A Pliocene boxfish (Tetraodontiformes, Ostraciidae) from New Zealand – a preview of future environmental change?

ABSTRACT

We report on an articulated fossil boxfish (Tetraodontiformes, Ostraciidae) recently recovered from the Pliocene of the North Island of New Zealand. The specimen was collected from the Tangahoe Formation, a mid-Pliocene (c. 3.0-3.4 Ma) shallow marine deposit, at Waihi Beach, South Taranaki. The fossil boxfish measures 10.7 cm in standard length, with an estimated total length of c. 13–14 cm (the caudal fin is not preserved). The fish is preserved in right lateral view, lying on its side, and has an intact body covering of fused hydroxyapatite plates that rigidly encase the fish, as is characteristic of boxfishes. The plates are hexagonal to subhexagonal in shape and largest close to the dorsal midline. Fossil boxfish have previously been recorded from Northern Hemisphere sites ranging in age from Palaeocene to Quaternary, but not from the Southern Hemisphere. Recent reports note that boxfishes and several other tropical Pacific fish species are now being seen off of northern New Zealand – the Pliocene boxfish from Taranaki, as well as an intriguing addition to New Zealand’s paleohistory, may also reflect how the ongoing impact of climate change will return New Zealand to a warmer marine ecosystem – similar to what prevailed during the Pliocene.

Introduction

The Boxfishes (Tetraodontiformes, OSTRACIOIDEA) are morphologically distinctive marine fishes that occur at tropical to mild temperate latitudes in the Atlantic, Pacific and Indian oceans, often on and around coral reefs. The clade is moderately diverse, with ca. 35–40 species in 11 or possibly 12 genera. Two families of boxfishes are generally recognised (Santini et al. 2013; Matsuura 2015) – the Aracanidae (‘deep water’ boxfishes) with +/− 13 species, and the Ostraciidae with +/− 24 species. Boxfishes (also variously called cowfish, trunkfish or turretfish) are unmistakable, with the extreme modification of having their bodies largely encased in a carapace of tightly fused hydroxyapatite plates that are most commonly hexagonal in shape. This forms a rigid structure that greatly reduces their swimming speed and mobility but provides protection against predators (Brainerd and Patek 1998). The ‘box’ varies from triangular to rectangular to rounded in cross-sectional shape, and also, depending on the taxon, can be associated with strongly developed keels, spines or spikes. Boxfishes lack pelvic and spiny dorsal fins, can be poisonous, and have small ventrally positioned and protuberant terminal mouths with relatively few conical teeth, which they use to feed primarily on invertebrates and algae.

The fossil record of ostracioids is relatively sparse in occurrences but broad temporally and geographically. Specimens assigned to the group are known from the following: the Paleocene of Denmark (Schwarzhans 2003, based on otoliths) and South Carolina, USA (Weems 1998); the Eocene of Monte Bolca, Italy (Tyler and Santini 2002; Bannikov 2014) and Alabama, USA (Ebersole et al. 2019, based on otoliths, and Thurmond and Jones 1981); the Oligocene of the Czech Republic (Tyler and Gregorova 1991); an unpublished record from the Pliocene of Florida (Paleobiology Database paleobiodb.org); and the Pleistocene of Georgia, USA (Hulbert and Pratt 1998). Subfossil Holocene records have also been recorded, from the Marshall Islands in the Pacific and the Turks and Caicos in the Caribbean (paleobiodb.org). While not extensive, these records are broadly consistent with the hypothesised early Cenozoic origin of the group (Santini et al. 2013) and the present-day broad distribution of boxfishes in the Atlantic, Indian and Pacific at mild temperate to tropical latitudes.

We describe here a recently discovered and nearly complete articulated fossil boxfish from the Pliocene of New Zealand. The specimen preserves a nearly complete covering of mainly hexagonal plates forming a rigid carapace and other features that readily identify it as an ostracioid. This is the first pre-Holocene record of the group from the Pacific, and the first fossil record from the Southern Hemisphere as a whole. The specimen adds a distinctive new taxon to the emerging Pliocene record from New Zealand, which has recently included the first discovery of a fossil monk seal from the Southern Hemisphere (Rule et al. 2020). The fossil boxfish discovery also resonates with respect to considering how New Zealand’s marine ecosystem and faunal dynamics may be impacted by ongoing climate change.

