sulcus (plesiomorphic). The fenestra cochleae is oval shaped and deep. The straight cochlear canal with a ridge on the ventral side strongly resembles that of eutriconodontans like Priacodon fruitaensis from the Upper Jurassic of North America. However, thick and extensive venous channels in the pars cochlearis are otherwise known from docodontans. In the Teete petrosal the channels are even more developed, and resemble the pattern recently reported from possible haramiyidan petrosals from the Middle Jurassic of western Siberia (Russia). Both eutriconodontan and haramiyidan dental remains are known from the Teete locality beside that of tritylodontids and docodontans.
... Much more complete material of small vertebrates, including mammals, has been found in sites in western Colorado (Fruita Paleontological Area, Callison 1987) and northeastern Utah (Dinosaur National Monument, Engelmann and Callison 1998), but only a few of these specimens have been formally described. Notable among these, the "plagiaulacidan" multituberculate Glirodon grandis is known by a partial skull from Dinosaur National Monument (Engelmann and Callison 1999), a skull fragment and postcrania of the triconodontid Priacodon fruitaensis (Rasmussen and Callison 1981;Rougier et al. 1996;Engelmann and Callison 1998;Harper and Rougier 2019;Jäger et al. 2020), and a nearly complete, articulated skeleton of the highly specialized and enigmatic Fruitafossor windscheffeli Luo and Wible, 2005, have been described from Fruita (Luo and Wible 2005). ...
We describe two skull fragments of a new morganucodontan from the Cisco Mammal Quarry (Upper Jurassic Morrison
Formation), preserving portions of the palate and snout in excellent 3D detail as well as the complete upper postcanine
dentition. Morganucodontans are best known by isolated elements and relatively complete skulls of several species of
Morganucodon from the Lower Jurassic of Wales and China; this group is fundamental to our understanding of the
early evolution of mammals. Cifellilestes ciscoensis gen. et sp. nov. possesses derived features of the snout paired with
plesiomorphic construction of the molars; the distal premolars are complex and there is an unusually low count (two) of
strongly imbricated molars. This character combination expands craniodental variation for the group. We sampled mud�stone from the Cisco Mammal Quarry for ash-fall zircon analysis and obtained a date of 151.50 ± 0.28 Ma. This dates the
locality to the earliest Tithonian and slightly younger than other major dated mammal-bearing localities in the Morrison
Formation. Cifellilestes represents one of the youngest members of this group and extends the record of morganucodon�tans in North America by more than 30 Ma. Morganucodontans are a rare component of Late Jurassic faunas but display
surprising dental diversity through variations in a tooth count and cusp morphology of a deeply conserved, generalized
mammalian tooth pattern, which was fully established in brasilodontid (non-mammalian) ancestors at least 80 my prior.
... Several changes in inner ear morphology (e.g. presence of ossified primary and secondary laminae, ossified ganglion canal, absence of a lagena macula) have been reconstructed for Cladotheria based on the presences of these features in dryolestoids and stem therians [16][17][18][19]. However, direct evidence from Early Cretaceous therians is still limited. ...
... Cokotherium thus bridges a gap between inner ear morphology of stem therians (e.g. dryolestoids, Vincelestes, Höover petrosals) [17][18][19]40] and Late Cretaceous eutherians including Kulbeckia, Zalambdalestes, Ukhaatherium, Uchkudukodon, zhelestids and eutherians from Bug Creek Anthills [41][42][43][44][45]. Similar to most Cretaceous eutherians the cochlear canal of Cokotherium completes a single coil (360°). This fairly consistent degree of coiling is only exceeded by some zhelestids and Kulbeckia. ...
... By contrast to Prokennalestes and other basal cladotherians, Cokotherium has an extrabullar course of the internal carotid artery similar to that of the Late Cretaceous Ukhaatherium, Asioryctes, Kennalestes and Zalambdalestes [10]. Cokotherium retains the plesiomorphic condition of a large inferior petrosal sinus that wraps around the cochlear canal similar to more basal mammals [19,46,47]. An intrapetrosal inferior petrosal sinus has also been reconstructed for Prokennalestes but a comparable structure is not known for extant therians [39]. ...
