Contributions to Zoology, 71 (4) (2002)Sven Lange; Frederick R. Schram: Possible lattice organs in Cretaceous Thylacocephala

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Reconstruction of the morphology of a possible lattice organ

An attempt to reconstruct the original morphology of the possible lattice organs of these Thylacocephala may be derived from the descriptions above (Fig. 3D). The tanned areas observed with light-microscopy to surround the possible lattice organs are not recognizable in SEM and are therefore not be included in the reconstruction. (Cuticle of a similar color occurs in connection with other structures such as the characteristic pit organs (Fig. 1B and F) or along the carapace margins, and may simply indicate a change in thickness or some other property of the cuticle).

An individual possible lattice organ from the posterior region of the carapace can therefore be recognized based on its general relief. It covers an elongated sub-oval area of approximately 120-150 µm parallel to the longitudinal axis of the animal and a width ranging from 60 – 80 µm. Superficially, each possible lattice organ consists of a central elongated trench flanked by low ridges. Though the ridges appear clearest along the ‘long sides’ of the trench, they could actually represent the remains of only one ridge completely encircling each trench. To the external side, the ridges gently grade into the carapace cuticle. On the internal side the slopes pass into the trench. The floor of the trench is situated below the external level of the ordinary carapace cuticle. Slopes also lead up from the floor of the trench to the carapace at both the anterior and posterior ends. The ridges seem to project up to 20 µm above the carapace surface in its immediate surroundings.

In most cases, the trench has a distinct club shape with the wider anterior part reaching 15–20 µm while the width of the more narrow posterior 2/3 of the trench predominantly is less than 10 µm. The total length of the floor is difficult to asses precisely because it grades into the carapace cuticle, but typically it surpasses 100 µm.

When not covered by sediment, the broader anterior part of the trench often appears folded. Typically there are one to a few small and short folds (Fig. 4A and B). Slighter folds were observed, but rarely, in the narrow posterior part of the trench.

The search for associated pores did not yield a consistent result. There were no pores appearing in the same location in all investigated possible lattice organs. However, in a few cases pore-like grooves were observed, near both the posterior and the anterior end of the trench (Fig 4B and C). In other of the inspected possible lattice organs, however, these locations lacked recognizable pores (Fig 4A).

Additional oblong structures were noted in P. hilgendorfi (specimen BM J 622-1 and BM PI IC 82) and probably in P. damesi (specimen BM IC 91-1). Reaching sizes around 500–600 µm they are considerably larger than adjacent possible lattice organs, though not dissimilar in consisting of two juxtapositioned longitudinal ridges. These oblong structures were found in only one pair per individual, situated anteriorly immediately ventral to the series of possible lattice organs in a bilaterally symmetrical arrangement (Fig. 1D). Furthermore, the carapace of Dollocaris also possesses a pair of elongate structures, albeit in a more ventral position; they are slightly curved and seemingly made up by two parallel ridges (Fig. 5).

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