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WhetherNanotyrannus lancensisrepresents a distinct taxon or an immatureTyrannosaurus rexis a decades-long controversy. TheN. lancensisholotype (CMNH 7541) is an isolated skull and ceratobranchials, but limb osteohistology ofNanotyrannus-like individuals implies that these individuals were immatureTyrannosaurus, suggesting that theN. lancensisholotype is also immature. We demonstrate that ceratobranchial (“hyoid”) histology is useful for ontogenetic assessment in extant and extinct archosaurs. The ceratobranchial histology of theN. lancensisholotype indicates that it was nearing or had reached skeletal maturity, suggesting that it is taxonomically distinct from the coevalTyrannosaurus rexand that Hell Creek (and equivalent) ecosystems supported a diverse assemblage of predatory dinosaurs approaching the Cretaceous-Paleogene extinction. 

Not AvailableThe earliest record of tooth antecedents and the tissue dentine1,2, an early-vertebrate novelty, has been controversially represented by fragmentary Cambrian fossils identified as Anatolepis heintzi3,4,5. Anatolepis exoskeletons have the characteristic tubules of dentine that prompted their interpretation as the first precursors of teeth3, known as odontodes. Debates over whether Anatolepis is a legitimate vertebrate6,7,8 have arisen because of limitations in imaging and the lack of comparative exoskeletal tissues. Here, to resolve this controversy and understand the origin of dental tissues, we synchrotron-scanned diverse extinct and extant vertebrate and invertebrate exoskeletons. We find that the tubules of Anatolepis have been misidentified as dentine tubules and instead represent aglaspidid arthropod sensory sensilla structures9,10. Synchrotron scanning reveals that deep ultrastructural similarities between odontodes and sensory structures also extend to definitive vertebrate tissues. External odontodes of the Ordovician vertebrate Eriptychius11,12,13 feature large dentine tubules1 that are morphologically convergent with invertebrate sensilla. Immunofluorescence analysis shows that the external odontodes of extant chondrichthyans and teleosts retain extensive innervation suggestive of a sensory function akin to teeth14,15,16. These patterns of convergence and innervation reveal that dentine evolved as a sensory tissue in the exoskeleton of early vertebrates, a function retained in modern vertebrate teeth16. Middle-Ordovician fossils now represent the oldest known evidence for vertebrate dental tissues. 

{"Abstract":["Whether Nanotyrannus lancensis, a putative species of small tyrannosaurid,\n represents a distinct taxon or an immature Tyrannosaurus rex is a\n decades-long controversy. The N. lancesis holotype is an isolated skull\n and ceratobranchials, but limb osteohistology of Nanotyrannus-like\n individuals implies that these individuals were immature Tyrannosaurus,\n suggesting that the Nanotyrannus holotype is also immature. We demonstrate\n for the first time that ceratobranchial (‘hyoid’) histology is useful for\n ontogenetic assessment in extant and extinct archosaurs. The\n ceratobranchial histology of the N. lancensis holotype indicates that it\n was nearing or had reached skeletal maturity, suggesting that it is\n taxonomically distinct from the coeval Tyrannosaurus rex and that Hell\n Creek (and equivalent) ecosystems supported a diverse assemblage of\n predatory dinosaurs approaching the K-Pg extinction event."],"TechnicalInfo":["# Data for: A diminutive Tyrannosaur lived alongside *Tyrannosaurus rex*\n [https://doi.org/10.5061/dryad.sqv9s4ndn](https://doi.org/10.5061/dryad.sqv9s4ndn) ## Description of the data and file structure The filenames of the associated whole-slide histological images reflect the taxon, specimen number, element, sampling location (if applicable), light regime, and magnification; "Genus_species_Museum####_Element_SamplingLocation_LightAbbreviation_Magnification.tif" 'Hyoid' as an element refers to ceratobranchial I. Light regime abbreviations: PP - Plane-polarized light XP - Cross-polarized light XPR - Cross-polarized light with a lambda retarder R - Reflected light OSEM - Optical shadow effect mode (a feature in the Keyence VHX-7100 digital microscope) Light - in addition to one of the above light regimes, this scan included either increasing brightness either digitally or by wetting the slide Example: Struthio_camelus_YPM112434_rightHyoid_cranialsample_XPR_200x.tif Ostrich (*Struthio camelus*), specimen YPM 112434, right hyoid (ceratobranchial I), the cranial sampling location, cross-polarized light with a lambda retarder, 200x magnification All filenames that include "Wagner" are images of slides made via automated grinder by Wagner Petrographics, Inc. All other slides were made by hand. ### Files and variables #### File: Alligator_mississippiensis.zip **Description:** Whole-slide images of *Alligator mississippiensis* bone histology #### File: Struthio_camelus.zip **Description:** Whole-slide images of *Struthio camelus* bone histology #### File: Allosaurus_fragilis.zip **Description:** Whole-slide images of *Allosaurus fragilis* bone histology #### File: Allosaurus_jimmadseni.zip **Description:** Whole-slide images of *Allosaurus jimmadseni* bone histology #### File: Coelophysis_bauri.zip **Description:** Whole-slide images of *Coelophysis bauri* bone histology #### File: Edmontosaurus_sp.zip **Description:** Whole-slide images of *Edmontosaurus sp.* bone histology #### File: Paleosuchus_palpebrosus.zip **Description:** Whole-slide images of *Paleosuchus palpebrosus* bone histology #### File: Eutyrannosaurians.zip **Description:** Whole-slide images of *Tyrannosaurus rex* (LACM specimens) and an indeterminate eutyrannosaur (SWAU individual) #### File: Nanotyrannus_lancensis.zip **Description:** Whole-slide images of *Nanotyrannus lancensis* bone histology. One whole slide in the "Caudal" subfolder, with "CGMtraced" in the file name, contains a whole slide image with a cortical growth mark (CGM) traced around the cortex, as in Fig. S1. The whole-slide version of this figure permits others to trace this CGM on the large, high-resolution image."]} 

