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![Pikaia gracilens, as reconstructed by Conway Morris and Caron[1].The head bears a pair of tentacles, probably sensory in nature, and paired rows of ventrolateral projections that may be gills. Not shown: the expanded anterior (pharyngeal) region of the digestive tract, and the dorsal shield-like structure, the anterior dorsal unit, that lies above it. The boxed detail shows the main axial features: the dorsal organ (do), and the putative notochord (not) and digestive tract (dt). The size range among specimens is 1.5 to 6 cm, which makes this animal very close in size to the adult stage of modern lancelets (amphioxus)](publication/225306766/figure/fig1/AS:196127681978372@1423771891129/Pikaia-gracilens-as-reconstructed-by-Conway-Morris-and-Caron1The-head-bears-a-pair-of.png)
Pikaia gracilens, as reconstructed by Conway Morris and Caron[1].The head bears a pair of tentacles, probably sensory in nature, and paired rows of ventrolateral projections that may be gills. Not shown: the expanded anterior (pharyngeal) region of the digestive tract, and the dorsal shield-like structure, the anterior dorsal unit, that lies above it. The boxed detail shows the main axial features: the dorsal organ (do), and the putative notochord (not) and digestive tract (dt). The size range among specimens is 1.5 to 6 cm, which makes this animal very close in size to the adult stage of modern lancelets (amphioxus)
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![Pikaia gracilens, as reconstructed by Conway Morris and Caron[1].The...](publication/225306766/figure/fig1/AS:196127681978372@1423771891129/Pikaia-gracilens-as-reconstructed-by-Conway-Morris-and-Caron1The-head-bears-a-pair-of_Q320.jpg)
Conway Morris and Caron (2012) have recently published an account of virtually all the available information on Pikaia gracilens, a well-known Cambrian fossil and supposed basal chordate, and propose on this basis some new ideas about Pikaia's anatomy and evolutionary significance. Chief among its chordate-like features are the putative myomeres, a...
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... This begs the question of why a notochord would be needed at all when the basal lamina separating the putative myotomes is as sturdy as it appears to be in Pikaia, since a series of box-like chambers, given sufficient internal hydrostatic pressure, would support the body by themselves. Because the shape of the myotomes suggests that Pikaia did not engage in rapid escape swimming [7], it may not have required a particularly robust support system in any case, and there are, in addition, other ways to support the body during undulatory swimming (see Comments to [7]). Coupled with these objections is the fact that the best-preserved axial traces are ventral, identified by CMC as a possible blood vessel (their Figs. ...
... This begs the question of why a notochord would be needed at all when the basal lamina separating the putative myotomes is as sturdy as it appears to be in Pikaia, since a series of box-like chambers, given sufficient internal hydrostatic pressure, would support the body by themselves. Because the shape of the myotomes suggests that Pikaia did not engage in rapid escape swimming [7], it may not have required a particularly robust support system in any case, and there are, in addition, other ways to support the body during undulatory swimming (see Comments to [7]). Coupled with these objections is the fact that the best-preserved axial traces are ventral, identified by CMC as a possible blood vessel (their Figs. ...
The Middle Cambrian fossil Pikaia has a regular series of vertical bands that, assuming chordate affinities, can be interpreted as septa positioned between serial myotomes. Whether Pikaia has a notochord and nerve cord is less certain, as the dorsal organ, which has no obvious counterpart in living chordates, is the only clearly defined axial structure extending the length of the body. Without a notochord to serve as a reference point, the location of the nerve cord is then conjectural, which begs the question of how a dorsal neural center devoted to somite innervation would first have arisen from a more diffuse ancestral plexus of intraepithelial nerves. This question is examined using hemichordates as a reference point, first for the information they provide on the organization of the ancestral deuterostome nervous system, and second, extending the analysis of E. E. Ruppert, to explain why neural infoldings like the enteropneust collar cord would first have evolved. Both implicate the medial surface of the anterior-most part of the metacoel as the likely site for the evolution of the first somites. The analysis highlights the importance of the somatobranchial condition in chordates, meaning the linkage between the anterior trunk, hox1 expression, and the beginning of the gill series and somites. This feature is arguably a valid criterion by which to assess extinct taxa from the Cambrian that resemble chordates (e.g., vetulicolians and yunnanozoans), but may be unrelated to them. In a more speculative vein, the nature of the dorsal organ is discussed, including the possibility that it is an expanded neural tube combining neural and support functions in one structure.