Here we report on a new Early Cretaceous eutherian represented by a partial skeleton from the Jiufotang Formation at Sihedang site, Lingyuan City, Liaoning Province that fills a crucial gap between the earliest eutherians from the Yixian Formation and later Cretaceous eutherians. The new specimen reveals, to our knowledge for the first time in eutherians, that the Meckelian cartilage was ossified but reduced in size, confirming a complete detachment of the middle ear from the lower jaw. Seven hyoid elements, including paired stylohyals, epihyals and thyrohyals and the single basihyal are preserved. For the inner ear the ossified primary lamina, base of the secondary lamina, ossified cochlear ganglion and secondary crus commune are present and the cochlear canal is coiled through 360°. In addition, plesiomorphic features of the dentition include weak conules, lack of pre- and post-cingula and less expanded protocones on the upper molars and height differential between the trigonid and talonid, a large protoconid and a small paraconid on the lower molars. The new taxon displays an alternating pattern of tooth replacement with P3 being the last upper premolar to erupt similar to the basal eutherian Juramaia . Parsimony analysis places the new taxon with Montanalestes , Sinodelphys and Ambolestes as a sister group to other eutherians.
This article is part of the theme issue ‘The impact of Chinese palaeontology on evolutionary research’.
... 6d) and even greater in the paulchoffatiid Pseudobolodon (180°; Schultz and Martin 2015). Although some variation in the degree of curvature of the cochlear canal appears to be present in multituberculates, the cochlear canal appears to be much less curved in multituberculates than in gondwanatherians (210°, Hoffmann and Kirk 2020), basal cladotherians (> 270°, Rougier et al. 1992;Ruf et al. 2009;Luo et al. 2011Luo et al. , 2012Harper and Rougier 2019), monotremes (> 140°, Schultz et al. 2017), and docodontans (Ruf et al. 2013;Panciroli et al. 2018), but is greater than in the eutriconodontan Priacodon and the stem therian Höövör petrosals (Harper and Rougier 2019). The relatively gentle bending of the cochlear canal in some derived multituberculates might be an apomorphic feature of these groups as lateral bending to a greater degree (< 140°) is common in mammaliaforms and appears to also be present in the most basal multituberculates, paulchoffatiids. ...
... 6d) and even greater in the paulchoffatiid Pseudobolodon (180°; Schultz and Martin 2015). Although some variation in the degree of curvature of the cochlear canal appears to be present in multituberculates, the cochlear canal appears to be much less curved in multituberculates than in gondwanatherians (210°, Hoffmann and Kirk 2020), basal cladotherians (> 270°, Rougier et al. 1992;Ruf et al. 2009;Luo et al. 2011Luo et al. , 2012Harper and Rougier 2019), monotremes (> 140°, Schultz et al. 2017), and docodontans (Ruf et al. 2013;Panciroli et al. 2018), but is greater than in the eutriconodontan Priacodon and the stem therian Höövör petrosals (Harper and Rougier 2019). The relatively gentle bending of the cochlear canal in some derived multituberculates might be an apomorphic feature of these groups as lateral bending to a greater degree (< 140°) is common in mammaliaforms and appears to also be present in the most basal multituberculates, paulchoffatiids. ...
Taeniolabis taoensis is an iconic multituberculate mammal of early Paleocene (Puercan 3) age from the Western Interior of North America. Here we report the discovery of significant new skull material (one nearly complete cranium, two partial crania, one nearly complete dentary) of T. taoensis in phosphatic concretions from the Corral Bluffs study area, Denver Formation (Danian portion), Denver Basin, Colorado. The new skull material provides the first record of the species from the Denver Basin, where the lowest in situ specimen occurs in river channel deposits ~730,000 years after the Cretaceous-Paleogene boundary, roughly coincident with the first appearance of legumes in the basin. The new material, in combination with several previously described and undescribed specimens from the Nacimiento Formation of the San Juan Basin, New Mexico, is the subject of detailed anatomical study, aided by micro-computed tomography. Our analyses reveal many previously unknown aspects of skull anatomy. Several regions (e.g., anterior portions of premaxilla, orbit, cranial roof, occiput) preserved in the Corral Bluffs specimens allow considerable revision of previous reconstructions of the external cranial morphology of T. taoensis. Similarly, anatomical details of the ascending process of the dentary are altered in light of the new material. Although details of internal cranial anatomy (e.g., nasal and endocranial cavities) are difficult to discern in the available specimens, we provide, based on UCMP 98083 and DMNH.EPV 95284, the best evidence to date for inner ear structure in a taeniolabidoid multituberculate. The cochlear canal of T. taoensis is elongate and gently curved and the vestibule is enlarged, although to a lesser degree than in Lambdopsalis.
