Alalcomenaeus

Alalcomenaeus is one of the most widespread and longest-surviving arthropod genera of the Early and Middle Cambrian.^[1] Known from over 300 specimens in the Burgess Shale and the Chengjiang biota. It is a member of the family Leanchoiliidae^[2] in the group Megacheira.

Alalcomenaeus had three median eyes; two stalked, more lateral eyes; a triflagellate great appendage; and two more head appendages posterior to that. Like its body appendages, these were biramous—their inner branch was spiny, segmented, flexible and leg-like, while the outer portion had a large surface area and resembled a flap.

Alalcomenaeus reached about 6 cm in length, although many smaller specimens are known.^[1] Its head was covered with a shield, and its eleven body segments were also covered with an exoskeleton. Its body terminated with a paddle-like telson ("tail") which probably helped to propel the organism; this ended with long flat spikes in the plane of the tail fin.^[1]

The organism probably swam, wafting its outer flap-limbs in waves along its body to gain propulsion, in the manner of Anomalocaris.^[1] Its inner limb branches do not appear to be optimised for walking, although it is possible they helped the organism move along the sea floor.^[1] A variety of other functions have been suggested, such as clinging to algal fronds; they seem best suited to grabbing onto, and tearing up, other animals, suggesting that the organism was probably a scavenger.^[1] However, its large eyes and the long flagella on its great appendages, combined with its large feeding apparatus and the spines on its inner limb branches, are more consistent with a predatory lifestyle, and the most recent interpretation has it feeding on organisms that lived on or in the surface of the sea floor.^[1]

Inicialmente se pensó que Alalcomenaeus pertenecía al grupo troncal de los crustáceos y se lo colocó en un clado con Leanchoilia , Actaeus y Yohoia . ^{[ 1 ]} Posteriormente se sugirió que estaba relacionado con los opabiníidos , principalmente en base a su gran apéndice. ^{[ 1 ]} Análisis más recientes lo ubican junto con Leanchoilia y Yohoia , así como con Sanctacaris , Habelia , Sarotrocercus y Sidneyia , en algún lugar dentro de los aracnomorfos . La cercanía de Sanctacaris y Leanchoilia ha sido respaldada por trabajos posteriores, ^{[ 3 ]} y Actaeus a menudo se excluye de dichos trabajos debido a que es poco conocido. ^{[ 1 ]} Actualmente, se incluye dentro de Megacheira . ^{[ 4 ]}

Alalcomenaeus es muy raro en la cantera Walcott de la Formación Burgess Shale , por lo que fue descrito basándose en media docena de especímenes. A medida que se desenterraron otros afloramientos de la Formación Burgess Shale, se hizo evidente que la criatura era, de hecho, un miembro dominante de la fauna. También se ha descubierto en Chengjiang y en Utah , lo que le confiere un amplio rango estratigráfico . ^{[ 1 ]} Incluyendo 596 juveniles, se conocen 618 especímenes de Alalcomenaeus del lecho Greater Phyllopod , donde constituyen el 1,2% de la comunidad. ^{[ 5 ]}

• " Alalcomenaeus cambricus " . Galería de fósiles de Burgess Shale . Museo Virtual de Canadá. 2011. Archivado del original el 12 de noviembre de 2020. Consultado el 21 de enero de 2023 .
• Imágenes de fósiles del río Chengjiang.