The origin and phylogenetic relationships of the New Zealand ravens

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Highlights

  • Our phylogenetic analyses supports the monophyly of the New Zealand ravens.

  • New Zealand ravens likely arrived in New Zealand as recently as the late Pliocene.

  • The distribution of remains of this species and ecological niche modelling suggests coastal habitats during the Holocene.

  • Unique features of the skull compared to other species of Corvus imply a unique dietary specialisation.

  • Our genetic analysis only support a single species with one or two subspecies, as opposed to the currently recognised two species and two subspecies.

Abstract

The relationships of the extinct New Zealand ravens (Corvus spp.) are poorly understood. We sequenced the mitogenomes of the two currently recognised species and found they were sister-taxa to a clade comprising the Australian raven, little raven, and forest raven (C.coronoides, C. mellori and C. tasmanicus respectively). The divergence between the New Zealand ravens and Australian raven clade occurred in the latest Pliocene, which coincides with the onset of glacial deforestation. We also found that the divergence between the two putative New Zealand species C. antipodum and C. moriorum probably occurred in the late Pleistocene making their separation as species untenable. Consequently, we consider Corax antipodum (Forbes, 1893) to be a subspecies of Corvus moriorum Forbes, 1892. We re-examine the osteological evidence that led 19th century researchers to assign the New Zealand taxa to a separate genus, and re-assess these features in light of our new phylogenetic hypotheses. Like previous researchers, we conclude that the morphology of the palate of C. moriorum is unique among the genus Corvus, and suggest this may be an adaptation for a specialist diet.

Introduction

Recent studies have argued that the core Corvoidea (crows, jays, and magpies) originated in what is now Wallacea in the late Oligocene/early Miocene and dispersed via the Indo-Pacific archipelago to the rest of the world (Jønsson et al., 2011). Subsequent work has concluded that the crows (genus Corvus) originated in the Palaearctic in the early Miocene and dispersed to North America and the Caribbean, Africa and Australasia (Jønsson et al., 2012). The colonization of Wallacea by the crows is likely to have taken place in the late Miocene, leading to further colonization of Australo-Papua around 5 Ma (Jønsson et al., 2012, Jønsson et al., 2016). Evidence for four colonization events of the Pacific from Asia and Australia have been found (Jønsson et al., 2012).

Following the work of Jønsson and colleagues, two clades of the genus Corvus are now recognised in Australia: the Australian ravens, comprising the Australian raven (C. coronoides), little raven (C. mellori), and forest raven (C. tasmanicus); and the Australian crows, which include the little crow (Corvus bennetti), Torresian crow (Corvus orru), along with the Bismarck crow (Corvus insularis). All the recent genetic work on the evolution of the genus Corvus, however, has been based on the extant species. The status of the recently extinct New Zealand ravens remains unresolved.

The New Zealand (NZ) ravens have received little study since two species were described by Henry Ogg Forbes (Forbes, 1892a, Forbes, 1892c, Forbes, 1893) based on partial sub-fossil skeletons. The two species, one found on the main islands of New Zealand (Corvus antipodum) and the other on the Chatham Islands (Corvus moriorum), appear to have been slightly smaller than the common raven (Corvus corax) but larger than the Australian raven of southern and eastern Australia. Forbes (1892b) suggested that supposedly unique characters of the cranium warranted separating the NZ ravens into their own genus, which he named Palaeocorax. Although the distinctiveness of some of Forbes’ cranial features were questioned (Pycraft, 1911), no author formally recommended the synonymy of Palaeocorax and Corvus until Brodkorb (1978). However, this recommendation was not widely accepted until Gill (2003) reviewed the New Zealand taxa and concluded that Palaeocorax should be regarded as a junior synonym of Corvus. Further, Gill’s (2003) analyses of overall proportions and osteology led him to conclude that “New Zealand ravens probably evolved following the invasion of New Zealand by C. coronoides or a population of crows ancestral to C. coronoides.”

Very little is known about the biology of NZ ravens. Pycraft (1911) found that Chatham Island ravens, in addition to having smaller wings than northern ravens, had a shallower sternal keel and smaller pygostyle, indicating a slightly reduced flight capacity. Brooke (2000) suggested that damage found on the shells of snails of the genus Placostylus in the Far North of New Zealand may have been due to raven predation. Tennyson and Martinson (2007) proposed that the coastal nature of the majority of NZ raven fossil sites and the prevalence of raven bones with the bones of adult and immature sealion (Phocarctos hookeri) may have meant a predator-prey relationship between the two taxa.

In the present study, we use molecular, osteological and distributional data to resolve the origins and phylogenetic relationships of these mysterious extinct New Zealand birds and to give further insight into the palaeobiology and distribution of ravens on the mainland of New Zealand.

Section snippets

Ancient DNA extraction and sequencing

We obtained bone samples of both Corvus antipodum (CM Av 12546, Marfells Beach, Marlborough) and Corvus moriorum (CM Av5468, Chatham Island) from the collections of the Canterbury Museum, New Zealand. Both samples are from dune deposits and are of late Holocene age. All DNA extraction and library preparation steps were performed in a purpose-built, physically isolated, ancient DNA laboratory at the Australian Centre for Ancient DNA, University of Adelaide.

Digestion of bone samples and DNA

Phylogenetic analyses

Our phylogenetic analyses (Fig. 1, Supplementary Information Tables 1&2) supported the monophyly of the NZ ravens (Bayesian posterior probability [BPP] = 1.0, maximum likelihood bootstrap [MLB] = 100%), and strongly supported a sister-taxon relationship between this endemic New Zealand lineage and the clade comprising the Australian raven, little raven, and forest raven (BPP = 1.0. MLB = 95%). This combined clade fell within the Australasian/Wallacean radiation of large ravens (“Clade VII”) previously

Size variation

Due to the similarity of the postcranial skeleton of. C. antipodum and C. moriorum, and the absence of intact cranial material attributable to C. antipodum, distinguishing between the two species based on osteological features is not possible (Gill, 2003). Forbes (1893) considered that the smaller size of North Island birds warranted their separation as a different species. The separation of mainland NZ raven and Chatham Island raven was not questioned until they were lumped into C. moriorum,

Conclusions

Our phylogenetic analyses supports the monophyly of the NZ ravens and that the NZ ravens likely arrived in New Zealand as recently as the late Pliocene. The distribution of remains of this species and ecological niche modelling suggests coastal habitats during the Holocene. Unique features of the skull compared to other species of Corvus imply a unique dietary specialisation and we suggest that this species may have been a facultative scavenger at the vast sea-lion rookeries that once lined the

Acknowledgements

We thank the Canterbury Museum for access to specimens, to Trevor Worthy for data on New Zealand sub-fossil Corvus distribution and N. Zelenkov and A. Mangold for discussion on fossil status. Grid computing facilities were provided by CIPRES (Cyberinfrastructure for Phylogenetic Research). Funding was provided by the Royal Society of New Zealand’s Marsden Fund and the Australian Research Council.

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