... One outstanding feature of the basic organization of chordates was the construction of a central axis with affixed muscles, which makes the motoric system quite effective. The crucial innovation was the chorda-myomere system (Koob & Long, 2000;Lacalli, 2012;Satoh et al., 2012;Fig. 3). ...
Studies of macroevolution have revealed various trends in evolution – which have been documented and discussed. There is, however, no consensus on this topic. Since Darwin's time one presumption has persisted: that throughout evolution organisms increase their independence from and stability towards environmental influences. Although this principle has often been stated in the literature, it played no role in mainstream theory. In a closer examination, we studied this particular feature and described that many of the major transitions in animal evolution have been characterized by changes in the capacity for physiological regulation. Organisms gained in robustness, self‐regulation, homeostasis and stabilized self‐referential, intrinsic functions within their respective systems. This is associated with expanded environmental flexibility, such as new opportunities for movement and behaviour. Together, these aspects can be described as changes in the capacity for autonomy. There seems to be a large‐scale trajectory in evolution during which some organisms gained in autonomy and flexibility. At the same time, adaptations to the environment emerged that were a prerequisite for survival. Apparently, evolution produced differential combinations of autonomy traits and adaptations. These processes are described as modifications in relative autonomy because numerous interconnections with the environment and dependencies upon it were retained. Also, it is not a linear trend, but rather an outcome of all the diverse processes which have been involved during evolutionary changes. Since the principle of regulation is a core element of physiology, the concept of autonomy is suitable to build a bridge from physiology to evolutionary research. image
... Altogether, we conclude that the hypothesis of a prechordal nature of the elongated amphioxus notochord is consistent with the evidence presented. Members of the phylum chordata are characterized by sharing structural components of the body plan, such as pharyngeal gill slits, segmental muscles, a dorsal nerve cord and the notochord (chordamesoderm) [Brusca and Brusca, 1990;Lacalli, 2012; Di Gregorio, 2020]. Chordate animals can be divided into three subphyla: cephalochordates (amphioxus or lancelets), urochordates (tunicates) and vertebrates (whose most early-branching lineage relative to the human one is represented by the agnatha, that is, hagfishes and lampreys). ...
... Modern studies also have corroborated this conclusion [Jurand 1974;Morris -Kay and Tuckett, 1987;Sausedo and Schoenwolf, 1994;Sulik et al., 1994;Barteczko and Jacob, 2002;Puelles et al., 2012;Puelles and Rubenstein 2015]. The notochord proper is thus strictly coextensive with the neural floorplate, this being a causal relationship, since floorplate specification and differentiation is selectively induced by notochordal morphogens [Marti et al., 1995;Placzek, 1995;Ericson et al., 1996;Harland and Gerhart, 1997;Dodd et al., 1998;Placzek et al., 2000;Stemple, 2005;Sanchez-Arrones et al., 2009, 2012. Consequently, the developing notochord of vertebrates never appears associated to the infundibular (neurohypophysis), tuberal, chiasmatic, preoptic (lamina terminalis) or septal (anterior commissure) median forebrain regions, which collectively represent the rostromedian acroterminal domain influenced inductively in a temporal sequence first by the basal plate-attached prechordal plate, and later by dispersed migratory prechordal cells related to the alar portion. ...
This essay re-examines the singular case of the supposedly unique rostrally elongated notochord described classically in amphioxus. We start from our previous observations in hpf 21 larvae [Albuixech-Crespo et al., 2017] indicating that the brain vesicle has rostrally a rather standard hypothalamic molecular configuration. This correlates with the notochord across a possible rostromedian acroterminal hypothalamic domain . The notochord shows some molecular differences that specifically characterize its pre-acroterminal extension beyond its normal rostral end under the mamillary region. We explored an alternative interpretation that the putative extension of this notochord actually represents a variant form of the prechordal plate in amphioxus, some of whose cells would adopt the notochordal typology, but would lack notochordal patterning properties, and might have some (but not all) prechordal ones instead. We survey in detail the classic and recent literature on gastrulation, prechordal plate and notochord formation in amphioxus, compared the observed patterns with those of some other vertebrates of interest, and re-examine the literature on differential gene expression patterns in this rostralmost area of the head. We noted that previous literature failed at identifying the amphioxus prechordal primordia at appropriate stages. Under this interpretation, a consistent picture can be drawn for cephalochordates, tunicates, and vertebrates. Moreover, there is little evidence for an intrinsic capacity of the early notochord to grow rostralwards (it normally elongates caudalwards). Altogether, we conclude that the hypothesis of a prechordal nature of the elongated amphioxus notochord is consistent with the evidence presented.