... Several studies provide additional information about cladotheres, in particular the exquisite descriptions of the ear regions of Henkelotherium (Ruf et al. 2009) and Dryolestes leirensis (Luo et al. 2012) and other cladotheres (Hughes et al. 2015). Harper and Rougier (2019) reviewed middle ear morphology in stem therians. We have incorporated characters and states reflecting these contributions. ...
We describe the first maxillae and additional new specimens of Reigitherium bunodontum, a small meridiolestidan from the Late Cretaceous La Colonia Formation, Patagonia, Argentina. The new material supports a dental formula of I?, C1, P4, M3, resolves postcanine positional uncertainty and corrects previous interpretations. Our phylogeny recovers Reigitherium as a meridiolestidan allied to other bunodont Mesungulatoidea, as the sister group of the Paleocene Peligrotherium. Posterior premolars/molars of Reigitherium, and to a smaller degree Peligrotherium, are dominated by an incomplete transverse ridge running between the protoconid-metaconid in the lowers and the paracone-stylocone in the uppers, semi-symmetrical basins developing mesially and distally from these central ridges. The trigonid-derived single transverse crest results from a mesial shift of the robust metaconid, an enhancement of the basin crest stretching from the protoconid/metaconid, and a shallower trigonid basin. The mesungulatoid condition, with its complete absence of talonid, contrasts sharply with that of therians with lophs, or transverse ridges, which involved at least one talonid-derived loph resulting in two transverse crests per tooth. Mesungulatoid meridiolestidans achieved complex tooth-on-tooth occlusion with a predicted increase in herbivory/omnivory, departing from the traditional sharp-cusp insectivores plesiomorphic for meridiolestidans and Mesozoic mammals in general. Reigitherium’s dramatic remodeling of the primitive meridiolestidan molar morphology, extensive continuous occlusal surface, accessory cuspules, and highly textured crenulated enamel illustrates one of most distinctive adaptations to herbivory among Mesozoic mammals.
... Dr. J. R. Wible has pioneered the incorporation of these characters in 1 3 the evolutionary study of mammals; we see our paper here as a direct consequence of his influence and a fitting tribute to an exceptional scientist. Extraction of a digital endocast of the bony labyrinth within the petrosal can be used to study morphological evolution and, under certain conditions, auditory capabilities can be estimated (Ekdale 2016;Harper and Rougier 2019) and locomotor habits inferred (Pfaff et al. 2015;Ekdale 2016). In fact, the advent of CT technology has resulted in a resurgence in the study of petrosal anatomy by opening classic and historic specimens to endocast extraction and a great increase in available data. ...
... Living monotremes, on the other hand, have a straight osseous cochlear canal containing a hook-shaped cochlear duct (Denker 1901;Alexander 1904;Schultz et al. 2016). Stem mammalian groups possess a similarly straight or slightly curved cochlear duct, as do some extinct Crown mammalian lineages (e.g., Ruf et al. 2009;Luo et al. 2011a, b;Hughes et al. 2015;Harper and Rougier 2019). Juramaia is a therian of reported Jurassic age (Luo et al. 2011b). ...
... Eutriconodontans.-The eutriconodontan Priacodon is known by a relatively well-preserved petrosal from the Morrison Formation in the nearby Fruita Paleontological Area in Colorado (Rougier et al. 1996;Harper and Rougier 2019; Fig. 9d here). The perilymphatic groove is open to the middle ear cavity lateral to the perilymphatic foramen, but the promontorium is tubular and contains a straight cochlear canal. ...
