Phylogeny

The parsimony analysis resulted in two MPTs of 132 steps (consistency index = 0.41 and retention index = 0,52; Fig. 4). The only topological difference between these two trees is the placement of Metacryphaeus branisai. The strict consensus is presented in Fig. 5, along with bootstrap probabilities and Bremer decay indices for each node.

Figure 4 (a,b) Two most parsimonious trees (132 long and consistency index of 0.41) calculated in the present phylogenetic analysis. Full size image

Figure 5 Strict consensus of the two MPTs with bootstrap values (using 1000 replicates; below) and Bremer support (above) indicated for each node. Full size image

Plesiomalvinella boulei and P. pujravii were found deeply nested within a clade of Metacryphaeus species. Accordingly, those two species are here referred to that genus, as previously proposed by Wolfart13. Metacryphaeus (including M. boulei and M. pujravii) is here supported by two synapomorphies: frontal lobe projecting beyond the cephalic anterior border in dorsal view (character 4) and uniformly divergent axial furrows from SO to the cephalic margin (character 19).

In contrast to Lieberman5, Clarkeaspis gouldi (Lieberman, 1993) and C. padillaensis (Lieberman, 1993) were grouped into a clade supported by four synapomorphies (Figs 4 and 5): cephalic anterior border (cranidial) extended and pointed (characters 2 and 3); pentagonal glabella (character 6); 60 to 70% ratio between the basal glabellar width and the glabellar length (character 9). Clarkeaspis is here placed closer to Metacryphaeus (as its sister group) than in Lieberman5. The Metacryphaeus + Clarkeaspis clade is supported by a single synapomorphy (character 9, 0 → 1) and shows low bootstrap support (Fig. 5). The placement of Malvinocooperella pregiganteus (Lieberman, 1993) and Wolfartaspis cornutus (Wolfart 1968) as successive outgroups of the Metacryphaeus + Clarkeaspis (Figs 4 and 5), also differs from the arrangement seen in Lieberman5.

The present analysis recovered the clades formed by Metacryphaeus giganteus + M. parana (Figs 4 and 5) and M. boulei + M. pujravii (Figs 4 and 5), previously recognized by Lieberman5. Synapomorphies of the M. giganteus + M. parana clade are: 60 to 70% ratio between the basal glabellar width and the glabellar length (character 9), convergently acquired in Clarkeaspis; slender genal spine (character 36); dorsoventral height of the pygidium gradually decreasing posteriorly (character 39); 0.65 to 0.80 ratio between the maximum pygidial axial width and the maximum pygidial axial length (character 42). The M. boulei + M. pujravii clade is supported by six synapomorphies which are related to the presence of two symmetrical rows of sagittal spines on the posterior part of the glabella (character 15), the presence of one or two spines on L1 and L2 (characters 17 and 18), 0.15 and 0.25 ratio between the distance of posterior margin of the eyes to the axial furrow and the maximum glabellar width (character 25), the presence of four or five spines on the thoracic axial rings (character 37), and the prosopon covered by spines (character 48).

Our study also recovered new hypotheses for the relationships of Metacryphaeus, including a clade formed by M. allardyceae, M. caffer, M. australis, M. meloi, M. kegeli, and M. tuberculatus. This is supported by four synapomorphies related to the shape and extension of the (cranidial) cephalic anterior border (characters 2 and 3), the ratio between the sagittal length of L1 and the glabellar sagittal length (character 14), and the incision of the occipital furrow medially (character 29). The clade including M. caffer, M. australis, M. meloi, M. kegeli, and M. tuberculatus is supported by four synapomorphies (Fig. 4a): glabella posteriorly elevated and declined anteriorly to S3 (character 8); 65 to 75° α angle (character 22); rounded pygidial terminus (character 45); no spine on the pygidial terminus (character 46). Also, the clade formed by M. tuberculatus, M. meloi, and M. kegeli is supported by four synapomorphies related to L2 and L3 that do not merge distally (character 13), 55 to 64° β angles (character 23), the connection of S2 and the axial furrow (character 24), and the lack of connection between the anterior margin of the eyes and the axial furrow (character 26) (Fig. 5). Two synapomorphies support the M. caffer plus M. australis clade: characters 9 (reverted to the plesiomorphic condition) and 41, which are respectively related to a ratio greater than 80% between the basal glabellar width and the glabellar length, and to 0.25 to 0.35 ratios between the maximum pygidial axial width and the maximum pygidial width.