Systematic palaeontology

Order TETRAODONTIFORMES Berg, 1940

Suborder OSTRACOIDEI sensu Betancur-R et al. 20173

Superfamily OSTRACIOIDEA sensu Tyler, 1980

Family OSTRACIIDAE sensu Tyler, 1980

Genus Tetrosomus (Linnaeus 1758)

cf. Tetrosomus sp.

Material. – A single articulated and nearly complete specimen catalogued in the Museum of New Zealand TePapa Tongarewa as NMNZ S.46695, collected and prepared in 2013/2014 by John Buchanan-Brown.

Locality and Geologic and Faunal Context. – The specimen is preserved in a mudstone concretion collected from the Tangahoe Formation, a mid-late Pliocene ((Piacenzian or Waipipian New Zealand Stage, age estimated as ca. 3.36–3.06 Ma) shallow marine deposit, at Waihi Beach, South Taranaki, North Island of New Zealand (39⁰36.02’ South, 174⁰13.82’ East). The Tangahoe Formation comprises a 270 m thick sequence of shallow shelf to shoreline sediments that represent alternating transgressive and regressive phases related to glacio-eustatic sea-level changes (Naish et al. 2005). Collecting efforts focused on the Tangahoe Formation have resulted in an intriguing range of discoveries, including a growing diversity of procellariid seabirds (Tennyson and Salvador 2023 and references therein), penguins (Thomas et al. 2020, 2023), and the first record of a monk seal from the Southern Hemisphere (Rule et al. 2020). The specimens collected to date include a number of articulated teleost fish specimens reposited in the collections of the Museum of New Zealand TePapa Tongarewa (author’s pers. obs.) that are just beginning to be investigated, including the boxfish that is the focus of this contribution.

Description. – The fossil boxfish (Figure 1) is preserved in right lateral aspect, with some degree of flattening of the ventral portion of the carapace, and as preserved measures 11.91 cm in greatest length with a maximum body depth of 5.41 cm. Most strikingly, nearly the entire body is encased in tightly connected hexagonal to sub-hexagonal to pentagonal plates of hydroxyapatitie, the largest of which, in the mid-body region, measures 1.2 cm in length. There are 7–8 rows of plates on the lateral surface of the ‘box’ and 4 rows of plates preserved on the ventral surface – the margin between the lateral and ventral surfaces is indicated by a thickened ridge or keel that marks the ventral edge of the laterally positioned plates, and we note that there has been some degree of flattening out of the specimen in this region. The plates are largest dorsal to the midline and in the posterior half of the body, and are generally smaller further ventrally. The plates that are well-preserved have a rounded and raised central area, and a light ornament of ridges that approximately follow the outline shape of the plate, with irregular but discrete small rounded bumps. A portion of the lateral line is visible as a series of raised linear ridges that pass through the middle of the three largest plates in the dorsal mid-body region (Figure 1).

Figure 1.

Pliocene boxfish (NMNZ S.46695) from the Tangahoe Formation. Waihi Beach, South Taranaki, New Zealand.

A prominent spine is located along the dorsal edge of the carapace (Figure 1), and two spines are situated along the division between the lateral and ventral portions of the carapace. These spines are strongly developed and extend at an angle from the body, with the dorsal spine and the larger and more posteriorly positioned ventral spine ornamented with fine sub-parallel ridges that converge at the apex of each spine. The two larger spines are conspicuously more strongly developed than the smaller more anterior ventral spine, and positioned opposite one another at just over 50% of the body length posterior to the head. The dorsal spine measures 2 cm in length while the larger and more posterior ventral spine is 1.7 cm long, and the smaller and more anterior ventral spine is 0.8 cm long.

The anterior portion of the specimen includes a small terminal and ventrally positioned mouth opening that was likely slightly protuberant, although this area is not as well-preserved. Under magnification a small subrectangular toothed element (∼0.65 cm wide) is visible dorsal and anterior to the mouth opening. This element bears four very small black incisiform teeth, which face dorsally and away from the mouth opening (Figure 1). This is most likely a premaxilla that has been displaced and rotated away from the mouth opening. No other jaw or other skull elements are visible.

The anterior dorsal portion of the body has a thickened ridge extending above the area where the orbit would be located (the eye is not preserved). This would have formed a supraorbital ‘brow’ extending above the orbital region, a common feature on boxfishes, but it is not (apparently) associated with an anteriorly prolonged spine as prominently occurs on the boxfish genus Lactoria (cowfish).

