The producer and ecological inferences

The overall shape, internal structure, and phosphatic composition of the specimens (Figs. 1 and 2) imply that they are coprolites. Cylindrical and elongated coprolites are known from various vertebrate and invertebrate producers (e.g., Häntzschel, El-Baz & Amstutz, 1968; Hunt & Lucas, 2012). The faecal pellets of the latter, however, are commonly smaller than the specimens from Wierzbica (Häntzschel, El-Baz & Amstutz, 1968), rendering a vertebrate producer probable. The preservation of the pterosaur tracks in the intertidal environment indicates a fast burial, implying that the droppings preserved on the same surface were deposited at a similar time to the track formation. Furthermore, both the size and shape of the coprolites are very similar to the so far only described pterosaur coprolite (Hone et al., 2015) and the coprolite sizes match those of the footprint maker(s) at Wierzbica (Elgh, Pieńkowski & Niedźwiedzki, 2019). Taken together, these facts suggest that pterosaurs were most likely scat producers.

Given the concentrations of foraminifera, minute shelly animals and probable polychaete bristles in the coprolites, it seems probable that these organisms were deliberately targeted rather than accidentally ingested. A reasonable explanation for how a pterosaur big enough to have produced the droppings effectively could have captured such small prey is through filter feeding. The modern filter-feeding Chilean flamingo (Phoenicopterus chilensis) has been shown to produce droppings rich in foraminifera (and also copepods and polychaetes), in similar to the coprolites described here (Tobar et al., 2014).

A glance at the pterosaur body fossil record reveals that several potential filter feeders belonging to the group Ctenochasmatidae were around at this time, both regionally (e.g., Ctenochasma and Gnathosaurus) and elsewhere (Liaodactylus); see Zhou et al. (2017) for phylogenetic interrelationships and inferred ghost lineages of the taxa. Filter feeding in these Jurassic ctenochasmatids is supported by their long rostra and many slender, closely-spaced teeth (Wellnhofer, 1970; Bennett, 2007; Zhou et al., 2017). It has been argued that they were not filter feeders, or at least not exclusively so, with reference to the more specialized jaw apparatus (a sieving basket consisting of many long, slender teeth sitting in upcurved dentaries) found in the Cretaceous taxon Pterodaustro (Sanderson & Wassersug, 1993; Witton, 2013). However, evidence from computed models of digitized pterosaur skulls support that Ctenochasma and Gnathosaurus, in similar to Pterodaustro, had very low bite forces, but fast closing jaws, which would facilitate feeding on small evasive prey/filter feeding (Henderson, 2018).

The combined evidence of the pterosaur tracks, which are possibly ctenochasmatid (Elgh, Pieńkowski & Niedźwiedzki, 2019), and the coprolites, which were most likely made by a filter feeder, leads us to conclude that filter-feeding ctenochasmatids were probable producers of both tracks and droppings. Our findings significantly expands the bromalite record for this pterosaur group, which was previously only known from gastroliths found in Pterodaustro (Codorniú, Chiappe & Cid, 2013), and lends further support to filter-feeding among these Jurassic taxa.

Moreover, the mesh size implied by the size of the coprolite inclusions can be matched not only in flamingos but also in Ctenochasma. The functional sieve in the Chilean flamingo has been shown to range from 80 to 959 µm across the proximal-distal axis of the beak (Mascitti & Kravetz, 2002). Most of the Wierzbica coprolite inclusions fall within this size range (most specimens being around 300 µm in size) indicating a similar mesh size. Such a small mesh-size was probably present in adult specimens of Ctenochasma elegans, evidenced by a tooth-spacing of 7.36 teeth/cm in one specimen (Bennett, 2007; Fig. 3). Comparable mesh sizes and feeding environments might explain the similar dietary contents of the Chilean flamingo and these ctenochasmatid pterosaurs. It should be noted that the Jurassic ctenochasmatids were perhaps not capable of active pumping, in contrast to recent flamingos, and possibly Pterodaustro (e.g., Sanderson & Wassersug, 1993).

Figure 3: Feeding apparatus of Ctenochasma elegans and Phoenicopterus chilensis. Ctenochasma elegans (redrawn from Schematic drawing of (A) the jaws and teeth of(redrawn from Bennett, 2007 ) and (B) the beak of the recent Chilean Flamingo (redrawn from Mascitti & Kravetz, 2002 ).

The laminated grey/brownish clayey marls from facies association 3, where the traces occur (track layer 1), revealed strongly impoverished benthic microorganisms, composed of scarce foraminifera tests (Spirilina sp. and Lenticulina sp.), rare ostracod carapaces and broken echinoid spines. Collectively, these finds point to relatively shallow water condition with marine benthic association (Pieńkowski & Niedźwiedzki, 2005) for the pterosaur track and coprolite horizon. The high abundance and disparity of the foraminifera in the coprolites further suggest that they were strongly targeted by the coprolite producers and/or that they derive from another feeding environment.

The size of the foraminifera indicates a high abundance of benthic forms in the coprolites, as pelagic ones are normally less than 100 µm in size (Armstrong & Brasier, 2005). However, there are possibly some small, but not very well-preserved specimens in the coprolites as well. The fact that the shells and tests at all are present in the coprolites also provides some clues about the digestive system of the producers. Calcitic, and likely aragonitic, material is typically completely dissolved in the digestive system of recent crocodiles, leaving keratinous structures but no bones or shells in their faeces (Fisher, 1981; Milàn, 2012). However, bones of various degree of etching are commonly found in coprolites of other archosaurs such as theropod dinosaurs (Thulborn, 1991; Chin et al., 1998; Qvarnström, Ahlberg & Niedźwiedzki, 2019a). It appears therefore that the pterosaurs at Wierzbica, in similar to some archosaurs, had shorter food retention time and/or weaker stomach acids than seen in recent crocodiles.

Specimen MUZ PGI 1663.II.15a was likely produced by a larger individual than that/those which produced the other two coprolites since animal size and diameter of faeces positively correlate (e.g., Milàn, 2012). Size variation is also seen in the footprint record of Wierzbica (Elgh, Pieńkowski & Niedźwiedzki, 2019), altogether suggesting the presence of a pterosaur flock with individuals of different ontogenetic stages and/or sympatric species. The higher relative abundance of foraminifera in the biggest coprolite might suggest an ontogenetic switch to a more specialized filter feeding in adults, whereas younger individuals relied more on eating soft-bodied organisms from the sediments. Such a dietary switch is consistent with the addition of more teeth across the ontogenetic series of Ctenochasma elegans (Bennett, 2007).