The clade that includes all Metacryphaeus except for M. convexus, M. curvigena, and M. branisai (Fig. 4a) is supported by three synapomorphies related to a 0.15 to 0.25 ratio between the distance from the posterior margin of the eyes to the axial furrow and the maximum glabellar width (character 25), occipital furrow weakly incised medially (character 29), and 130 to 160° γ angle (character 34). Three synapomorphies support the group formed by M. giganteus, M. parana, M. allardyceae, M. australis, M. caffer, M. meloi, M. kegeli, and M. tuberculatus (Fig. 4a): 0.25 to 0.34 ratio between sagittal length of L1 glabellar lobe and glabellar sagittal length (character 14), 0.3 to 0.4 ratio between the maximum exsagittal eyes length and the glabellar sagittal length (character 27), 0.60 to 0.80 ratio between maximal sagittal pygidial length and maximal transverse pygidial width (character 40). The position of Metacryphaeus branisai is variable in the two MPTs (Fig. 4), probably because its pygidium is unknown, implying the non-codification for characters 38 to 47.

In the phylogeny modelled by Lieberman5, Metacryphaeus convexus and M. curvigena are not considered sister taxa to all other Metacryphaeus. Instead, M. curvigena is considered the sister taxon to M. branisai and M. convexus the sister taxon to both (Fig. 1a). In our analysis, the clade formed by M. convexus and M. curvigena is supported by five synapomorphies: inclination of 10–20° of S3 in relation to SO (character 12); L2 and L3 not merged distally (character 13); cephalic axial furrows deep and broad (characters 20 and 21); evident connection between S2 and the axial furrow. Likewise, the affinities of M. meloi and M. kegeli are supported by four synapomorphies. This is interesting because these species are endemic to the Parnaíba Basin (Brazil), as is their sister-taxon M. tuberculatus, the only other species of the genus known to that basin.

Palaeobiogeography

Likelihood Ratio Test supports DEC M2 (w and j set as free parameters) as the best-fit model to our data (Table 1). The palaeobiogeographic reconstructions differ only slightly for the two MPTs, so we focus the discussion on the first MPT. The summary of biogeographic stochastic mapping (BSM) counts (Table 2) shows a predominance of dispersals among range change events (33.6% of total events) and, among those, founder events (19.6%) are slightly more frequent than anagenetic dispersals (14.1%). Vicariance was very uncommon according to our model, accounting only for 3.9% of the events (Table 2). Most dispersals occurred from Bolivia and Peru (A) to other areas, more frequently to the Paraná (B) and Parnaíba (E) basins (Table 3).

Table 1 Pairwise comparison of the results of the ancestral area reconstructions of nested DEC models on tree 1. Full size table

Table 2 Summary of BSM (Biogeographic Stochastic Mapping) counts based on DEC M2 model showing the mean, standard deviations (SD), and percentage of different types of biogeographic events. Full size table

Table 3 Counts (and standard deviations in parentheses) of dispersal events averaged across 100 biogeographics stochastic mappings based on the biogeographic history of Metacrypheus according to DEC M2 model. Full size table

All three models estimate a 100% probability for Bolivia and Peru (A) as the ancestral area for the Metacryphaeus clade, as well as for most of its internal clades (Fig. 6; Supplementary Supple 3). The earliest Metacryphaeus records in this area are from the early Pragian4,5, but three range changes were estimated to have occurred earlier, during the late Lochkovian (Fig. 6): 1- the ancestor of M. parana and M. giganteus expanded its occurrence to encompass the Paraná Basin (B), with the former species maintaining this broader distribution and the latter restricted to B (subset sympatry) - in an alternative scenario, the ancestor of this clade is present only in Bolivia and Peru (A), with M. parana expanding its range to also the Paraná Basin (B); 2- M. allardyceae dispersed to the Falklands area (D); 3- the ancestor of M. australis and M. caffer dispersed to the Paraná Basin (B). During the early Pragian, M. caffer dispersed from the Paraná Basin to South Africa (C). It is interesting to note that those dispersal and expansion events likely occurred before the transgressive events on western Gondwana14,15,16,17 dated between the late Pragian and the early Emsian (Fig. 6). Those areas (A, B, C, D) were eventually connected by transgressive-regressive cycles (Fig. 6), which promoted the faunal similarity observed among the Malvinokaffric fauna of the Early Devonian15,18.

