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Morphogenesis of the Metazoa

Goal: See Dynamic Paleontology (Springer, 2016) for details.

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Mark McMenamin
added a research item
An undescribed new species of vetulicolian (Vetulicolian n. sp.) from the Early Cambrian Chengjiang Biota (Burgess Shale-Type (BST) deposit; Maotianshan Shale, 518 Ma) represents the earliest example of ‘angelfish’ body form in a nectobenthic deuterostome. The preserved length of the animal is 5 cm, with an anterior body region 4 cm deep. Sagittal plane expansion of Vetulicolian n. sp. provides a striking morphological contrast to the dorsal-ventral flattening of the anterior section of the Burgess Shale vetulicolid Banffia constricta, thereby expanding our knowledge of the range of form in this important group of early deuterostomes. A unique ‘dorsal fin’ in Vetulicolian n. sp. mirrors the profile of the ventral cuticular plate. The anterior end of the latter forms a ‘wedge beak’ similar to that described from Vetulicola cuneata. The wedge beak-bearing cuticular plate consists of three articulating sections, implying a dentary-like mobility in what appears to be the earliest beak-like ‘jaw’ structure reported from a deuterostome. The triangular, angelfish-like body form of Vetulicolian n. sp., complete with a protruding lower ‘jaw,’ is well known from the freshwater angelfish Pterophyllum, the marine angelfish Pomacanthus, and a variety of fossil fishes including Eoplatex papilio and the pycnodont Proscinetes. Lacking pectoral fins or their functional equivalents, Vetulicolian n. sp. would presumably have been incapable of labriform swimming, and would have relied instead on less-efficient tail propulsion.
Mark McMenamin
added 2 research items
Marsupials first appear in Cretaceous North America, their ancestors having arrived from Asia during an eastward migration of Mesozoic metatherians. By the end of the Mesozoic, the North American metatherians had developed into large (over one meter long) animals with a powerful bite force, partly a function of hypertrophied premolars (p3) in some species. The inflated premolar is associated with reappearance of zahnreihen in a Cretaceous metatherian mammal, Didelphodon coyi. The metatherian migration begun in Asia continued through North America, to South America, to Antarctica and on to Australia where marsupials underwent a well known adaptive radiation in ‘splendid isolation’. Until recently it was thought that terrestrial mammaliaforms never reached New Zealand, as New Zealand had tectonically rifted away from Antarctica at 82 million years ago, supposedly before marsupials had reached Antarctica. Recent discoveries from limited exposures of Miocene strata in New Zealand near Otago show that mammaliaforms had indeed colonized and diversified in New Zealand apart from their ancestors in the rest of the Gondwanan continental diaspora. These zealanditherians inhabited the newly characterized continent Zealandia, and were apparently driven to extinction by habitat loss when most of Zealandia was submerged by the sea.
New specimens of the procolophonoid parareptile Barasaurus from the Permo-Triassic Sakamena Group of Madagascar show skin preservation in the form of scale patches. Based on its appendicular skeletal anatomy, Barasaurus was an aquatic form, the only known aquatic procolophonoid. Its squamation consisting of large (up to 4 mm greatest dimension on an animal approximately 30 cm in length), skink-like ventral scales suggest that this ventral scale configuration was well suited for existence in an aquatic habitat. The Barasaurus lifestyle was comparable to that of the crab-eating modern Madagascan skink (Amphiglossus astrolabi). Although they had the potential to do so since they survived the Permo-Triassic mass extinction, barasaurian procolophonoids did not diversify into a major group of Mesozoic Marine tetrapods.
Mark McMenamin
added 2 research items
A diverse fauna of Ediacarans from the Clemente Formation of northwestern Sonora, México, includes Pteridinium cf. P. simplex, a recumbent sand frond Beothukis cf. B. mistakensis, the kimberellomorph Kimberella cf. K. quadrata, the solzid kimberellomorph Zirabagtaria ovata n. gen. n. sp., the praecambridiid Palankiras palmeri n. gen. n. sp., Vendamonia truncata n. gen. n. sp., and the aculiferans Clementechiton sonorensis McMenamin and Fleury, 2016 and Korifogrammia clementensis n. gen. n. sp. The Clemente biota provides new data regarding the Ediacaran Cuticle Paradox, which holds that in spite of their apparent simplicity, the Ediacaran cuticle in fact hosted a highly complex morphogenetic field. In a corollary of Williston’s Law, this cuticle underwent successive simplification at the end of the Proterozoic.
