Atlas of Crustacean Larvae

3. Fossil Larvae (Head Larvae, Nauplii, and Others) from the Cambrian in Orsten Preservation

Carolin Haug Joachim T. Haug Andreas Maas Dieter Waloszek Carolin Haug, Joachim T. Haug, Andreas Maas, and Dieter Waloszek general: The Orsten is a special type of phosphatic preservation of mainly arthropod fossils from the Cambrian (approximately 525 to 488 million years old) that provides an exceptional view on the early evolution of the Crustacea and their larval types. Many workers (e.g., Stein et al. 2008; J. Haug et al. 2009b, 2010a, 2010b) use the term Crustacea sensu lato (abbreviated s. l.) to encompass extant crustaceans and all earliest representatives of the Crustacea (the “Pan-Crustacea” of Walossek and Müller 1990), and that designation is also used here. Among these fossils are many crustacean larvae. The fossils are preserved in an uncompressed state, that is, in three dimensions and in extremely fine detail, with preservation of microstructures such as setae and setules, membranous areas (e.g., limb joints, mouth, and anus), and even faceted compound eyes (Maas et al. 2006). The full life cycle for most of these animals is unknown, but at least in some groups it included a very long series of gradually changing stages (anamorphic development). larval types: Apart from a few putative adult forms (e.g., species of †Skara; see K. Müller and Walossek 1985; Walossek and Müller 1998a, 1998b; Liu and Dong 2007), the majority of Orsten fossils are different larval and immature stages of crustacean species for which the adult most often is unknown (a putative exception is †Bredocaris admirabilis; K. Müller and Walossek 1988; Walossek and Müller 1998a). Head Larvae: Head larvae have an ocular somite, four appendage-bearing somites, and an undifferentiated trunk bud. The mouth is located at the rear of a prominent ventral sclerotization (the hypostome), another plesiomorphic feature. The anterior three appendages mainly work in conjunction for locomotion and feeding, a character new to the head larva of Crustacea s. l., in contrast to the euarthropod condition. This functional change was facilitated by the evolution of an exopod on the two post-antennular limbs, both with inwardly directed setation (figs. 3.1F, J; 3.2A, C; 3.3 C). This is one of the autapomorphies of Crustacea s. l. (J. Haug et al. 2009b, 2010a). This setation must have been very efficient , since it has been retained in the early larvae of all extant crustaceans. The fourth limb of the head larva (last head limb of the adult, shaped like the trunk limbs) was also functional and may have assisted in locomotion. Another limb-associated autapomorphy of Crustacea s. l., developed first in post–head larvae, is a special setiferous endite, the proximal endite, located medioproximally to the basipod (e.g., fig. 3.2A, C). The proximal endite might have facilitated better food transport toward the mouth along the ventral body surface. Subsequently, during crustacean evolution , the proximal endite became pre-displaced, appearing as a developed feature in the hatching head larvae, such as in †Martinssonia elongata (fig. 3.2K; J. Haug et al. 2010a). Development itself was strictly anamorphic, where new segments and appendages appeared progressively during ontogeny at the anterior end of the budding zone in front of the telson. Such head larvae are retained among Crustacea s. l. until the level of the Labrophora, the taxon containing the exclusively Cambrian Phosphatocopina and the crustacean crown group, the Eucrustacea (sensu Walossek 1999, corresponding to “Crustacea ” of various authors; also see J. Haug et al. 2010a; Olesen et al. 2011). Nauplius (Including Orthonauplius and Metanauplius): In our view, the Eucrustacea—not Crustacea s. l.—is characterized by a new type of hatching larva, the nauplius or orthonauplius. The orthonauplius possesses only four segments—the ocular segment and three appendage-bearing segments (Walossek and Müller 1990; Maas et al. 2003)—and a short hindbody with a dorsocaudal spine and initial furcal spines. This stage is followed by a number of metanauplius stages, where further segments develop progressively and the primordia of further limbs appear. This phase is then followed by a juvenile phase, leading to the adult. Larval and immature stages can be distinguished by their number of segments and appendages and by the progressive development of the adult morphology. The demarcation between the metanaupliar and 3 Fossil Larvae (Head Larvae, Nauplii, and Others) from the Cambrian in Orsten Preservation 18 Carolin Haug, Joachim T. Haug, Andreas Maas, and Dieter Waloszek juvenile phases is not always clear and depends on the taxon (and author). Additional differences between the plesiomorphic head larva and...