Figure 6 Ancestral area reconstructions based on DEC M2 model on the tree 1 (top), sea-level changes curves from Lochkovian to Frasnian (middle) based on Haq & Schutter55, and Lower Devonian palaeomap of Southern Gondwana (bottom) modified from Torsvik & Cocks56. Arrows on the palaeomap indicate inferred Lochkovian (full arrow) and Pragian (dashed arrow) dispersal routes for Metacryphaeus taxa. Additional abbreviations: DML, Dronning Maud Land, Antarctica; EWM, Ellsworth-Whitmore Mountains, Antarctica; MT, Mexican terranes; P, Precordillera Terrane, Argentina; Pat., Patagonia. Full size image

The last common ancestor of Metacryphaeus meloi, M. kegeli, and M. tuberculatus, and the node including only the latter two taxa were reconstructed with two almost equal probable ranges, either restricted to Bolivia and Peru (A) or a joint distribution (Fig. 6) also including the Parnaíba and Paraná basins (ABE). These different ancestral range reconstructions imply distinct processes of range changes, respectively: 1 - successive dispersals from Bolivia and Peru to the other areas (for an ancestral with distribution restricted to A), 2 - distribution expansions inferred as founder events (for an ancestral widely distributed in ABE). Nevertheless, in all cases M. meloi and M. kegeli became restricted to the Parnaíba Basin (E), whereas M. tuberculatus maintained (or reached) a widespread distribution (ABE), even though its earliest records, dated as late Eifelian and early Givetian, do not include the Parnaíba Basin4,5,6,11,12. Alternatively, but with lower statistical support, the ancestral range reconstruction hypothesized for the clades M. meloi + (M. tuberculatus + M. kegeli) and M. tuberculatus + M. kegeli could be AB, encompassing only their older records. This would imply expansion events towards the Parnaíba Basin (E) after the arrival of ancestors in the Paraná Basin (B).

The arrival of Metacryphaeus in the Parnaíba Basin may have occurred via two alternative routes (Fig. 6). A northern route (surrounding the northern margin of the South American continent) would impose no continental (landmass) barriers, but there would be climatic barriers related to the warmer waters the animals would need to overcome, as the Malvinokaffric Realm marks cooler areas. Also, faunas of this age on the northern margin of South-America belong to other realms, which lack Metacryphaeus. On the other hand, a route through the Amazon Basin (Fig. 6) would have presented no climatic or faunal barriers (cf.15,18,19). Even a continental barrier might not have been in place, as there were transgression events possibly connecting that basin to Bolivia and Peru. The lack of fossils of this age in the Amazon Basin, which could confirm such a dispersal route, is related to the depositional gap present in the upper Lochkovian and lower Emsian of the basin (cf.20,21,22,23,24,25). This absence of Lochkovian–lower Emsian rocks is also observed in the Parnaíba Basin20,21,24, which hinders palaeobiogeographical inferences related to the presence/absence of Metacryphaeus in the Lower Devonian of this basin.

Other trilobite genera also have a broad Gondwanan distribution during the Devonian, e.g. the calmoniid Eldredgeia, with occurrences in the Bolivia, Brazil (Amazon and Parnaíba basins), and South Africa, and the homalonotid Burmeisteria, with records in the Brazil (Amazon, Parnaíba, and Paraná basins), Falkland Islands, South Africa, and Ghana1,15,19,26. Furthermore, the distribution of the brachiopods Tropidoleptus carinatus (Conrad, 1839) and Australocoelia palmata (Mooris & Sharpe, 1846), and the crinoids Exaesiodiscus Moore & Jeffords, 1968, Laudonomphalus Moore & Jeffords, 1968, Monstrocrinus Schmidt, 1941, and Marettocrinus Le Menn15,27,28,29,30,31,32,33,34, also reinforce that connections between the Bolivian-Peruvian region and the Amazon, Parnaíba, and Paraná basins were recurrent by the Middle Devonian (e.g.15,26). However, the dispersal and range expansion events highlighted in our biogeographic analyses (except that related to M. caffer dispersal from the Paraná Basin to South Africa) occurred during the late Lochkovian (Fig. 6). As such, our data suggest an earlier connection between all those Gondwanan regions, allowing Metacryphaeus trilobites to expand into the Paraná and Parnaíba basins via southeastern and northern/northeastern routes, respectively (Fig. 6). Another interesting fact is the diversification of Metacryphaeus in South America occurring earlier than its dispersal to South Africa (where it is represented by M. caffer). This was temporally the latest dispersal of the genus, taking place during the Pragian, and a separate event from the dispersal of M. allardyceae in the same direction (to the Falkland Islands), which occurred earlier.