Patterns of ecological diversification in thelodonts

Abstract

Here we explore the spatial, temporal and phylogenetic patterns of ecological diversification for the entire clade of thelodonts, one of the earliest groups of vertebrates and longest lasting of the Palaeozoic agnathans in the fossil record. Parsimony and maximum‐likelihood methods are used to reconstruct ancestral states of their geographical distributions, habitats and lifestyles. Our results support the concept that thelodonts originated during the Middle?–Late Ordovician probably in marine open waters of Laurasia, with a demersal lifestyle on hard substrates being the ancestral condition for the whole clade. Later, thelodonts underwent a complex ecological diversification and palaeobiogeographical history, comparable in many aspects to those of some major groups of living fishes. Different modes of life evolved repeatedly and a wide range of habitats were colonized by distinct groups, including deep waters and brackish marine and/or freshwater environments. Diadromous strategies presumably appeared on nine different occasions. The palaeobiogeographical history of thelodonts reveals significant differences in the dispersal potential of some major groups. Dispersal of thelodontiforms entailed displacements over long distances and the crossing of deep‐water biogeographical barriers, whereas those of furcacaudiforms were always limited to areas interconnected by shallow platforms. We propose that the evolution of pelagic larval stages in thelodontiforms might explain this biogeographical pattern and could satisfactorily account for the greater evolutionary success of this group.

Evolutionary analysis of swimming speed in early vertebrates challenges the ‘New Head Hypothesis’

“…2 and S4 ). This provides robust support for previous proposals of more active lifestyles in thelodonts and anaspids 25 , 26 , with higher potential for migration, dispersal and colonization of a wider range of habitats 26 , 42 . Therefore, this finding offers a causal framework for understanding the contrasting palaeobioegeographic patterns that characterize each of these groups, where heavily armoured lineages have distributions marked by a strong endemism limited to areas connected by shallow continental shelfs, whereas microsquamous taxa had more cosmopolitan distributions resulting from the crossing of deep-water oceanic basins 43 – 48 .…”

“…Each of the obtained trees was time calibrated 100 times using the R package ‘paleotree’ 63 by randomizing the tip age of every species within the chronostratigraphic unit, at age or subperiod rank, where their first appearance occurs. A minimum age constraint was set in the ancestral nodes of the main clades of stem-gnathostomes considering their first appearance in the fossil record (Conodonta, Furongian 64 ; Anaspida, Llandovery 20 ; Pteraspidomorphi, Darriwilian-Sandbian 65 , 66 ; Thelodonti, Sandbian 26 ; Osteostraci, Aeronian 67 ; Placodermi, Telychian–Wenlock) 68 .…”

Middle Triassic sharks from the Catalan Coastal ranges (NE Spain) and faunal colonization patterns during the westward transgression of Tethys

“…In this paper, we document for the first time the presence of chondrichthyans in the Middle Triassic rocks of the Catalan Coastal ranges (CCR). This paper is complementary to Pla et al's (2013) and Manzanares et al's (2018) studies on Middle-Late Triassic chondrichthyans from other areas of the Iberian Peninsula, i.e. the Iberian (IR) and Betic ranges (BR), respectively.…”

“…A larval stage is present in agnathan lampreys (but not in mixines) and consists of a large sedentary burrowed larva in freshwater streams (e.g. Kelly and King, 2001), and could to be present in other extinct Palaeozoic agnathan fishes (Ferrón et al 2018). The cosmopolitanism and high dispersal of Palaeozoic conodonts might also agree with a planktic dispersion drifted by ocean currents; however, further and more extensive studies on this topic are necessary to support this idea.…”

Silurian vertebrate remains from the Oslo Region, Norway, and their implications for regional biostratigraphy

“…The usually more abundant record of disarticulated scales and fragments of dermal bone, recovered by acid preparation, provides greater potential for high-resolution biostratigraphy and correlations (Long, 1994;Blieck & Turner, 2000;Märss et al, 2007;Märss & Männik, 2013). Thelodonts and, more continuously so, acanthodians have been shown to be particularly useful in this context because their taxonomically identifiable isolated scales are widespread in the Silurian and Devonian of most palaeogeographical provinces (e.g., Turner, 1997Turner, , 1999Valiukevičius, 1998Valiukevičius, , 2005Märss et al, 2006Märss et al, , 2007Hairapetian et al, 2016;Ferrón et al, 2018). In recent years, detailed Worsley et al (1983) showing the districts discussed in the text, as well as all Silurian outcrops in black.…”