Abstract Squamata is the most diverse clade of terrestrial vertebrates. Although the origin of pan-squamates lies in the Triassic, the oldest undisputed members of extant clades known from nearly complete, uncrushed material come from the Cretaceous. Here, we describe three-dimensionally preserved partial skulls of two new crown lizards from the Late Jurassic of North America. Both species are placed at the base of the skink, girdled, and night lizard clade Pan-Scincoidea, which consistently occupies a position deep inside the squamate crown in both morphological and molecular phylogenies. The new lizards show that several features uniting pan-scincoids with another major lizard clade, the pan-lacertoids, in trees using morphology were convergently acquired as predicted by molecular analyses. Further, the palate of one new lizard bears a handful of ancestral saurian characteristics lost in nearly all extant squamates, revealing an underappreciated degree of complex morphological evolution in the early squamate crown. We find strong evidence for close relationships between the two new species and Cretaceous taxa from Eurasia. Together, these results suggest that early crown squamates had a wide geographic distribution and experienced complicated morphological evolution even while the Rhynchocephalia, now solely represented by the tuatara, was the dominant clade of lepidosaurs. 

Abstract Chemosensation is the most ubiquitous sense in animals, enacted by the products of complex gene families that detect environmental chemical cues and larger-scale sensory structures that process these cues. While there is a general conception that olfactory receptor (OR) genes evolve rapidly, the universality of this phenomenon across vertebrates, and its magnitude, are unclear. The supposed correlation between molecular rates of chemosensory evolution and phenotypic diversity of chemosensory systems is largely untested. We combine comparative genomics and sensory morphology to test whether OR genes and olfactory phenotypic traits evolve at faster rates than other genes or traits. Using published genomes, we identified ORs in 21 tetrapods, including amphibians, reptiles, birds, and mammals and compared their rates of evolution to those of orthologous non-OR protein-coding genes. We found that, for all clades investigated, most OR genes evolve nearly an order of magnitude faster than other protein-coding genes, with many OR genes showing signatures of diversifying selection across nearly all taxa in this study. This rapid rate of evolution suggests that chemoreceptor genes are in “evolutionary overdrive,” perhaps evolving in response to the ever-changing chemical space of the environment. To obtain complementary morphological data, we stained whole fixed specimens with iodine, µCT-scanned the specimens, and digitally segmented chemosensory and nonchemosensory brain regions. We then estimated phenotypic variation within traits and among tetrapods. While we found considerable variation in chemosensory structures, they were no more diverse than nonchemosensory regions. We suggest chemoreceptor genes evolve quickly in reflection of an ever-changing chemical space, whereas chemosensory phenotypes and processing regions are more conserved because they use a standardized or constrained architecture to receive and process a range of chemical cues.