The pectoral fin is not preserved on the specimen. Several faintly preserved partial fin rays pertaining to the dorsal fin are present, posterior to the dorsal spine and relatively close to the caudal peduncle. These are sufficient to indicate the relatively far posterior position of the fin, but do not show the fin’s shape. A portion of the anal fin is preserved, consisting of ca. 10 partial fin rays arrayed along the ventroposterior margin of the body. These suggest a relatively long-based anal fin ca. 2 cm in length that is tucked into the posteroventral curve of the body, ventral and anterior to the anterior end of the caudal peduncle. The anterior portion of the caudal peduncle is represented, extending straight out and parallel to the long axis of the body – the preserved portion measures 2.2 cm in length. The external covering of the caudal peduncle is incomplete and not well-preserved, and it is not clear if discrete individual plates are present. No portion of the caudal fin itself is preserved.

Taxonomic Assignment. – NMNZ S.46695 can clearly and unambiguously be assigned to the OSTRACIOIDEA [boxfishes] based on the rigid carapace covering of hexagonal to subhexagonal hydroxyapatite plates, a defining feature for the group [see e.g. Tyler and Gregorova 1991], along with other boxfish features including the lack of a spiny dorsal or pelvic fin and a box-like body in cross-section. Within OSTRACIOIDEA, we provisionally place the specimen in the extant genus Tetrosomus (Family Ostraciidae) primarily based on the strongly developed and ornamented dorsal and ventral spines, and the rectangular cross-sectional shape of the ‘box,’ as seen in the extant species T. gibbosus (Matsuura 2014). Extant Tetrosomus can reach up to 30 cm in total length but are more typically 15-20 cm total length as adults – in comparison the fossil boxfish is estimated to have had a total length 13-14 cm with the caudal fin included. We note that all three extant species of Tetrosomus (T. concatenatus, T. gibbosus, and T. reipublicae) occur in Australian waters (Matsuura 2014) and therefore, in relative terms, they are geographically proximate to New Zealand.

Discussion

The presence of a boxfish adds a distinctive new record to our emerging understanding of New Zealand’s Pliocene marine biodiversity, and as the first fossil record of a boxfish in the Southern Hemisphere it is a significant expansion of the paleogeographic distribution of the clade. It indicates that boxfishes reached well into the Southern Hemisphere (latitude ∼40⁰ South) by the Pliocene, no later than 3–4 million years ago [molecular data suggest an initial divergence of boxfishes from other tetraodontiforms as early as the late Cretaceous ∼65–70 Ma; Arcila et al. 2015]. The group’s broader range of occurrences, both geographically and temporally, in the Northern Hemisphere suggests an origin and initial diversification of the group in that hemisphere, although this is of course subject to change if there are further fossil discoveries from southern ocean sites. The data available are not sufficient to consider if boxfishes made one or possibly multiple transhemispheric crossings through the course of the Cenozoic.

The Pliocene boxfish naturally leads to consideration of what implications it may have, in a predictive context, for anticipating the consequences of future climate and environmental change around New Zealand. As already evidenced by e.g. the presence of a Monk seal (Rule et al. 2020) and relatively warm-water molluscs (Beu 1974), the marine environment around New Zealand was several degrees warmer 3–4 million years ago than it is today. As the present-day climate continues to warm further it may, in the not too distant future – and within a human historical timescale – revert back to warmer and more equable conditions much like those in the Pliocene (see e.g. Warne 2005). Recent data indicate increasingly more intense heatwaves impacting the marine environment around New Zealand, including ‘unparalleled’ warming in 2021/22 (Salinger et al. 2023). Boxfishes have been anecdotally (Hueber 2011) and formally (Matsuura and Middleton 2017) reported by divers around the northeastern coast of the North Island in recent years. These records include a pelagic late stage larval individual of the aracanid Kentrocapros sp. (Matsuura and Middleton 2017) that suggests the possibility of larval recruitment in New Zealand waters. Other subtropical to tropical species have been reported from northern New Zealand waters (Francis et al. 1999; Dunn et al. 2022), so additional boxfish records may not be very surprising. As the more intensively warming climate continues to alter ecosystems and distributional patterns we might think in terms of a ‘once and future New Zealand’ as more tropical species, including boxfishes, are increasingly able to invade and survive in a near-future ‘warm world’ New Zealand marine environment that resembles conditions in the Pliocene. This has the potential to displace more familiar New Zealand fishes, including commercially valuable species such as Orange Roughy and Blue Cod, which may in future struggle to hold on in a less temperate and more tropically influenced marine environment.

Disclosure statement

No potential conflict of interest was reported by the author(s).