... The legacy of this caudal appendage from the early chordates is its use as a propulsive apparatus in many vertebrate clades to move through the aquatic medium. Swimming by basal chordates originated with serial activation of myomeres to produce a wriggling movement of the body and tail for swimming (Webb 1973;Stokes 1997;Lacalli 2012). From the basal chordates, movement through water by the majority of fishes used body and caudal fin (BCF) swimming (Breder 1926;Webb 1975Webb , 1982Webb , 1984Lindsey 1978;Sfakiotakis et al. 1999). ...
Synopsis Secondary aquatic vertebrates exhibit a diversity of swimming modes that use paired limbs and/or the tail. Various secondarily aquatic tetrapod clades, including amphibians, reptiles, and mammals employ transverse undulations or oscillations of the tail for swimming. These movements have often been classified according to a kinematic gradient that was established for fishes, but may not be appropriate to describe the swimming motions of tetrapods. To understand the evolution of movements and design of the tail in aquatic tetrapods, we categorize the types of tails used for swimming and examine swimming kinematics and hydrodynamics. From a foundation of a narrow, elongate ancestral tail, the tails used for swimming by aquatic tetrapods are classified as tapered, keeled, paddle, and lunate. Tail undulations are associated with tapered, keeled, and paddle tails for a diversity of taxa. Propulsive undulatory waves move down the tail with increasing amplitude toward the tail tip, while moving posteriorly at a velocity faster than the anterior motion of the body indicating that the tail is used for thrust generation. Aquatic propulsion is associated with the transfer of momentum to the water from the swimming movements of the tail, particularly at the trailing edge. The addition of transverse extensions and flattening of the tail increases the mass of water accelerated posteriorly and affects vorticity shed into the wake for more aquatically adapted animals. DPIV (Digital Particle Image Velocimetry) reveals differences were exhibited in the vortex wake between the morphological and kinematic extremes of the alligator with a tapering undulating tail and the dolphin with oscillating wing-like flukes that generate thrust. In addition to exploring the relationship between shape of undulating tails and swimming performance across aquatic tetrapods, the role of tail reduction or loss of a tail in aquatic-tetrapod swimming was also explored. For aquatic tetrapods, reduction would have been due to factors including locomotor and defensive specializations and phylogenetic and physiological constraints. Possession of a thrust-generating tail for swimming, or lack thereof, guided various lineages of secondarily aquatic vertebrates into different evolutionary trajectories for effective aquatic propulsion (i.e., speed, efficiency, acceleration).
... In Pikaia (Fig. 1A), the myomeral configuration has segmented muscle blocks, which are gentlycurved (Conway Morris and Caron 2012). These have been interpreted as narrow slow-twitch fibers (Lacalli 2012). Myomeres in Metaspriggina ( Fig. 1B and C), on the other hand, have a clear chevron or V-shaped arrangement and the caudal region shows myomeres which are more steeply inclined (Conway Morris and Caron 2014). ...
... Myomeres in Metaspriggina ( Fig. 1B and C), on the other hand, have a clear chevron or V-shaped arrangement and the caudal region shows myomeres which are more steeply inclined (Conway Morris and Caron 2014). This configuration is close to Branchiostoma (Fig. 1D) (Lacalli 2012) and more directly comparable to fishes (Van Leeuwen 1999), suggesting that Metaspriggina would have likely been capable of swimming rapidly operating in the fasttwitch mode for escape. Since Metaspriggina is considered a true vertebrate (Conway Morris and Caron 2014), the myomere configuration offers an opportunity to illuminate the origin of high-performance swimming in basal fishes. ...
We use a series of hydrodynamic experiments on abstracted models to explorewhether primitive vertebrates may have swum under various conditions without a clearly-differentiated tail fin. Cambrian vertebrates had post-anal stubby tails, some had single dorsal and ventral fins, but none had yet evolved a clearly differentiated caudal fin typical of post-Cambrian fishes, and must have relied on their long and flexible laterally-compressed bodies for locomotion, i.e. by bending their bodies side-to-side in order to propagate waves from head to tail. We approach this problem experimentally based on an abstracted model of Metaspriggina walcotti from the 506-million-year old Burgess Shale by using oscillating thin flexible plates while varying the tail fin geometry from rectangular to uniform, and finally to a no tail-fin condition. Despite a missing tail fin, this study supports the observation that the abstracted Metaspriggina model can generate a strong propulsive force in cruise conditions, both away from, and near the sea bed (in ground effect). When considering acceleration from rest, we find that the Metaspriggina model's performance is not significantly different from other morphological models (abstracted truncate tail and abstracted heterocercal tail). In contrast, when the abstracted Metaspriggina model moves in ground effect, a weaker performance is observed, indicating that Metaspriggina may not necessarily have been optimized for swimming near the sea bed. Statistical analysis shows that morphological parameters, swimming modes, and ground effect all play significant roles in thrust performance. While the exact relationships of Cambrian vertebrates are still debated, as agnathans, they share some general characteristics with modern cyclostomes, in particular an elongate body akin to lampreys. Lampreys, as anguilliform swimmers, are considered to be some of the most efficient swimmers using a particular type of suction thrust induced by the traveling body wave as it travels from head to tail. Our current experiments suggest that Metaspriggina's ability in acceleration from rest, through possibly a similar type of suction thrust, which is defined as the ability to generate low pressure on upstream facing sections of the body, might have evolved early in response to increasing predator pressure during the Cambrian Explosion.
