Parasites are common in modern ecosystems and are also known from the fossil record. One of the best preserved and easily recognisable examples of parasitism in the fossil record concerns isopod-induced swellings in the branchial chamber of marine decapod crustaceans. However, very limited quantitative data on the variability of infestation percentages at the species, genus, and family levels are available. Here we provide this type of data for a mid-Cretaceous (upper Lower Cretaceous, upper Albian) reef setting at Koskobilo, northern Spain, on the basis of 874 specimens of anomurans and brachyurans. Thirty-seven specimens (4.2%), arranged in ten species, are infested. Anomurans are more heavily infested than brachyurans, variability can be high within genera, and a relationship may exist between the number of specimens and infestation percentage per taxon, possibly suggesting host-specificity. We have also investigated quantitative patterns of infestation through geological time based on 88 infested species (25 anomurans, 55 brachyurans, seven lobsters, and one shrimp), to show that the highest number of infested species can be found in the Late Jurassic, also when corrected for the unequal duration of epochs. The same Late Jurassic peak is observed for the percentage of infested decapod species per epoch. This acme is caused entirely by infested anomurans and brachyurans. Biases (taphonomic and otherwise) and causes of variability with regard to the Koskobilo assemblage and infestation patterns through time are discussed. Finally, a new ichnogenus and -species, Kanthyloma crusta, are erected to accommodate such swellings or embedment structures (bioclaustrations).

Abundant fossil material of extinct brachyurans has revealed morphological details hitherto rarely used inpalaeontological studies. Detailed comparisons between extant and extinct material have been carried out, with anemphasis on thoracic sternum, abdomen and appendages. Documented for the first time is the unique character ofRaninoidea De Haan, 1839, their ‘gymnopleurity’, which is not found in their predecessors, the PalaeocorystidaeLőrenthey in Lőrenthey & Beurlen, 1929. Palaeocorystidae, together with four other families (CamarocarcinidaeFeldmann, Li & Schweitzer, 2008; Cenomanocarcinidae Guinot, Vega & Van Bakel, 2008; Necrocarcinidae Förster, 1968emend.; and Orithopsidae Schweitzer, Feldmann, Fam, Hessin, Hetrick, Nyborg & Ross, 2003a emend.), is assigned hereto the superfamily Palaeocorystoidea, of similar rank to Raninoidea. Both Raninoidea and Palaeocorystoidea are affordeda subsection rank and referred to as subsection Raninoidia De Haan, 1839 emend. New or emended diagnoses areprovided for all higher taxonomic levels, and all members of Raninoidia are listed in an appendix. A unique abdominalholding structure, the double peg, is described for the first time. Its gradual evolution is documented and the phylogeneticimplications are discussed. Comparative morphology of the thoracic sternum, abdominal holding structures, the sternum-pterygostome configuration, respiratory physiology and spermathecae, all reveal polarities of the raninoidian clade. Theconfiguration of the sternum with the pterygostome, which is related to body strength and respiratory physiologicalefficiency, differs significantly between the two superfamilies, Raninoidea showing a derived condition. An evolutionarylineage, leading from Palaeocorystidae, via Lyreididae to Raninidae is recognised, and an intermediate form, Marylyreiduspunctatus n. comb., is discussed. Several hitherto unknown structures in extant raninoids, an obstruction system for theabdomen and a telson protection valve, are documented. The cryptic spermathecal apertures of raninoids, so far barelyunderstood, are re-examined and compared to those of palaeocorystoids. The phylogeny of Podotremata, often debated inthe recent literature, is discussed anew on the basis of these observations. A position of Raninoidea within Eubrachyura,recently claimed by several authors, cannot be maintained, an observation supported by documentation of the basalcondition of Raninoidia. A new basal lyreidid clade, Marylyreidinae n. subfam., is erected, whereas new genera andspecies include Antonioranina n. gen. (Cyrtorhininae), Bournelyreidus teodorii n. gen., n. sp. (Lyreidinae), Cenocorystesbretoni n. sp. (Palaeocorystidae), Cenomanocarcinus cantabricus n. sp. (Cenomanocarcinidae), Eosymethis aragonensisn. gen., n. sp. (Symethinae), Eucorystes iserbyti n. sp., Eucorystes navarrensis n. sp. (both Palaeocorystidae),Ferroranina tamilnadu n. gen., n. sp. (Palaeocorystidae), Joeranina gaspari n. gen., n. sp. (Palaeocorystidae),Marylyreidus n. gen. (Marylyreidinae n. subfam.), Paranecrocarcinus balla n. sp. (Paranecrocarcininae), Symethoidesmonmouthorum n. gen., n. sp. (Symethinae) and Vegaranina n. gen. (Ranininae). Several raninoid and palaeocorystoid genera are revised, and emended diagnoses given.

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