There are two extant (living) groups of agnathans (jawless fishes), lampreys and hagfish. Their ancestral relatives were thought to have been very basal on the tree of vertebrates, hence, their evolutionary history is of immense scientific interest. The oldest fossil lampreys, known from Waterloo Farm, have challenged some of the established textbook theory about the origins of jawless fishes.
Priscomyzon riniensis (‘ancient sucker from Rini’) is an extinct lamprey known from fossils found at Waterloo Farm. It is the world’s oldest fossil lamprey by around 35 million years. This species was described in Nature (1), and it has important implications for the early evolution of lampreys. A subsequent paper in Nature (2), described a near-complete ontogenetic series of this species from Waterloo Farm, which represents the most complete Palaeozoic lamprey ontogeny.
The holotype (representing the only presumed adult) is 42mm long. It has a large oral disc, surrounded by 14 small teeth and a soft outer lip. The arrangement of the teeth is similar to some modern lampreys, although they generally have more teeth. Like extant lampreys it has seven gill pouches, whereas jawed fishes have only five. Modern lampreys are parasitic or predatory, possessing a large round sucker disk surrounded by small hooked teeth, with which they latch onto other fishes. A rasping tongue is then used to access the bodily fluids of the host, which are sucked out with a pumping action. Priscomyzon preserves a prominent sucker disc, suggesting that since early in their evolution lampreys have maintained a parasitic existence. In this manner, they have survived four of the ‘big five’ extinction events. Textbook theories challenged! Lampreys and their larvae were long thought to preserve the characteristics of creatures spanning the divide between invertebrates and vertebrates. This perception has however been falsified based on fossils of Priscomyzon from Waterloo Farm, with supporting evidence from three other (Carboniferous) fossil lamprey genera (2). Larvae of current day lampreys, commonly known as ammocoetes, lack many vertebrate characteristics such as vertebrae and proper eyes. They bury themselves in sediment in freshwater streams in order to filter feed, and somewhat resemble invertebrate chordates such as Amphioxus – which pursues a similar lifestyle. They then transform, through a number of stages, into the parasitic, clearly vertebrate, adults with well-developed eyes, vertebral elements and characteristic mouths. This pattern of development appeared to re-enact an early transitional phase in the origin of vertebrates, between filter feeding Amphioxus-like invertebrate creatures and early vertebrates. This apparent re-enactment is called the ontogenetic recapitulation of phylogeny (recapitulation theory). We now know that this is not demonstrated by lampreys. None of the oldest fossil genera had a filter feeding larval stage. From Waterloo Farm a full ontogenetic series including (yolk-bearing) hatchlings of Priscomyzon demonstrates this. The ammocete larvae of modern forms is a derived character, acquired late in their evolutionary history, and does not reflect vertebrate origins. Their resemblance to Amphioxus is simply an adaptation to a similar lifestyle. Lampreys themselves are probably not ‘primitively’ naked and cartilaginous but rather descended from bonier jawless ancestors [2]. References:
1) Gess, R. W., Coates, M. I., & Rubidge, B. S. (2006). A lamprey from the Devonian period of South Africa. Nature, 443(7114), 981. 2) Miyashita, T., Gess, R. W., Tietjen, K., Coates, K. I. (2021). Non-ammocoete larvae of Palaeozoic stem lampreys. Nature, 591, 408-412. |
|