... Myomeres seem to be denser in the trunk and caudal region than in the front part. Their size and shape gradually change along with the body, and caudal myomeres are more steeply inclined than the anterior ones (Lacalli, 2012). Basal to Haikouichthys is Pikaia gracilens (Walcott, 1911), known from fossil remains of the Middle Cambrian. ...
... Due to the presence of numerous sigmoidal myomeres, P. gracilens has been classified as a stem chordate (Morris & Caron, 2012) or as basal members of the chordate lineage (Lacalli, 2012). Myomeres in P. gracilens resemble slow muscle fibers in fishes. ...
It has long been assumed that serial homologues are ancestrally similar-polysomerism resulting from a "duplication" or "repetition" of forms-and then often diverge-anisomerism, for example, as they become adapted to perform different tasks as is the case with the forelimb and hind limbs of humans. However, such an assumption, with crucial implications for comparative, evolutionary, and developmental biology, and for evolutionary developmental biology, has in general not really been tested by a broad analysis of the available empirical data. Perhaps not surprisingly, more recent anatomical comparisons, as well as molecular knowledge of how, for example, serial appendicular structures are patterned along with different anteroposterior regions of the body axis of bilateral animals, and how "homologous" patterning domains do not necessarily mark "homologous" morphological domains, are putting in question this paradigm. In fact, apart from showing that many so-called "serial homologues" might not be similar at all, recent works have shown that in at least some cases some "serial" structures are indeed more similar to each other in derived taxa than in phylogenetically more ancestral ones, as pointed out by authors such as Owen. In this article, we are taking a step back to question whether such assumptions are actually correct at all, in the first place. In particular, we review other cases of so-called "serial homologues" such as insect wings, arthropod walking appendages, Dipteran thoracic bristles, and the vertebrae, ribs, teeth, myomeres, feathers, and hairs of chordate animals. We show that: (a) there are almost never cases of true ancestral similarity; (b) in evolution, such structures-for example, vertebra-and/or their subparts-for example, "transverse processes"-many times display trends toward less similarity while in many others display trends toward more similarity, that is, one cannot say that there is a clear, overall trend to anisomerism.
... Fossil records have traced the origins of the notochord to the Middle Cambrian ($510-495 mya), and the extinct eel-shaped Pikaia gracilens is still argued to be either a basal chordate or a more specialized, divergent one (Lacalli, 2012;Mallatt & Holland, 2013;Morris & Caron, 2012). Structures that could represent evolutionary precursors of the notochord have been sought in non-chordate phyla; the stomochord of different species of hemichordates (acorn worms) has been repeatedly probed for notochord marker genes, and is currently considered more closely related to chordate organs of pharyngeal origin than to an ancestral notochord (Peterson, Cameron, Tagawa, Satoh, & Davidson, 1999;Satoh et al., 2014). ...
The notochord is a structure required for support and patterning of all chordate embryos, from sea squirts to humans. An increasing amount of information on notochord development and on the molecular strategies that ensure its proper morphogenesis has been gleaned through studies in the sea squirt Ciona. This invertebrate chordate offers a fortunate combination of experimental advantages, ranging from translucent, fast-developing embryos to a compact genome and impressive biomolecular resources. These assets have enabled the rapid identification of numerous notochord genes and cis-regulatory regions, and provide a rather unique opportunity to reconstruct the gene regulatory network that controls the formation of this developmental and evolutionary chordate landmark. This chapter summarizes the morphogenetic milestones that punctuate notochord formation in Ciona, their molecular effectors, and the current knowledge of the gene regulatory network that ensures the accurate spatial and temporal orchestration of these processes.
... body's long axis, so that unilateral contractions of these muscles will cause the trunk to bend sideways (Lacalli, 2012). The mouth is located on the left side of the head in larval amphioxus but then migrates closer to the midline at metamorphosis. ...
