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

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Materials and methods

Four Cretaceous thylacocephalan species were examined: Protozoea hilgendorfi Dames, 1886; P. damesi Roger, 1946; Pseuderichthus cretaceus Dames, 1886, and Thylacocephalus cymolopos Lange et al. (2001).

Series of possible lattice organs were found in eight specimens of: Protozoea hilgendorfi: [BM: J 469-1, J 622-1; OR 59245 (2)-1, (3)-1, OR 59554 (3). MB: A 693.3. MSNM: i12172, i20556] and one specimen of P. damesi [BM: OR 59672 (4)] (see Table 1).

Table 1. Information on thylacocephalan fossils possessing well preserved series of lattice like organs including length (* carapace length without rostrum and posterior spines, in mm); body type (**according to Schram et al., 1999); position of organs (ant – anterior, mid – mid dorsal, post – posterior); number of organs and distance between neighboring organs (in mm).
P. hilgendorft	Carapace length	Body type** mm (estimated*)	Position of series along midline	Number of llo’s in each series	Average distance between illo’s

BM J469-1	(40)	globose	ant	3	0.5
			mid	10	0.55
			most	7	0.85
BM J622-1	78	elongate	ant	12	0.5
			mid	7	0.85
			post	11	1.5
BM OR 59245 (2)-1	44	elongate	mid	~20	0.5
BM OR 59245 (3)-1	(50)	intermediate	post	12	0.82
BM OR 59554 (3)	(32)	globose	mid – post	15 – 20	0.4 – 0.5
MB A 693.3	(24)	globose	mid	~13	0.6
MSNM il2172	42	intermediate	mid	5	0.8
MSNM i20556	(30)	globose	post	4-5	0.7

P. damesi
BM OR 59672 (4)	15		post	8	0.2 – 0.3

Similar structures occurred sporadically in some four specimens of P. hilgendorfi: [BM: PI IC 82; In 42507a; OR 59672 (16)] and an additional specimen MSNM: i7586-3, which is of uncertain thylacocephalan affinity.

Further possible lattice organs were noted, but their poor preservation prevents unequivocal identification: P. hilgendorfi: [BM: PI IC 76, 83; In 48832 (2), (3); OR 59554 (2), OR 59672 (14), (15) and (24). MB: A 693.4, 693.5. MSNM: i7586-1]; P. damesi: [BM: IC 91-1; OR 59245 (4)-1, OR 59672 (25).]

A section (indicated in Fig. 1A) of the posterior carapace along the dorsal midline of one especially well-preserved specimen (BM: J 622-1) was carefully removed. The section containing four pairs of possible lattice organs was subjected to standard high voltage SEM (JEOL JSM-35C), and images were saved in the Semaphore digital image storage system.

In addition, specimens of the Jurassic thylacocephalan Dollocaris ingens van Straelen, 1923 were examined. Structures reminiscent of possible lattice organs were noted on two specimens (MNHN: R.50930 and R.50937).

Specimen repositories are abbreviated as follows: BM – The Natural History Museum, London; MB – Museum für Naturkunde, Berlin; MNHN – Museum Nationale d’Histoire Naturelle, Paris; MSNM – Museo Civico di Storia Naturale, Milan.