The Morrison Formation bears one of the most diverse assemblages of Late Jurassic terrestrial vertebrates worldwide. A recently discovered site in eastern Utah, the Cisco Mammal Quarry (CMQ), shows excellent preservation and small vertebrates (particularly mammals) collected thus far are very diverse. Two isolated petrosals from the CMQ, representing the same taxon, are described here based on CT data. Several plesiomorphic mammalian characters are present, including a horizontal crista interfenestralis, unfloored cavum supracochleare, a perilymphatic foramen, and an open perilymphatic groove. By contrast, a well-developed tractus foraminosus is present for distribution of cochlear nerve fibers and the cochlear endocast makes one full turn, as in early therians such as Prokennalestes. This latter derived condition is unrecorded in the Jurassic; the mammal fauna characteristic of the Morrison is dominated by docodontans, eutriconodontans, dryolestoids (dryolestids and paurodontids), and multituberculates. All currently-known examples of these high-level taxa possess a relatively short, straight or curved cochlea. Therians are not known from the Morrison Formation, but these petrosals invite comparison with some stem therians of South America, the meridiolestidans, which also bear a full coil, but differ from the Morrison petrosals in aspects of external morphology. While the taxonomic affinity of the specimens we describe here remains uncertain, the unique combination of petrosal characters is evidence that our current view of petrosal/inner ear evolution is oversimplified. With the presence of derived and plesiomorphic features character conflict is unavoidable, pointing to a complex interplay of external petrosal osteology under the influence of neurovascular and middle ear evolution and the (perhaps more independent) biophysical demands of inner ear function.
... Recently, intensive descriptive morphology has gained a renewed importance in phenomic systematics (e.g., O'Leary, 2010;Muizon et al., 2018;Harper and Rougier, 2019). This approach requires thorough examination of relevant extant taxa, for which exhaustive detail can be collected on both hard and soft tissues. ...
... Here used to refer to both arteria and vena diploetica magna (vasa, L. "vessels"). As defined by Tandler (1899), arteria diploetica magna is an anastomotic link between occipital artery and caudal branch of stapedial ramus superior, supplying (depending on taxon) cranioorbital artery, parietosquamosal arteries, branches to dura mater, and others (see Hyrtl, 1854;Wible, 1984Wible, , 1987Harper and Rougier, 2019). Wible (1987) defines arteria diploetica magna as an inherent part of stapedial system, but morphologically it is better understood as a link with the latter and the occipital artery (or posterior auricular artery in some cases). ...
In 1933 George G. Simpson described a remarkably complete skull of Trigonostylops, an Eocene South American native ungulate (SANU) whose relationships were, in his mind, quite uncertain. Although some authorities, such as Florentino Ameghino and William B. Scott, thought that a case could be made for regarding Trigonostylops as an astrapothere, Simpson took a different position, emphasizing what would now be regarded as autapomorphies. He pointed out a number of features of the skull of Trigonostylops that he thought were not represented in other major clades of SANUs, and regarded these as evidence of its phyletic uniqueness. Arguing that the lineage that Trigonostylops represented must have departed at an early point from lineages that gave rise to other SANU orders, Simpson reserved the possibility that Astrapotheriidae might still qualify (in modern terms) as its sister group. Even so, he argued that the next logical step was to place Trigonostylops and its few known allies in a separate order, Trigonostylopoidea, coordinate with Astrapotheria, Notoungulata, Litopterna, and Pyrotheria. Simpson's classification was not favored by most later authors, and in recent decades trigonostylopids have been almost universally assigned to Astrapotheria. However, his evaluation of the allegedly unique characters of Trigonostylops and its allies has never been systematically treated, which is the objective of this paper. Using computed tomography, the skull of Trigonostylops is compared, structure by structure, to a variety of representative SANUs as well as extant perissodactylans (which together comprise the clade Panperissodactyla) and the "condylarthran" Meniscotherium. In addition to placing Simpson's character evaluations in a comparative context, we also provide detailed assessments of many vascular and pneumatization-related feature of panperissodactylans never previously explored. Overall, we found that this new assessment strengthened the placement of Trigonostylops within a monophyletic group that includes Astrapotherium and Astraponotus, to the exclusion of other SANU clades. Although Trigonostylops cannot be considered as morphologically distinct or unusual as Simpson thought, our comparative and phylogenetic analyses have helped to generate a number of hypotheses about character evolution and function in SANUs that may now be fruitfully tested using other taxon combinations.
