麻豆传媒

Exquisitely preserved monograptid faunas from the Cape Phillips Formation, Arctic Canada, provide valuable information regarding their morphologic and taxonomic characteristics. These chemically isolated monograptids assist in refining the identification of monograptid genera and their phylogenetic relationships within the Early Silurian radiation event. Without astogenetic criteria to define monograptid groups, definition of monograptid genera through the identification of derived thecal characteristics becomes the most reliable method. Monograptid genera and their defining apomorphies include: Atavograptus- simple geniculate thecae with a large dorso-ventral overlap; Pribylograptus-lateral apertural thecal lappets; Coronograptus-flared thecal apertures; Lagarograptus-ventral apertural processes (lips); New Genus - dorsal hooks (retroverted dorsal thecal wall only); Monoclimacis- geniculate thecae with dorsal hoods; and Pristiograptus- simple, slightly to non-geniculate thecae with a minimal dorso-ventral overlap.

Phylogenetic relationships of these genera are derived using stratophenetics -- a method of parsimoniously plotting the distribution of derived thecal characters against stratigraphy to define evolutionary lineages. Atavaograptus, the oldest and most morphologically primitive monograptid genus, forms the monophyletic base on which the Monograptidae arose. The first of two distinct lineages within Atavograptus, A. ceryx - A. atavus, defined on their relatively short siculae, sigmoidal thecal shape and lack of genicular hoods, likely gave rise to Pribylograptus through Pb. n. sp. A, a species with thecae identical to A. atavus except for the apertural lappets, and to Pristiograptus by the loss of geniculation and the reduction of dorso-ventral thecal overlap. The second Atavograptus lineage, A. primitivus - A. praestrachani - A. strachani - A. pristinus, likely gave rise to Coronograptus and Lagarograptus through a conservation of long siculae in both, the development of thecal apertural flare and conservation of simple, genicular hoods in Coronograptus, and through a derivation of ventral apertural processes in Lagarograptus. It is possible that Lagarograptus represents a set of independently derived species from different ancestors. New Genus likely arose from A. strachani through a derivation of dorsal hooks and a conservation of sicular size. Monoclimacis likely arose, monophyletically, from a New Genus ancestor (N. Gen. n. sp. A) through a retraction of the dorsal hooks in one lineage (Mcl. n. sp. a - Mcl. imago - Mcl. n. sp. B), and through a conservation of dorsal hooks proximally with the derivation of dorsally hooded thecae distally (Mcl. n. sp. C - Mcl. crenularis). In each case, "monoclimacid" thecae became strongly geniculate. The phylogenetic analysis suggests a monophyletic origin for each Early to Middle Llandovery monograptid genus comprising this study.

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