Paleosuchus undetermined

Clarac, Francois, Campos, Zilca & Marquis, Olivier, 2024, The extended osteoderm shield in Paleosuchus sp.: a dwarf crocodylian adaptation to the equatorial forest ecosystem?, Comptes Rendus Palevol 23 (12), pp. 161-170 : 164-166

RESULTS

THE DIFFERENTIAL EXPRESSION OF BONE ORNAMENTATION ON THE OSTEODERM SHIELD

The crocodylian osteoderm ornamentation is composed of a pattern of repetitive excavated pits that are separated by ridges ( de Buffrénil 1982; Clarac 2021). Here we can observe that these pits are systematically deep on the dorsal part of both the trunk and the neck ( Fig. 1 ; Table 1 ). Among the osteoderms which come from the flanks, the tail, the belly and the limbs; only 3.7% (n = 39) are deeply pitted, 15.1% (n = 161) show shallow pits and 81.1 % (n = 861) are deprived of ornamentation ( Fig. 1 ; Table 1 ).

MICROANATOMICAL COMPARATIVE ANALYSES

Within our cross-section subsample that is composed of 12 osteoderms, the osteoderm porosity varies between 0.04 and 0.16 with a mean value of 0.071 and a median value of 0.06 ( Table 2 ; Figs 2 ; 3 ). Within the dorsal osteoderms, which compose half of the cross-section subsample (Cat. 1; n = 6), the porosity varies between 0.06 and 0.16 with a mean value of 0.09 and a median value of 0.085 ( Table 2 ; Figs 2A ; 3A ). Within the non-dorsal osteoderms, which compose the second half of the cross-section subsample (Cat. 2; n = 6), the porosity varies between 0.08 and 0.054 with a mean value of 0.053 and a median value of 0.05 ( Table 2 ; Figs 2B ; 3A ). The Mann-Whitney test shows a significant difference between the median porosity of the dorsal osteoderms (Cat. 1) and the median porosity of the non-dorsal osteoderms (Cat. 2; Table 3 ) which plots a lower median value (median Cat. 2 = 0.05 <median Cat. 1 = 0.085).

We have compared the full dataset (Cat. 1 and 2; n = 12) with a previous dataset that is composed of an interspecific sample which includes both extant and extinct pseudosuchian species that lived during the Triassic and later periods ( Clarac et al. 2020). Within this previous study, the sample was split in two sub-samples: 1) the terrestrial taxa which lived during the Mesozoic; and 2) the semi-aquatic taxa that appeared at the lower-Jurassic and who present some descendants that are still present in the extant nature (i.e., Crocodylia; Benton & Clark 1988). Here we have added a new dataset as a third sub-sample ( Table 2 ; n = 12) in order to perform statistical comparisons via two-sampled non-parametric tests (i.e., Mann-Whitney). This test revealed that the median porosity is significantly different between P. palpebrosus and the other semi-aquatic forms whereas it is not with the terrestrial forms ( Table 4 ; Fig. 3B ). The test also revealed a significant difference of the median porosity between the semi-aquatic and the terrestrial forms ( Table 4 ; Fig. 3B ). The median value is indeed lower in both the terrestrial forms (Median = 0.05) and in P. palpebrosus (Median = 0.06) than in the semi-aquatic interspecific sample (Med = 0.175). The second and the third percentile show a narrower range in P. palpebrosus than either in the terrestrial forms or in the semi-aquatic interspecific group ( Fig. 3B ).

Figures

FIG. 1. — A, Specimen of interest (Paleosuchus palpebrosus (Cuvier, 1807)) in its vivarium at the Parc zoologique de Paris; B, diagram of the ornamental pit distribution on the osteoderm shield of P. palpebrosus: red, the deeply pitted osteoderms; yellow, the shallow pitted osteoderms; green, the non-ornamented osteoderms; blue circle, a deeply pitted dorsal osteoderm. Scale bars: 1 cm. Credits: photo by François-Gilles Grandin. Scale bars: 1 cm.

FIG. 2. — Osteoderm transversal cross-sections: A, dorsal osteoderms (Cat. 1; n = 6); B, non-dorsal osteoderms (Cat 2; n = 6). A dorsal view of each osteoderm is presented next to each cross section. Scale bars: 50 mm.

FIG. 3. — Boxplots of the osteoderm porosity summarizing the mean,the quartiles,the minimum and the maximum values:A, intraspecimen sample (Paleosuchus palpebrosus (Cuvier, 1807)); B, interspecific comparison. Abbreviation: S.aquatic, semi-aquatic (“Terrestrial forms” and “S.aquatic forms” both represent taxa that lived during the Jurassic and later periods, the data were previously published in Clarac et al. 2020).

Tables

TABLE 4. — Statistical non-parametric test (Mann-Whitney): interspecific comparison between the porosity of the Paleosuchus palpebrosus (Cuvier, 1807) osteoderms with a set of both terrestrial and semi-aquatic pseudosuchian osteoderms (see Clarac et al. 2020 for details). Abbreviation: Cat., category.

Mann-Whitney test	Terrestrial pseudosuchians	Semi-aquatic pseudosuchians
	(n = 11; after Clarac et al. 2020)	(n = 20; after Clarac et al. 2020)
Paleosuchus palpebrosus	Non-Significant	Significant
(MNHN-RA-2022.0064)	(Pvalue = 0.90)	(P= 2.77. 10–5) value
Total sample (Cat. 1 and 2; n = 12)