... They are chevron-shaped in amphioxus, but W-shaped or straight in some of the fossil chordates. These differences are functionally significant, because chevron-and Wshaped muscle segments allow the contractions of individual muscle fibers to sum more effectively than straight muscle segments, thus generating stronger bending forces and faster swimming (Lacalli, 2012). Still, we can conclude that the earliest vertebrates did not invent a radically new form of locomotion, though they might have improved its efficiency. ...
Some time in the Ediacaran or early Cambrian period, the first vertebrates emerged. Compared to the invertebrate chordates, early vertebrates were active predators, rather than suspension feeders. This change in behavior was facilitated by several major morphological innovations, including pharyngeal muscles that pump water through the pharynx, vascularized gills, paired image-forming eyes, a complex vestibular apparatus, lateral line receptors, taste buds, and a well-developed olfactory system. Early vertebrates also evolved several new brain regions, notably the telencephalon and the midbrain. Developmentally, most of these innovations were linked to the emergence of two novel embryonic tissues, namely placodes and neural crest. Although these tissues and their adult derivatives did not evolve “out of nothing,” they represent genuine innovations that contributed substantially to the evolutionary success of the vertebrate lineage.
... Large specimens of Pikaia reach 4 cm in length. Originally thought [35] to bear a notochord, the structure running along the animal's back is now called the 'dorsal organ', and a notochord and/or notochord plus nerve cord may occur in a position ventral to the dorsal organ [42]. True notochords of course do occur in modern Amphioxus and Branchiostoma. ...
... An analysis of swim mechanics in Pikaia concluded that it must have been a slow swimmer because it lacked the fast-twitch fibers that allow rapid motion in modern fish and other living chordates [42]. ...
Deuterostomes make a sudden appearance in the fossil record during the early Cambrian. Two bilaterian groups, the chordates and the vetulicolians, are of particular interest for understanding early deuterostome evolution, and the main objective of this review is to examine the Cambrian diversity of these two deuterostome groups. The subject is of particular interest because of the link to vertebrates, and because of the enigmatic nature of vetulicolians. Lagerstätten in China and elsewhere have dramatically improved our understanding of the range of variation in these ancient animals. Cephalochordate and vertebrate body plans are well established at least by Cambrian Series 2. Taken together, roughly a dozen chordate genera and fifteen vetulicolian genera document part of the explosive radiation of deuterostomes at the base of the Cambrian. The advent of deuterostomes near the Cambrian boundary involved both a reversal of gut polarity and potentially a two-sided retinoic acid gradient, with a gradient discontinuity at the midpoint of the organism that is reflected in the sharp division of vetulicolians into anterior and posterior sections. A new vetulicolian (Shenzianyuloma yunnanense nov. gen. nov. sp.) with a laterally flattened, polygonal anterior section provides significant new data regarding vetulicolians. Its unsegmented posterior region (‘tail’) bears a notochord and a gut trace with diverticula, both surrounded by myotome cones.
... Large specimens of Pikaia reach 4 cm in length. Originally thought [35] to bear a notochord, the structure running along the animal's back is now called the 'dorsal organ', and a notochord and/or notochord plus nerve cord may occur in a position ventral to the dorsal organ [42]. True notochords of course do occur in modern Amphioxus and Branchiostoma. ...
... An analysis of swim mechanics in Pikaia concluded that it must have been a slow swimmer because it lacked the fast-twitch fibers that allow rapid motion in modern fish and other living chordates [42]. ...
Deuterostomes make a sudden appearance in the fossil record during the Early Cambrian. Two deuterostome groups, the chordates and the vetulicolians, are of particular interest for understanding the evolutionary dynamics of the Cambrian evolutionary event. Lagerstätten in China and elsewhere have dramatically improved our understanding of the range of variation in these ancient animals. Cephalochordate and vertebrate body plans are well established at least by Cambrian Series 2. Taken together, roughly a dozen chordate genera and fifteen vetulicolian genera document an explosive radiation of deuterostomes at the base of the Cambrian. A new vetulicolian with a polygonal anterior section and a narrow, unsegmented posterior region (‘tail’) bearing possible myotomes provides new insight into the affinities of the various body plans that emerged during the Early Cambrian. It seems clear that the advent of deuterostomes near the Cambrian boundary involved both a reversal of gut polarity and a two-sided retinoic acid gradient, with a gradient discontinuity at the midpoint of the organism that is reflected in the sharp division of vetulicolians into anterior and posterior sections.
