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1.4. Bredocaris admirabilis Müller, 1983

Presumed to be a maxillopod related to the Thecostraca sensu Grygier (1984), possibly the sister taxon to all living descendant, Bredocaris admirabilis (= Orstenocarida Müller & Walossek, 1988) is known from a series of 5 metanaupliar stages and the possible adult, approximately 0.85 mm long. Its phylogenetic affinities with the thecostracan lineage are founded on very specific details of the larval series, such as that not only the series is very condensed but that all so-called nauplii show a suppression of their trunk segmentation and suppression of their limb development – best seen in Ibla quadrivalvis, a basal thoracican cirriped. That means that the trunk is not distinctly segmented in larvae and the limbs are buds at most.

Extant forms moult thereafter into a so-called cyproid stage (cypris is an autapomorphy of Cirripedia) which has all trunk limbs developed at one step. Also the latest stage of B. admirabilis has all limbs developed, but since all thoracic segments still lack a clear demarcation, the abdomen is not segmented, the thoracopods are ill-divided, and the furca is not articulated off from the telson, as in the extant tantulocarids, ascothoracids, factetotectans, and cirripeds, this suggests that the 850 µm-stage is not a larva, but its design is that of a special adult, i.e. these characters are all autapomorphies.

Moreover, the series of up to six 'nauplii' of thecostracans – better to say larvae because only the first stage is a true nauplius (last carries already the cyproid under its skin, corresponding to about a THS-6-stage, 6 thoracomeres are developed; reference Rehbachiella ontogeny) – does not means correspond to the six "nauplii" of Copepoda. Again here only the first is a nauplius, but then the series is very gradual (see Walossek 1993), and the first copepodid of Copepoda has 2–3 thoracomeres developed, while the cyproid bears a complete thorax region plus abdomen, thus as much as the future adult.

B. admirabilis differs from extant forms in two major features: in the trunk-limb shaped maxilla 2 and in the well-developed 7th thoracopod, which is present only in males (if at all) and there modified to a penis structure. The ground pattern of 7 thoracomeres with 7 pairs of limbs for Maxillopoda was first hypothesized by Grygier in 1983. Now his presupposition could be validated since B. admirabilis shows exactly this morphology. B. admirabilis is the second record of maxillopod Crustacea from the Orsten material.

Morphology: Head shield of moderate size, domed and roof-shaped, reaching to the segment of the maxillula (mx1) at metanauplius 5 but including mx2-segment at adult stage. Dorsal organ, also in the adult, recognizable by four pits. The largest stage, the preseumed adult has 12 pairs of limbs: 5 head limbs (maxilla 2 looks exactly like a trunk limb, but has a slightly better developed proximal endite) and 7 pairs of thoracopods for swimming, the number suggested for the groundplan of Maxillopoda by Grygier (1983, 1984).

Eyes are present in front of the large labrum, which is adorned with small curved rows of denticles. Antennula (A1) with sclerotic bars on outer side, soft posteriorly, Swimming setae at inner distal edge and a terminal set on more rod-shaped dsistal podomeres. Antenna (A2) and mandible (Md) similar, coxal gnathite well developed in the latter limb. Distal ends of endopods of these two limbs strikingly similar to the distal end of a1. Again exopods with rows of fine denticles on outer annulations, as is characteristic for eucrustaceans. Only one more "mouthpart" developed, as in the ground pattern of Eucrustacea: maxillula with four endites on the protopod (= proximal endite pe and 3 on basipod), a 4-segmented endopod and a short, one-segmented rod-shaped exopod with two terminal setae. Maxilla of the shape of a trunk limb. This limb could be recognized as a head limb because we have found a specimen with nice preservation having bent its head down and exposing the tagma boundary between head and trunk.

All thoracopods with ill-developed demarcations of the portions and podomeres of the endopods. Inner edge with endites, which carry a fairly poor set of one pair of short spines each. This pattern is succeeded along the inner side of the endopod suggestive of its former subdivision into podomeres. Endopods and exopods paddle-shaped and almost alike, arising in parallel from the distal end of the basipod (typical for swimming legs, but not a character that holds to presume a larval character of the largest stage or even a systematic position in the stemline of what-so-ever prior to Eucrustacea!). Limbs slightly decreasing in size and armature towards the posterior.

Trunk surface with many tiny folds but no sharp segment boundaries (recognizable, thus, only by the legs). Abdominal portion completely unsegmented, no telson demarcated-off, but a small area with fine transverse furrows may point to an original segmentation. Also the furcal rami are not set-off from the tail, being rod-shaped and carrying a set for four setae posteriorly. Anus dorsal from and between the furcal rami (terminal, as is characteristic for Eucrustacea).

Ontogeny starts with larva 1, approximately 0.19 mm long, with eyes present, a neck or dorsal organ, and bifid buds (anlagen) of the maxillula (mx1) hence representing already a metanauplius (L3 as compared to Rehbachiella). Trunk elongates during further development but remains unsegmented. At stage 2 two more tiny humps appear on the trunk while mx1 is developed (enditic armature still less developed than later). Stage 3 adds one more limb bud, and stage 4 another. At stage 5 six buds, incl. mx2 as a bud can be recognized on the trunk, all undeveloped, as is characteristic for thecostracans in generell (see above).

Life habits: first metanauplius already feeding and swimming (limbs, labrum, mouth and anus developed, possibly all stages lived within or, at most, on the flocculent bottom zone. Features such as the non-segmentation of the thorax and abdomen, ill-developed subdivision of the trunk limbs, and non-separation of the furcal rami point to meiofauna specialization of B. admirabilis.

Additions: the paper by Müller and Walossek (1988) includes the presentation of a 40 cm long clay model to improve the reconstruction of the feeding area and motion of the trunk limbs (pictures 1 , 2 , 3 , 4 , 5 , 6 ). The authors also discuss the status of the Maxillopoda, recognized first by Dahl (1954), but not founded on phylogenetic systematics analysis), as a valid monophylum.

Besides the Skaracarida, another presumed maxillopod, most likely the sister taxon to all "crown groupers", is currently under study: the "headless" Dala peilertae Müller, 1983.
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