An agglutinated animal (or possibly protist) of the Late Proterozoic Clemente Formation biota cemented an array of tourmaline crystals (trigonal prisms; schorl/dravite composition) to its dorsal surface, presumably as ballast, in the earliest known case of an agglutinated animal. Quantitative confirmation of the spatial association (clustering) of the tiny crystals is demonstrated here by means of Kappa (K) value analysis. This discovery of this Proterozoic “crystal creature” reveals both the earliest known case of agglutination, the earliest known case of monomineralogic agglutination, and the earliest known bioaccumulation of tabular crystals of the same mineral. This case compares to the preferential selection of ilmenite to form an agglutinated exoskeleton in the Cambrian agmatan Volborthella. It also compares to the preferential selection of muscovite flakes to form the agglutinated Cambrian worm tube Onuphionella. Agglutinated dorsal skeletons of the Proterozoic and Early Cambrian utilized selected mineral types, including white mica, ilmenite, anatase, and tourmaline. All of these minerals potentially afford protection from UV-B radiation, and may have been deployed on the dorsal surfaces of these early animals to serve as sunscreen.
Mark McMenamin
added 2 research items
Acid maceration and thin section analysis of archaeocyathan limestones of the Cerro Clemente and Cerro Rajón, Sonora, México, have yielded some of the most ancient sponge fossils reported from the North Ameri-can Cordillera. The sponge fossils are from Unit 3 of the Puerto Blanco Formation. The fossils include one of the earliest known specimens of Kiwetinokia WALCOTT. These new data indicate that diverse archaeocyaths and sponges occur in close association within Lower Cambrian limestones of Sonora. The new specimen of Kiwetinokia sp. provides evidence that geometrically complex spicules in Cambrian sponges evolved by the fusion of simpler spicule types. These are among the earliest sponges known to have lived in reef habitats.
Mark McMenamin
added 2 research items
An archaic human mandible ("Penghu Man" or Penghu 1, here named Homo tsaichangensis n. sp.) dredged from the Penghu Channel off the west coast of Taiwan shows intriguing similarities to a newly reported jaw fragment from Tegal, central Java, here assigned to Gigantopithecus cf. G. blacki. These Pleistocene creatures may have inhabited similar environments as part of what is referred to here as the Tegal Penghu Biogeographic Province. In addition to the primates, this province hosted the proboscidean Stegodon and the hyaenid Crocuta. Morphological similarity between the mandibular dentition of H. tsaichangensis n. sp. and Gigantopithecus blacki von Koenigswald, 1935 suggest shared dietary preferences; indeed, the increased size of teeth in H. tsaichangensis n. sp. contrasts sharply with the typical evolutionary trend of "dentognathic reduction" (Chang et al. 2015) in successive species within the genus Homo. The similarities represent an unusual case of convergent evolution between the genus Homo and an unrelated group of primates.
In addition to the traditional four classes of tetrapods, amphibians (Class Amphibia), reptiles (Class Reptilia), birds (Class Aves) and mammals (Class Mammalia), herein is introduced a new, fifth class of tetrapods—Class Paramphibia. Paramphibians are characterized by a unique morphogenetic field with field lines that diverge and converge in alternating bundles. Restricted to Lower Permian terrestrial strata, paramphibians are known from a single species, Permodiadonta oklahoma n. gen. n. sp., from the Ryan Formation in Oklahoma, USA. The fossil locality is west of the Waurika fossil site. Waurika is well known for its abundant, if typically disarticulated, Permian vertebrate fossils. The unique jaw and punctate tooth morphology of Permodiadonta shares an odd mix of characteristics with fish, amphibians and reptiles. This mixture of traits precludes placement of the new species into any of these well-known vertebrate groups. In addition to punctate teeth (unusual in a tetrapod), there is a particularly unusual feature seen in Permodiadonta's jaw—morphogenetic field lines that alternately converge and distend as they run across the edge of the jaw (as revealed by placement of the animal's dentition). This strange configuration of zahnreihen (considered here as a proxy for the animal's morphogenetic field) reflects a body form that is as unique for vertebrates as the twisted spindle morphology of the strange Early Cambrian echinoderm Helicoplacus is for Phylum Echinodermata. Helicoplacoids are placed in their own class (Class Helicoplacoidea), thus the placement here of Permodiadonta in its own tetrapod class (Paramphibia) is amply justified by the available evidence.
Mark McMenamin
added a project goal
See Dynamic Paleontology (Springer, 2016) for details.